Guides & conversion know-how

Equivalent Aperture: Why f/2.8 on a Crop Sensor Isn't Full-Frame f/2.8

Crop factor changes more than focal length — it changes depth of field and light gathering too. Here's what 'equivalent aperture' really means, why your APS-C f/1.8 behaves like full-frame f/2.7, and where the analogy stops.

How New Tire Sizes Throw Off Your Speedometer and Odometer

Fit taller or shorter tires and your speedo starts lying — reading low when you go bigger, high when you go smaller. Here's the simple ratio behind the error, how to calculate your true speed, and why your mileage and fuel economy drift too.

Why Your Coffee Tastes Sour or Bitter — and How to Fix It With Ratio, Grind and Time

Sour, bitter or just flat? Learn how extraction actually works, what under- and over-extraction taste like, and the three dials — ratio, grind and contact time — that fix a bad cup.

How to Pace a Marathon: Even Splits, Negative Splits and the Riegel Reality Check

The most common way to ruin a marathon is starting too fast. Here's how to set a realistic goal pace from a shorter race, why negative splits win, and how to build a per-kilometre plan you can actually hold.

Why a Bigger Air Conditioner Makes You Less Comfortable (The Short-Cycling Trap)

An oversized AC cools the air fast but leaves the room clammy, wears out early and costs more to run. Here's why short cycling happens, how to spot it, and how to size a unit that's right instead of just big.

Build Your Own Unit Converter: The One-Base-Unit Trick That Handles Anything

Every unit converter, from kilometres to game currencies to lab reagents, runs on the same tiny idea: define one reference unit as 1 and express everything else against it. Here's how the factor method works and how to define your own units for workflows no off-the-shelf tool covers.

What a $5,000 Overseas Invoice Actually Pays You After FX, Fees and Tax

A cross-border freelance invoice loses money at three separate stages — the payment processor, the exchange-rate spread, and your local tax — applied in an order that matters. Here's how to trace a headline number down to what really lands in your account.

From Hourly Rate to Fixed Quote: The Buffers That Stop You Losing Money

Why multiplying your hourly rate by an estimate produces a quote that quietly loses money, and how a revisions buffer, expense pass-through and profit margin turn a raw number into a project price you can actually deliver against.

GH, KH, ppm and dKH: Reading Aquarium Water Numbers That Refuse to Match

Your test kit says 8 dKH, the fish profile wants 120 ppm, and the reef calculator asks for meq/L. They're all the same water measured three ways. Here's the single conversion factor that ties them together — and why GH and KH aren't interchangeable.

Bra Size Conversion and Sister Sizes: Why 34D, 32DD and 36C Are the Same Cup

UK, US, EU, AU and JP bras label the same body with different numbers, and the band and cup aren't independent. Here's how the conversion actually works from two measurements — and why a sister size fixes a bra that's close but not quite.

Max VG Isn't a Number You Choose — It's a Number Your Ingredients Leave You

Your nicotine base and flavour concentrates smuggle PG into every DIY e-liquid, so the highest VG you can hit is fixed before you add a drop of pure VG. Here's the arithmetic that decides it, and how to push the ceiling higher.

Reading a Fertilizer Label Right: Why NPK Percentages Aren't the PPM Your Plants See

A 6-2-4 fertilizer doesn't put 6, 2 and 4 into your tank. The P and K on the label are oxide forms that need converting, and PPM only means something once you know your tank volume and EC scale. Here's the full chain from bottle to root zone.

What a Resin Print Actually Costs: mL to Grams to Dollars, Without the Guesswork

Resin pricing looks simple until supports and density get involved. Here's the real chain — model volume × density for weight, bottle price ÷ bottle size for cost per mL — and why the support buffer, not the resin brand, is what moves the number.

Sticker Sizing for Sellers: Why Your 900px Design Isn't 3 Inches, and Bleed Eats Your Sheet

Pixels only become inches once you fix a DPI, and the gaps between stickers — bleed plus kiss-cut margin — quietly shrink how many fit on a sheet. Both feed directly into your cost per sticker and the price you should list.

What Does a 3D Print Actually Cost? Pricing Prints for Etsy Without Losing Money

Filament is the cheap part. The real cost of a print is electricity, machine wear, failed prints and your own labour — here's how to add them all up and set a price that pays you.

