Paint Calculator
Work out exactly how many gallons of paint your room needs — including doors, windows, and a second coat.
Measure your room, set the number of coats, and this tells you how many gallons to buy. It subtracts doors and windows so you're not paying for paint you'll never use.
Your details
Floor to ceiling. Most rooms are 8 ft; older homes are often 9 or 10.
Two is standard. Go to three if you're covering a dark colour with a light one.
Counted at 21 sq ft each (a standard 3 x 7 ft door).
Counted at 15 sq ft each (a standard 3 x 5 ft window).
Check your paint can. Most interior latex covers 350-400 sq ft per gallon.
Result
Rounded up — paint is sold by the can, and you want the same batch.
- Paintable wall areaSquare feet, after doors and windows come out.
- 365
- Total wall areaSquare feet, before subtracting openings.
- 416
- Gallons per coat
- 1.04
About this tool
How Much Paint Do I Need? A Straight AnswerSkip the guesswork at the paint counter. Here's the actual formula, what the can's coverage number really means, and a calculator that does it for you.
Free download
Room-by-Room Paint PlannerMeasure every room in one pass, then work out the paint before you get to the store. The assumptions below are the same ones the calculator uses.
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Paint Calculator infographicThe key numbers as one image — free to save, share, or embed on your own site with credit.
How this is calculated
The maths is simple: perimeter x wall height gives total wall area, then doors (21 sq ft each) and windows (15 sq ft each) come off. Multiply by the number of coats, divide by the coverage on your paint can, and round up to whole gallons. This covers walls only — not the ceiling, and not trim. A ceiling is just length x width, so add that separately if you're painting it. Trim and doors use so little that a single quart usually does a whole room. Two things worth knowing: bare drywall and deep colour changes drink more paint than the can claims, so if you're in either situation, buy the extra gallon. And buy it all at once — paint is mixed in batches, and a second trip can come back a shade off.
Common questions
- Should I round up or down?
- Up, always. Running out halfway through a wall means a second trip, a new batch, and a visible seam where the two meet. Leftover paint is also what you use for touch-ups later.
- Does this include the ceiling?
- No — this is walls only. For a ceiling, multiply room length by width and divide by your coverage figure. A 12 x 14 room is 168 sq ft, so well under one gallon.
- Why does the calculator subtract doors and windows?
- Because you don't paint them. On a small room with two windows and a door that's around 50 sq ft, which is a meaningful slice of a single gallon's coverage.
- Do I need primer as well?
- If you're going over bare drywall, patched areas, or a dramatically darker colour, yes. Primer covers roughly the same area per gallon, so run the numbers again with one coat to estimate it.
Take it further with AI
Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.
I used the Paint Calculator at https://www.bfcbrilliance.com/tools/paint-calculator.
What I entered:
- Room length (ft): ___
- Room width (ft): ___
- Wall height (ft): ___
- Coats: ___
- Doors: ___
- Windows: ___
- Coverage per gallon (sq ft): ___
What it calculated:
- Gallons to buy: ___
- Paintable wall area: ___
- Total wall area: ___
- Gallons per coat: ___
Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.
Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.Last updated
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Part of a bigger job
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