What is paint coverage?
Paint coverage is the area one gallon of paint covers in a single coat. It is printed on the back of every can, usually as a range — 350 to 400 square feet per gallon, or 10 to 12 square metres per litre. That one number turns a room into a shopping list, and it is the number most people never read before they get to the till.
Coverage is a claim about a good surface, not a law of physics. A primed, previously painted wall gives you the number on the label. Bare drywall drinks paint and can pull a gallon down to 250 square feet. Textured walls have more surface than their measurements suggest. Going dark over light needs a third coat, which is a coats problem rather than a coverage one, but it empties the can just the same.
The reason any of this matters is that paint is sold in whole containers. You cannot buy 1.7 gallons. A 12 by 10 foot bedroom with 8-foot walls, one door and two windows works out to 301 square feet of paintable wall and 1.7 gallons at two coats — and the answer you actually need is buy 2 gallons. Getting from the tape measure to that sentence is the whole job, and it is the one this Paint Calculator finishes rather than handing back a decimal.
How to use the Paint Calculator
Measure the room, count the openings, read the shopping line. It updates as you type, and there is nothing to dismiss before you start.
- Pick a mode. Room size wants a length, a width and a wall height — the three numbers a tape measure gets you. Known area takes a paintable square footage you already worked out, which is what you want for an L-shaped room, a stairwell, or a vaulted ceiling.
- Choose feet or metres. The toggle converts the values you already typed, so the same room gives the same physical answer either way.
- Type the room. Wall area is the perimeter times the height. You do not need to measure each wall separately unless the shape is genuinely odd.
- Count the doors and windows. Each door subtracts a standard 21 square feet, each window 15. Those are the trade-standard sizes and they are close enough for most houses.
- Turn on the ceiling if you are painting it. That adds length × width — often the difference between two cans and three.
- Leave coats at 2. One coat is the most common wrong assumption in this category, and it under-buys by half.
- Check the coverage against your can, then read the shopping line at the top. Copy takes all four numbers with you.
That is the entire flow. No postcode, no email, no "find a painter near you" waiting at the end of the result. The calculator most people find first is a lead form wearing a calculator's clothes — the arithmetic is free, and your phone number is the price. This one just does the arithmetic.
The formula behind paint quantity
Four lines, and you can check every one of them by hand.
walls = 2 × (length + width) × height
openings = doors × 21 sq ft + windows × 15 sq ft
paintable = (walls − openings) + (ceiling ? length × width : 0)
paint = paintable × coats ÷ coverage
Take the 12 by 10 foot bedroom with 8-foot walls, one door and two windows. The perimeter is 2 × (12 + 10) = 44 feet, times 8 feet of height, so the walls are 352 square feet. The door takes off 21 and the two windows take off 30, leaving 301 square feet of paintable wall. Two coats over 301 square feet is 602 square feet of painting, divided by 350 square feet per gallon: 1.72 gallons, shown as 1.7.
Then comes the part most paint calculators leave to you in the aisle. Let r be the fraction left over after the whole gallons. If r is zero, buy the whole gallons. If r is a quarter or less, buy the whole gallons plus one quart. Anything more, round up to the next gallon. Here r is 0.72, so the answer is buy 2 gallons.
The quarter-gallon line is a price decision, not a rounding convention. Below it, a quart is the cheaper way to finish the job. Above it, the gallon costs less per litre than the quarts you would need and leaves you real touch-up paint for the scuff you will put in that wall next year. Tick the ceiling on the same room and it adds 120 square feet: 421 square feet paintable, 2.4 gallons, buy 3.
How much paint common rooms need
Every row below uses 8-foot walls, one door, two windows, two coats, and 350 square feet per gallon — and comes straight out of the paint calculator above, not a rule of thumb.
| Room | Paintable walls | Paint needed | Buy | Buy with ceiling |
|---|---|---|---|---|
| Small bedroom, 10 × 10 ft | 269 sq ft | 1.5 gallons | 2 gallons | 2 gallons + 1 quart |
| Bedroom, 12 × 10 ft | 301 sq ft | 1.7 gallons | 2 gallons | 3 gallons |
| Large bedroom, 14 × 12 ft | 365 sq ft | 2.1 gallons | 2 gallons + 1 quart | 3 gallons + 1 quart |
| Living room, 16 × 14 ft | 429 sq ft | 2.5 gallons | 3 gallons | 4 gallons |
| Open living/dining, 20 × 15 ft | 509 sq ft | 2.9 gallons | 3 gallons | 5 gallons |
Two things jump out of that table. The first is the quart in row three: the 14 by 12 room needs 2.1 gallons, and a third full gallon for that last tenth would be money on a shelf. The second is the ceiling column. On the 20 by 15 room the ceiling adds 300 square feet — more than half a bedroom's worth of walls — and pushes the order from three cans to five.