Crochet Hook Sizes Decoded: US Letters, UK Numbers, Metric mm and Japan

Why the same hook has three different names, why UK numbers run backwards, how mm is the only measurement that matters, and how to match hook size to yarn weight without ruining your gauge.

How Far Should a Grow Light Be From Plants? The Inverse Square Law, Explained

Why a few inches of hang height changes your light intensity so drastically, how PPFD targets differ by growth stage, why lux meters mislead, and how to set canopy distance with confidence.

px, pt, rem, em, %: Which Font-Size Unit to Use and How to Convert Them

A plain-English guide to CSS font-size units — why pt is a print unit, how rem and em actually differ, the 62.5% trick, and how to convert any size to any other unit.

How Long Should a Podcast Episode Be? Format, Listening Context and File Size

Why episode length is set by format and where people listen, not by a magic number — plus how ad load eats content time and how to estimate MP3 file size from bitrate and duration.

How Much Fabric Do I Need? Yardage, Fabric Width and the Nap Nobody Warns You About

Why the same dress needs 30–50% more fabric on a narrow bolt, what 'with nap' really costs you, how pattern repeats add up, and how to estimate quilt top and backing yardage.

DIY E-Liquid Mixing: Why PG/VG Ratio and Nicotine Strength Fight Each Other

How DIY e-liquid mixing math works — nicotine dilution, the hidden PG your nic base and flavour concentrate already carry, and why some PG/VG targets simply aren't reachable.

Pixels Aren't a Size: DPI, Print Prep and the Bleed Nobody Explains

Why a pixel has no physical size until you attach a DPI, how to convert pixel dimensions to cm/mm/inches and back, and why bleed makes your print file bigger than the finished piece.

Sidechain Compression Timing: Why the Pump Depends on the Clock, Not the Ear

How to set sidechain attack and release in milliseconds from BPM and note divisions, why a fast attack and a beat-synced release create the pump, and how dotted and triplet values shift the feel.

Your 8-Bar Loop Is the Wrong Length: The Time Signature Trap in Sample Editing

Bars-to-seconds math looks like simple division, but compound time signatures redefine what a 'beat' even is — get that wrong and every trimmed loop drifts out of sync.

1 Hour in the Oven Isn't Always 4 Hours on High: The Real Math Behind Slow Cooker Conversions

The '1h oven = 4h high = 8h low = 20 min Instant Pot' rule of thumb only holds for one dish type. Stews, roasts, beans and rice each convert at a different ratio — here's why.

Why Your Homemade Curry Powder Never Tastes Like the Jar: Spice Blend Math Explained

Classic spice blends aren't a pinch of this and a dash of that — they're fixed ratios of parts. Scaling them by weight (not volume) is what makes a homemade blend taste consistent batch after batch.

Why Your Cask-Strength Whisky Math Is Off: The V₁×ABV₁ Rule (and Its Hidden Error)

Diluting a spirit to a target ABV isn't just 'add some water' — it's a mass-conservation equation, and skipping the contraction correction can put your final ABV off by half a point.

The Homebrew Number Nobody Trusts Until They Check It: Why Your OG Guess Is Usually Wrong

Predicted original gravity, beer color, and strike water temperature aren't guesswork — they're three separate formulas, and getting any one input wrong (especially efficiency) throws off the whole recipe.

Why '$4.99 a Sub' Isn't What Streamers Actually Make: The Twitch Payout Math

A Tier 1 Twitch sub costs viewers $4.99, but the streamer doesn't see all of it — and bits, ads, and Prime subs each follow a completely different payout formula. Here's the real math.

What's Actually Driving Your Electricity Bill? A Watts-to-Dollars Guide

Why wattage alone can't tell you what an appliance costs to run, the huge gap between always-on vampire loads and short heavy loads, and a worked comparison that shows which one actually wins on your monthly bill.

1:1 or 2:1? A Beekeeper's Guide to Mixing Sugar Syrup by Weight

Why beekeepers feed sugar syrup at all, the real difference between 1:1 and 2:1 mixes, and why weighing your sugar instead of measuring cups gets you a consistent batch every time.

