Paint Calculator
Work out how much paint a room needs, with doors and windows taken out.
Last reviewed September 19, 2026
Result
Enter room length and room width to see the result.
https://calcgallery.com/paint-drywall/paint-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure the length and width of the room, and the height of the walls floor to ceiling.
- Enter how many doors and windows are in the room. Their areas come out of the total.
- Choose the number of coats. Two is normal over a similar colour.
- Read the quantity. It is rounded up to the next quart, or the next litre in metric.
How it is calculated
The four walls of a rectangular room are two pairs, so their combined area is:
Wall area = 2 × (length + width) × heightOpenings come out of that, and the ceiling goes in if you are painting it:
Paintable = wall area − doors − windows + ceilingThen the paint itself, where coverage is the area one unit of paint covers in one coat:
Paint = paintable × coats ÷ coverageThe result is rounded up to something you can buy. In the United States that is the next quart, because paint is sold in quarts and gallons. In metric markets it is the next whole litre.
Coverage is the number most worth changing. The default is a typical figure for a smooth, primed surface, and your tin will state its own. Textured, porous or previously unpainted surfaces can use half as much again on the first coat.
Practical tips
Buy the whole job in one go. Paint is mixed in batches and two tins of the same colour can differ slightly. If the job needs more than one tin, buy them together and box them: pour the lot into a larger container, stir, and decant back. Then every wall gets the same colour.
The first coat on bare surfaces is not a real coat. New plaster, filler and bare drywall absorb paint rather than being covered by it. Prime first, or accept that the first coat is doing the priming and buy for three.
Cutting in uses less than you think, and takes longer than you think. The edges are maybe a tenth of the paint and half the time. Work out the total from the whole wall area, as this does, rather than trying to separate the two.
Leftovers are worth keeping, but not much. Half a litre in a sealed tin is enough for touch-ups for years. Half a tin of a colour you will never use again is money you spent on nothing, which is the argument for measuring rather than adding a safety tin.
Check the sheen, not just the colour. Matt, eggshell and satin in the same colour cover at slightly different rates, and a patch repair in the wrong sheen is as visible as one in the wrong colour.
With your numbers
- Wall area = 2 × (12 ft + 14 ft) × 8 ft
- Paintable = wall area − doors and windows = 365 ft²
- Paint = 365 ft² × 2 coats ÷ 92,460 ft² per gallon = 2.086 gal
- Rounded up to 2 gal 1 qt
Quick reference
Paint is sold by the tin, so this rounds up to the next quart. Buy the whole job at once for an even colour.
| Job | Paint to buy |
|---|---|
| 10 × 10 ft room, 8 ft walls, 2 coats | 1 gal 3 qt |
| 12 × 14 ft room, 8 ft walls, 2 coats | 2 gal 1 qt |
| 12 × 16 ft room, 9 ft walls, 2 coats | 2 gal 3 qt |
| 15 × 20 ft room, 8 ft walls, 2 coats | 3 gal |
| 3 × 4 m room, 2.4 m walls, 2 coats | 6 L |
| 4 × 5 m room, 2.4 m walls, 2 coats | 8 L |
| 5 × 6 m room, 2.7 m walls, 2 coats | 11 L |
Common questions
- How much paint do I need for a 12 by 14 foot room?
- About 2 gallons and 1 quart for two coats, assuming 8 ft walls, one door and two windows, and a spread rate of 350 ft² per gallon. That is the worked example above, and you can change any of those numbers.
- How many square feet does a gallon of paint cover?
- Around 350 to 400 ft² per coat on a smooth, primed surface. Bare drywall, render and anything textured drink considerably more, sometimes half as much again on the first coat. Your tin states its own figure, and you can put that in under More options.
- Do I need two coats?
- Usually. One coat over a similar colour can look acceptable until the light changes. Going lighter over a dark wall often needs three, or a tinted primer first, which is cheaper than a third coat of finish paint.
- Should I subtract doors and windows?
- Yes, and this calculator does. A door is about 21 ft² and a window about 15 ft². Skipping them on a small room can overstate the paint by a fifth, which is a whole extra tin you did not need.
- Does this include primer?
- No. Primer is a separate product with its own spread rate, usually a little lower. Work it out as one coat and buy it separately.
- What about the ceiling?
- There is a toggle for it. Ceilings are usually painted in a different product to the walls, so if you are buying two different paints, run the calculation twice rather than adding the ceiling to the wall total.