Notice also how little the room's shape changes things compared with its perimeter. A 16 by 14 room and a 20 by 15 room differ by 76 square feet of floor, but 80 square feet of wall. Wall paint is bought by the perimeter, and a long thin room costs more to paint than a square one of the same floor area. A 20 by 4 hallway with four doors has almost exactly the same paintable area as that 12 by 10 bedroom — 300 square feet against 301 — despite having a third of the floor.
Where paint estimates go wrong
The coverage default is the biggest single lever, and it is the one worth overriding. Run the same 301-square-foot bedroom at 250 square feet per gallon — a fair figure for bare drywall — and it needs 2.4 gallons instead of 1.7. The order goes from two cans to three on one input you could have read off the label.
Why it sometimes says "2.0 gallons" and then tells you to buy a quart. That bedroom with no doors or windows at all is 352 square feet, which needs 2.011 gallons. The display rounds to one decimal; the can rule does not. Two gallons would leave you about a metre of wall short, so it adds the quart. The shopping line is always built from the exact figure, never the rounded one.
A few other places the number moves. Ceiling paint is usually a different product from wall paint, so most people buy it separately rather than folding it into one total — run the room twice if that is you. Primer is its own calculation, typically 200 to 300 square feet per gallon at one coat. Exterior siding and stucco are thirstier than interior walls, so the room formula still applies but the coverage default does not. And if you enter so many doors and windows that they swallow the walls, the paint calculator stops and says so rather than quietly returning zero.
One honest limitation: this works a room at a time. For a whole house, run each room and add the cans up — averaging wall areas across rooms will under-buy the one with the fewest openings.
Related calculations
If your room is not a rectangle, work the area out first and bring the number back in Known area mode. The square footage calculator handles rooms made of several rectangles, and the area calculator covers circles, triangles and trapezoids for the gable end of a room with a sloped ceiling.
Wall paint is bought by the perimeter rather than the floor, so the perimeter calculator is the faster route when you have an odd-shaped room and only need the distance around it. If the job includes a splashback or a floor, the tile calculator works the same way for tiles, and the concrete calculator does it for a slab or a set of steps. Working from plans drawn in metres while shopping in gallons? The unit converter will get you between the two, though the feet/metres toggle here already handles it for this calculation.
Frequently asked questions
How much paint do I need for one wall?
Measure the wall, multiply length by height, and put that straight into Known area mode. A 12 by 8 foot wall is 96 square feet, which needs 0.5 gallons at two coats — so you buy 1 gallon, because there is nothing smaller than a quart and a quart will not cover it. Single walls are where the rounding feels most wasteful, which is a good reason to do the whole room in one order if you are painting it all eventually.
How much paint for the ceiling only?
The ceiling is length × width. A 12 by 10 ceiling is 120 square feet, which is 0.7 gallons at two coats and a single gallon to buy. Enter it in Known area mode with the ceiling paint's own coverage figure — ceiling paint is often a flat, high-build formulation with a different number on the can than the wall paint sitting next to it.
What if my room has a sloped or vaulted ceiling?
Work the walls out in pieces and add them up, then use Known area. The rectangular parts are straightforward; the triangular gable end is half the base times the height. The Paint Calculator's Room size mode assumes four walls of equal height, so it will under-count a vaulted room every time — it is the one shape where doing the geometry yourself is genuinely better than typing three numbers.
Why do the metric and imperial answers match?
Because the toggle converts the values rather than swapping one region's conventions for another's. US cans quote 350 to 400 square feet per gallon, European cans quote 10 to 12 square metres per litre, and those are not the same physical number — 350 sq ft/gal is about 8.6 m²/L. A calculator that relabels the units and sets metric to a flat 10 gives two different answers for one room. Here the 12 by 10 bedroom comes out at 28.0 m² and 6.5 litres, which buys one 5 L can plus two 1 L cans.
My can doesn't print a coverage figure. What should I use?
Start at 350 square feet per gallon for interior latex on a primed or previously painted wall, and drop it to 250 if the surface is bare drywall, new plaster or heavily textured. For primer, use 200 to 300 with coats set to 1. These are working defaults rather than guesses to be proud of — if the can has a number, that number wins.
Does it need an account, and does it store my measurements?
No account, no sign-up, nothing stored. The calculation runs in your browser and the numbers leave when you close the tab. It costs nothing to use, which is worth stating plainly rather than making a slogan of it: this is the full tool, not a sample of one, and there is no version behind a subscription that does the same arithmetic with more colours. Microapp gives 10% of every dollar it earns to charity, off the top, audited quarterly.