Candle Wax and Fragrance Load: The Two Numbers Everyone Gets Wrong

Why the headspace you leave at the top of a vessel changes your wax total, why fragrance load is a percentage of wax weight (not total volume), and how to work out both correctly with a worked example.

Canning at Altitude: Why You Must Adjust Time or Pressure (and How)

Water boils cooler as you climb, which quietly undermines home canning safety. Here's why USDA adjusts boiling-water processing time and pressure-canner PSI for altitude — with both official lookup tables and a worked example.

How Much Feed Does Your Flock Actually Need? A Chicken Keeper's Guide

The rule-of-thumb feed rates for bantams, standard layers and meat birds, why cold weather and free-ranging change the math, and how to turn a daily rate into a monthly or yearly feed budget.

$75/hr Contracting vs. $75/hr Salaried: Why They're Not the Same Money

The self-employment tax gap, the health insurance you'd suddenly have to buy yourself, the 401(k) match you'd forfeit, and the unpaid gaps between contracts — why an identical hourly number means very different take-home, and where the '1.25-1.5x' rule of thumb actually comes from.

What Does It Actually Cost to Charge an EV? Home vs. Public Charging

The real formula behind an EV charging bill — why $/kWh alone hides charging losses, demand charges and time-of-use rates, and how charger power (not battery size) sets your charging time. Worked example included.

Why Fence Quotes Go Wrong: The Concrete Math Almost Everyone Gets Wrong

Two silent errors blow up fence-post concrete estimates: counting posts as length ÷ spacing instead of spacing + 1, and treating hole size as a linear number when volume actually scales with the radius squared.

Stop Salting Sauerkraut 'To Taste': Why Fermentation Runs on Weight, Not Volume

Salt percentage by weight is what actually keeps lacto-fermented vegetables safe and crisp. Here's why a tablespoon lies, how the brine and dry-salt methods differ, and the safe ranges for sauerkraut, pickles, kimchi, hot sauce and olives.

Bigger Tires, Same Gears: Why Your Speedometer and Your Engine Both Get It Wrong

Fitting larger tires without regearing throws off your speedometer, odometer, fuel economy math and cruise control — and quietly robs the engine of RPM. Here's the math behind both problems and the fix.

Why 'Running Watts' Underestimates the Generator You Actually Need

Motors like fridge compressors, sump pumps, and AC units draw a brief 2-3x surge on startup. Here's the one-motor rule for sizing a generator that survives that moment instead of stalling.

How Many Fish Can Your Tank Hold? Why 'One Inch per Gallon' Is Wrong

Real aquarium stocking math — actual water volume, species bioload, filter turnover and schooling minimums — instead of the outdated inch-per-gallon rule.

How Much Hay Does a Horse Eat? Turning the 1.5–3% Rule into a Buying Plan

Why there's no single answer to how much hay a horse eats, the 1.5-3% of body weight rule equine nutritionists actually use, why horses need near-constant forage access, and how to turn a daily percentage into a real hay-buying budget.

Knitting Gauge: Why Your Sweater Came Out Huge (and How to Fix It)

What gauge really measures, how to swatch honestly, the math for converting a pattern's stitch counts to your tension, and which way to change needle size.

Why '3 Inches Deep' Means a Different Truckload for Every Material

The cubic-yards-from-square-feet-and-depth formula, why mulch weighs a fraction of what gravel does at the same depth, and when bagged material actually beats a bulk delivery.

MIG Welding Settings: Why There's No Single Correct Number

Why MIG amperage, voltage and wire feed speed are always a starting point to tune by ear and eye — how WFS actually sets your amperage, why voltage and wire speed have to move together, and how thickness and material change the target.

Mileage Deductions Explained: US IRS Standard Mileage vs UK HMRC AMAP

How the US flat cents-per-mile deduction and the UK's tiered 45p/25p mileage allowance actually work, who qualifies, and worked examples for both — plus why you must always check the current-year rate before filing.

Priming Sugar: How Much to Add at Bottling (and Why Temperature Matters)

How to calculate priming sugar for bottle carbonation — residual CO₂, target volumes by style, sugar-type conversion factors, and how to avoid bottle bombs.

