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

Stand in the paint aisle without a number and you’re guessing — and both kinds of guess cost you. Buy too little and you’re back the next day hunting for the same batch code, hoping the new can matches the half-dry first coat. Buy too much and you’ve got a part-used gallon slowly skinning over in the garage, money sunk into a colour you’ll never use again. The estimate that avoids both is almost embarrassingly simple: it’s wall area, a couple of multipliers, and one rounding rule. The trouble is that every step has a place where intuition quietly leads you wrong.

The core idea: paint is sold by area, bought by the can

Concrete is sold by volume and tile by the box, but paint is really an area problem wearing a volume label. A can tells you how many square feet or square metres it covers, and your job is to work out how much wall there is. Everything else — coats, primer, texture — is a multiplier on that one number.

The wall area of a rectangular room is its perimeter times its height:

wall area = 2 × (length + width) × height

A 14 ft × 12 ft room with 8 ft ceilings has 2 × (14 + 12) × 8 = 416 square feet of wall. That’s the figure paint is measured against — not the floor area, which is a number people reach for by habit and which has nothing to do with how much wall there is to cover.

Why doors and windows barely move the number

The instinct is to carefully measure every door and window and subtract them. In practice the savings are small, and the calculator uses sensible standard sizes rather than asking you to measure each opening: 21 square feet per door (about 1.95 m²) and 15 square feet per window (about 1.4 m²). A room with one door and two windows loses 21 + 30 = 51 sqft from its 416, leaving 365 sqft of net paintable wall.

Notice the scale: those openings shaved off roughly an eighth. That’s why obsessing over the exact dimensions of a window is wasted effort — but ignoring openings entirely in a room full of patio doors and picture windows will have you buying a whole extra can you don’t need. Subtract the typical openings; don’t agonise over them.

Coats are a multiplier, and two is the honest default

A single coat almost never looks finished. Roller marks, patchy sheen and the old colour ghosting through are all signs of a job that needed a second pass, which is why the calculator defaults to two coats. Coats multiply directly:

paint needed = net wall area × coats ÷ coverage per can

Going from a light colour to another light colour, two coats is right. Painting a deep or saturated colour, or going light-over-dark, plan for three — strong colours have weaker hiding pigments and the underlying tone fights through. Going dark-over-light is more forgiving and often closes up in two. Each coat is a full pass over the whole net area, so the third coat on that 365 sqft room is another 365 sqft of coverage to buy, not a touch-up.

Surface texture quietly eats your coverage

Here’s the variable most people never account for. A can’s coverage rating assumes a smooth wall, but texture increases the actual surface area the paint has to wrap around, so a rougher wall drinks more paint per square foot of apparent area. The calculator lets you pick the spread rate to match:

Surface	Coverage (sqft/gallon)
Smooth drywall / plaster	350
Light texture (orange peel, light knockdown)	300
Heavy texture (stucco, popcorn, masonry)	250

The jump from smooth to heavy is a 40% increase in paint for the same wall. Spray a textured ceiling or a stucco exterior off the smooth number and you’ll run dry well before the job is done. Match the spread rate to what you’re actually rolling onto.

A worked example: a two-room repaint

Say you’re repainting a living room (14 × 12 × 8 ft, one door, two windows) and a bedroom (12 × 10 × 8 ft, one door, one window), smooth walls, two coats, no ceilings:

1. Living room wall area: 2 × (14 + 12) × 8 = 416 sqft, minus 21 + 30 = 51 → 365 sqft net
2. Bedroom wall area: 2 × (12 + 10) × 8 = 352 sqft, minus 21 + 15 = 36 → 316 sqft net
3. Total net area: 365 + 316 = 681 sqft
4. Paint: 681 × 2 ÷ 350 = 3.89 gallons → round up to 4 gallons

That rounding step matters: you can’t buy 3.89 gallons, and a job that comes up a few hundred millilitres short on the final wall is a wasted trip and a visible lap mark where the fresh can meets the dried edge. The Paint Coverage Calculator does this for any number of rooms at once, in litres or gallons, and always rounds up to whole containers so the number you see is the number you buy.

Primer is a separate calculation

Bare drywall, patched repairs, glossy trim and dramatic colour changes all want primer first — and primer is not the same as a coat of your colour. The calculator treats it as one coat at about 200 sqft per gallon, a deliberately lower spread rate because primer is formulated to soak into and seal porous surfaces rather than glide. For the 681 sqft job above, that’s 681 ÷ 200 ≈ 3.4 → 4 gallons of primer, bought separately from your colour. Self-priming “paint and primer in one” can fold this into the main coats on previously painted walls in good condition, but on bare or repaired surfaces a dedicated primer coat still earns its place.

Don’t forget the ceiling — if you’re painting it

By default the estimate covers walls only, because plenty of repaints leave the ceiling alone. If you are doing the ceiling, it adds its floor footprint to the area:

ceiling area = length × width

That 14 × 12 living room adds 168 sqft per coat — and ceilings are very often textured, so drop to the 300 or even 250 spread rate when you include them. Toggle the ceiling option on and the calculator folds it into the total automatically.

Frequently asked questions

How much does a gallon of paint cover? On smooth interior walls, roughly 350 square feet for one coat — about 8.5 m² per litre. Light texture pulls that down to ~300 and heavy texture to ~250, and every figure assumes a single coat, so a standard two-coat job covers half as much wall per can as the headline number suggests.

Do I really need two coats? Almost always, yes. One coat leaves roller lap marks, uneven sheen and the old colour showing through. Two is the realistic default; deep or saturated colours and light-over-dark changes often need three because their hiding power is lower.

Should I subtract doors and windows? Subtract them using standard sizes — about 21 sqft per door, 15 sqft per window — but don’t measure each opening to the inch. The savings are modest, and the standard allowances are close enough that the rounding-up step absorbs the difference.

Is “paint and primer in one” enough to skip primer? On sound, previously painted walls of a similar colour, often yes. On bare drywall, fresh patches, glossy trim or a big colour swing, a dedicated primer coat still seals and grips far better — estimate it separately at the lower ~200 sqft/gallon spread rate.

Measure your rooms, drop the numbers into the Paint Coverage Calculator, and walk into the store with one figure instead of a guess. Sizing the rest of the project too? The Tile & Flooring Coverage Calculator and Concrete Volume & Bags Calculator do the same job-sizing math for floors and pours.