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Paint Calculator: How Much Paint for a Room
Enter the room dimensions, the area of doors and windows, the number of coats and the coverage printed on the can. The calculator estimates the paint needed for the walls and, if you choose, the ceiling.
Wall, ceiling and coverage maths
The four walls of a rectangular room form a band whose area is the perimeter times the height: 2 × (length + width) × height. Subtract the doors and windows you will not paint. The ceiling is length × width. Add them, multiply by the number of coats, and divide by the coverage on the can.
Coverage is the area one unit of paint covers in a single coat on a suitable surface. It is a manufacturer figure and the one input worth checking on your actual product.
Why real coverage is often lower
Label coverage assumes a smooth, sealed, previously painted surface. Bare plaster, new drywall, textured walls, brick and dark-to-light colour changes soak up or hide more paint, so a primer or an extra coat is common. Rolling thickly or spraying also changes how far a can goes. If you are unsure, enter a lower coverage figure to build in margin.
Estimating doors and windows
A single interior door is often around 20 square feet (about 1.8 m²) and a medium window is somewhere near 15 square feet (about 1.4 m²), though sizes vary. Measure for accuracy when it matters. Leaving openings out entirely gives a small safety margin, which many painters accept, and subtracting them gives the tighter figure.
Skirting boards, trim, cupboards and radiators are not included. Paint those separately from a small tin.
Buying the right amount
- Round up to the next full container.
- Buy all the paint for one room at once, and mix tins together to even out small colour differences.
- Keep a small amount for touch-ups, since colour matching later can be hard.
- Use a separate quantity for primer, which may cover a different area per unit.
Preparing and buying paint
Surface condition changes how much paint you use. Rough, porous or newly plastered walls absorb more than smooth, previously painted ones, so the coverage figure on the tin is a best case. Dark colours, strong colour changes and unprimed surfaces often need an extra coat. Buy enough for the whole room at once, or mix tins together, because colour can differ slightly between batches. Keep leftover paint with its colour code written on the lid so touch-ups match. Deduct large openings such as windows and doors, but small outlets and trim can be ignored, since they roughly offset the paint lost to brushes and rollers.
Worked example
A 14 ft by 12 ft room with 8 ft walls, 36 sq ft of doors and windows, two coats and the ceiling painted too, at 350 sq ft per gallon.
Inputs
| Units | Imperial (feet, square feet, gallons) |
|---|---|
| Room length (feet or meters) | 14 |
| Room width (feet or meters) | 12 |
| Wall height (feet or meters) | 8 |
| Total area of doors and windows (square feet or square meters) | 36 |
| Paint the ceiling too? | Yes, include the ceiling |
| Number of coats | 2 |
| Coverage of the paint | 350 |
Result
For 2 coats. Round up to the next full can.
| Wall area after openings | 380.0 ft² |
|---|---|
| Ceiling area | 168.0 ft² |
| Area for one coat | 548.0 ft² |
| Area over all coats | 1,096.0 ft² |
| Paint needed | 3.13 US gallons |
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How much paint do I need for a 12 by 14 foot room?
With 8 foot walls the wall area is about 416 square feet before doors and windows. Two coats at 350 square feet per gallon need roughly two gallons after allowing for openings.
How many coats of paint should I plan for?
Two coats is the usual plan for a colour change or new surface. A single coat can be enough over a similar existing colour with a good-quality paint.
Do I need to paint the ceiling with the same paint?
No. Ceilings are often painted with a flat or ceiling paint, so run the calculator once for the ceiling alone and once for the walls if you want separate quantities.
How many square meters does a liter of paint cover?
Many labels give somewhere around 8 to 12 square meters per liter per coat for standard interior paint. Use the exact number printed on your can.
Should I subtract doors and windows?
You can for a tighter estimate. Many people ignore small windows because the saved paint is minor, and the extra covers cutting in and touch-ups.