How Much Alcohol for a Wedding or Party? The Formula Bartenders Actually Use

How to turn guest count, event length and your crowd's drinker mix into an exact shopping list — bottles of wine, cases of beer, bottles of spirits and bags of ice — using the industry's 'one drink per person per hour' rule.

Watts to Speed: The Physics of How Fast Your Cycling Power Actually Takes You

Why the same 200 W gives wildly different speeds, how rolling resistance, aerodynamic drag and gravity split your power, why aero dominates on the flat and weight dominates on climbs, and how wind changes everything.

Video Bitrate to File Size: H.264 vs H.265 vs AV1 (and Why ProRes Is Huge)

How bitrate, duration and codec determine video file size, the recommended bitrates for each resolution and frame rate, why H.265 and AV1 halve your storage, and why editing codecs like ProRes are 20× bigger on purpose.

Yeast Substitution: Converting Fresh, Active Dry, Instant and Sourdough Starter

The industry-standard ratios for swapping fresh, active dry and instant yeast, why a gram of one is not a gram of another, how teaspoon measures betray you, and how to replace commercial yeast with sourdough starter.

How to Batch Cocktails for a Party: Scaling, Dilution and What Not to Pre-Mix

How to scale any cocktail recipe from a single serving to a full batch — including the water dilution stirred drinks need and why shaken citrus drinks should never be pre-batched.

How Much to Feed Your Dog: A Real Calorie-Based Portion Guide

Why the feeding chart on your dog food bag is only a rough starting point, how RER and life-stage multipliers actually determine a dog's daily calories, and how to adjust portions by body condition instead of guesswork.

Streaming Bitrate Explained: The Right OBS Settings for Twitch, YouTube and Kick

How platform bitrate caps, resolution, FPS and content motion combine to set the right streaming bitrate, why the 1.5x upload headroom rule exists, and how to avoid dropped frames without over-compressing your stream.

Why Your WAV Files Are So Huge: Sample Rate, Bit Depth and File Size

How sample rate, bit depth and channel count multiply into uncompressed audio file size, why FLAC isn't 'half the size' of WAV by a fixed ratio, and when 192kHz is actually worth the disk space.

IBU Explained: Why 60 Minutes in the Boil Isn't the Same as 15

How hop utilization actually works in the Tinseth formula, why gravity and boil time matter more than the AA% on the bag, and how to read BU:GU ratio so a beer's 'bitterness number' means something.

Finding Meeting Hours That Don't Wreck Someone's Sleep

How to actually find working-hour overlap across a distributed team, why DST breaks fixed offset math twice a year, and how to read a UTC overlap band instead of doing timezone arithmetic in your head.

Day Rate to Salary: What a Freelance Number Is Really Worth

Why a freelance day rate doesn't multiply straight into a salary, how billable days, tax, super and unpaid leave reshape the number, and how to price a day rate that actually matches an employed package.

FPS to Frame Time: What Milliseconds Per Frame Actually Buys You

Why frame rate is really a measure of time, how to convert any FPS to milliseconds per frame, why the jump from 60 to 144 feels bigger than 144 to 240, and how frame time ties into input lag.

Word Count to Pages: Why 'How Many Pages?' Has No Single Answer

Why page count depends entirely on format, what the 250-words-per-page manuscript standard really means, how trade paperbacks and web posts differ, and how to estimate pages for a novel, essay or blog.

Why Your 100 Mbps Connection Downloads at 12 MB/s: Bandwidth Explained

The bits-versus-bytes mix-up that makes downloads feel eight times slower than your plan, how to turn any connection speed and file size into a real download time, why ~10% overhead is normal, and decimal versus binary units.

Modular Type Scales: How One Ratio Sizes Your Whole Design

Why picking font sizes by feel produces a muddy hierarchy, how a modular scale multiplies a base size by a single ratio, what the musical-interval ratios mean, and how to export the result as CSS variables or a Tailwind block.

°F, °C, Gas Mark and Fan: How to Convert Any Oven Temperature

Why a recipe written for one oven scorches in another, the exact formula linking Fahrenheit and Celsius, where UK gas marks come from, and the 20°C rule for fan-forced ovens — a practical guide to oven temperature conversion.

