How it works
Every line in the box defines one unit as name=value_in_base, meaning how many base units fit inside it. Conversion runs through that base in two steps: base = value × from_factor, then result = base ÷ to_factor, which is algebraically value × (from ÷ to). The starting list makes the metre the base at 1, with foot=0.3048, inch=0.0254, yard=0.9144, furlong=201.168 and league=4828.032. Both dropdowns rebuild from your text as you type, and the table shows the value in every defined unit at once.
Assign 1 to whichever unit you want as the reference; every other factor is then that unit's size in base units. Values accept scientific notation, so 1e-9 parses fine, but they must be positive and finite, and any line whose value is zero, negative or non-numeric is dropped from the list without a warning. Lines beginning with # are treated as comments. Save units stores the whole textarea in your browser, Clear empties both the box and the stored copy, and each table row carries its own delete button.
This is a pure scale converter: one multiplication, no offset. Temperature cannot be expressed, because Celsius to Fahrenheit needs an added 32 that the format has no room for, and neither can derived units such as m/s or kg·m. Nothing checks dimensions either, so define kilogram and furlong in one list and it will convert between them without complaint. Definitions live in this browser only, and the row delete button matches lines starting exactly with name=.
Frequently asked questions
Can I define Celsius and Fahrenheit as custom units? +
No. Each line is a single multiplier against the base unit, and temperature scales also need an offset, since Fahrenheit is 1.8 × Celsius plus 32. The same limitation rules out any pair of scales that do not share a zero point.
Where are my saved unit definitions kept? +
In your own browser's local storage, under one key, written when you press Save units. Nothing goes to a server. Clear empties the textarea and removes the saved copy, and wiping your browser's site data for this site will lose the definitions too.
Which unit should I set equal to 1? +
Whichever you want as the reference, usually the smallest or most fundamental one, so the remaining factors are numbers above 1 and easier to sanity-check. The starting list uses meter=1, which is what makes foot 0.3048 and furlong 201.168.
Can I enter very small or very large factors? +
Yes, decimals and scientific notation both parse, so 1e-9 and 4.828032e3 are accepted. Values must be positive and finite; a line holding zero, a negative number or text where the number should be is skipped, and that unit never appears in the dropdowns.
How do I share my unit list with someone else? +
Copy the text out of the definition box and paste it into theirs, since the whole system is only those name=value lines. There is no export file and no shareable link, and each browser keeps its own separately saved copy.
Conversion factors, and what they cannot express
A custom conversion is a multiplication by a factor, which handles any pair of units measuring the same quantity on scales sharing a zero point. Length, mass, volume and most engineering units work this way.
Temperature does not, because Celsius and Fahrenheit have different zero points — converting between them requires an offset as well as a factor, which a pure ratio cannot express. Kelvin and Celsius share a scale but not a zero; Kelvin and Rankine share neither with the others.
Dimensional consistency is on you
A factor-based converter will happily convert between units of different quantities and produce a meaningless number. Converting a mass to a volume requires a density, and converting a flow rate to a total requires a time — those are physical facts, not conversion factors, and the tool cannot supply them.
Precision is the other consideration: some conversions are exact by definition (an inch is exactly 25.4 mm) and others are measured approximations. Carrying more decimal places than the underlying factor justifies gives false confidence, particularly when several conversions are chained and rounding compounds at each step.