Aspect Ratios Explained: Resize Without Stretching and Find the Ratio Behind Any Size

What an aspect ratio really is, how to resize an image so it never distorts, why locking width or height matters, and how the greatest common divisor turns 1920×1080 into a clean 16:9.

Barbell Plate Math: How to Load Any Target Weight Per Side

Why loading a barbell is a per-side problem, how the greedy 'heaviest plate first' method works, why bar weight and plate inventory change the answer, and how to handle weights your plates can't hit exactly.

How to Scale a Recipe Up or Down Without Wrecking It

The one ratio that scales every ingredient, why servings beat guesswork, which numbers you must never multiply, and how friendly fractions keep a doubled recipe actually measurable.

A4, Letter and the √2 Trick: How Paper Sizes Actually Work

Why A4 halves perfectly into A5, where the 841 × 1189 mm of A0 comes from, how the B and C series relate to A, and why US Letter refuses to scale — a practical guide to converting between every standard paper size.

Reading Regex Out Loud: Tokens, Quantifiers and Flags Demystified

Character classes, anchors, quantifiers, groups and the flags that change everything — how to read any regular expression left to right and stop pasting patterns you don't understand.

Training Zones, Explained: Heart Rate, FTP and Pace From One Threshold

Why every zone model hangs off a single threshold number, how Coggan power zones, Friel heart-rate zones and McMillan pace ranges differ, and how to turn your FTP, LTHR or threshold pace into Z1–Z5 you can actually train to.

How Many Tiles Do I Need? Area, Grout Lines and Pattern Waste

Why you can't just divide floor area by tile area: the grout-gap correction, pattern waste from straight to herringbone, rounding up to full boxes, and how to order tile without a second trip.

Board Feet, Explained: How Lumber Is Really Measured and Priced

What a board foot actually is, the thickness × width × length ÷ 144 formula, why nominal lumber is smaller than it says, and how to turn board feet into linear feet and a real shopping total.

Unix Time, Explained: Epoch Seconds, Timezones and the ISO Formats

What a Unix timestamp really counts, why the epoch starts in 1970, seconds versus milliseconds, how timezones and UTC fit in, ISO 8601 vs RFC 3339, and the 2038 problem — with a converter that does it both ways.

How Long Will a Battery Last? Turning mAh into Real Runtime

Why mAh alone can't tell you battery life, how watt-hours make different batteries comparable, and the efficiency factor that separates the spec sheet from the stopwatch — with the formula behind every runtime estimate.

Macros from Calories: Turning a Calorie Target into Grams of Protein, Fat and Carbs

How the 4-4-9 rule converts any calorie goal into grams, why protein is the macro you anchor first, and how diet presets like keto and high-protein actually change the split — with the simple arithmetic behind it all.

From Note to Hertz: How Equal Temperament Turns Pitch into a Number

The single exponential formula that maps every musical note to a frequency, why a semitone is the twelfth root of two, how A4 = 440 (or 432) sets the whole grid, and what cents and MIDI numbers really mean.

The Exposure Triangle: How ISO, Shutter and Aperture Trade Stops

What a 'stop' of light really is, how aperture, shutter speed and ISO each control it, and why every exposure has dozens of equivalent settings that all meter the same — with the one equation that ties them together.

Wilks, DOTS & IPF GL: How Powerlifting Scores Compare Lifters Across Weight Classes

Why a 600 kg total means something different at 60 kg than at 120 kg, how Wilks 2020, DOTS and IPF GL points each correct for body weight, and which coefficient to trust when you compare two lifters.

Crop Factor Explained: What '50mm Equivalent' Really Means

Why a 50mm lens frames differently on APS-C, Micro Four Thirds and medium format, how crop factor scales both field of view and depth of field, and the one thing it does not change — your exposure.

How Much Paint Do I Need? Walls, Coats and Coverage Explained

The simple area math that turns a room into litres or gallons of paint, why doors and windows barely help, how coats and surface texture change the number, and when to buy primer separately.

Mouse Sensitivity Conversion Between Games: cm/360, eDPI and Why Your Aim Resets

Why a 0.4 in Valorant isn't a 0.4 in CS2, what cm/360 and eDPI really measure, and the simple yaw-constant math that keeps your muscle memory intact when you switch games.

Shoe Size Conversion: How US, UK, EU and Japanese Sizes Actually Relate

Why a US 9 is a UK 8 and an EU 42, what foot length in centimetres has to do with all of it, and how to measure your feet so an online size chart actually fits.

Oven to Air Fryer Conversion: The −25°F Rule and the Foods That Break It

How to convert any oven recipe to air-fryer time and temperature with the standard −25°F / ×0.8 rule, why an air fryer cooks faster, and the food-specific overrides for wings, fries, fish and baked goods.

Tire Size Comparison and Plus Sizing: Reading the Numbers on the Sidewall

What 225/45R17 actually means, how to compare two tire sizes for diameter, speedometer error and clearance, and the rule for plus sizing without throwing off your speedo or rubbing the fender.

Hyperfocal Distance: How to Get a Landscape Sharp Front to Back

What hyperfocal distance means, the formula that finds it, how circle of confusion and sensor size change the answer, and why focusing at the hyperfocal distance gives you the deepest depth of field for any aperture.

What Time Should I Go to Bed? Sleeping in 90-Minute Cycles

Why waking up at the end of a sleep cycle leaves you alert and mid-cycle leaves you groggy — how to count back from your alarm in 90-minute cycles, account for fall-asleep time, and nap without the fog.

What Size Air Conditioner Do I Need? A BTU Sizing Guide

How to size an AC, heat pump or heater in BTUs from room area, ceiling height, insulation, climate and sun — and why bigger is not better. A plain-English Manual-J-lite walkthrough.

How to Pick the Right Series Resistor for an LED

Why an LED needs a resistor, the Ohm's-law formula that sizes it, how to read forward voltage and current from a datasheet, and how to choose a standard value and power rating that won't cook.

Password Entropy Explained: What 'Bits of Entropy' Really Mean

How password strength is actually measured — what bits of entropy are, how the character pool and length drive the math, why crack-time tables differ by attacker, and why a four-word passphrase beats P@ssw0rd1.

Running Pace Explained: Convert min/km to min/mi and Predict Your Race Times

How running pace, speed and race-time prediction actually work — converting between min/km and min/mi, reading km/h vs mph, and using the Riegel formula to forecast your 5K, 10K, half and full marathon.

How Much Concrete Do I Need? Slabs, Footings and Bags Explained

The simple geometry that turns a slab or post hole into cubic yards of concrete, how many bags of mix that really means, and when to stop bagging and order a ready-mix truck.

Coffee Brewing Ratios Explained: The Golden Ratio for Pour Over, Espresso and French Press

What the coffee-to-water ratio actually controls, why 1:16 became the 'golden ratio', and how to dial in pour over, espresso, AeroPress, French press and cold brew by weight instead of guesswork.

Understanding ABV: How Alcohol Content Is Calculated from OG and FG

What original and final gravity measure, the formula that turns them into alcohol by volume, and why your homebrew's strength is decided before fermentation even finishes.

3D Print Filament: Estimating Length, Weight and Cost Before You Print

How filament diameter and material density connect grams, metres and dollars — so you can price a print and know if the spool will last.

Cron Expressions Explained: From Every Minute to Once a Year

The five cron fields, what the special characters mean, and a cheat sheet of ready-to-use schedules — so you stop guessing whether your job runs when you think.

HEX, RGB, HSL and OKLCH: A Designer's Guide to Color Spaces

What each color notation actually means, why HSL lies about brightness, and how OKLCH finally makes perceptually even color ramps possible on the web.

Sous Vide Time and Temperature: A Practical Guide to Perfect Results

How sous vide temperature sets doneness and time sets tenderness, with a reference chart for steak, chicken, eggs and more — plus the food-safety basics.

How to Calculate Your One-Rep Max (and Why Every Formula Disagrees)

Epley, Brzycki, Lombardi and more — how 1RM estimation formulas work, which is most accurate, and how to use your max to program training.

BPM to Milliseconds: Setting Delay and Reverb Times by the Numbers

How to convert tempo to delay time in milliseconds, the formula behind note divisions, and why dotted and triplet delays sit in the groove.

Cups to Grams: The Complete Baking Conversion Guide

Why 1 cup of flour isn't 1 cup of sugar, how ingredient density changes the math, and a reference table for converting US cups to grams accurately.