Paint calculator
Enter the room measurements and see how much paint to buy.
Your inputs
Your inputs are processed on your device and are not sent to a server.
Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
You have already measured the room perimeter: the baseboard comes from that same figure.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Measure the room length and width, and the ceiling height.
- Say how many doors and windows there are: they get subtracted.
- Choose the number of coats. Two is normal over an already painted wall.
- If your tin states a different coverage, change it in the advanced options.
How it is calculated
There are four steps and none of them is mysterious:
Coverage is the only figure that does not come from your room: the manufacturer prints it on the tin, in square feet per gallon per coat. Interior emulsion typically covers 350-400 sq ft per gallon. Over new drywall, a very porous surface, or a light colour over a dark one, it covers less.
The default deductions are 17 sq ft per door (a standard 32 x 80 in leaf) and 16 sq ft per window. If you have large picture windows, change them: in a living room with ten feet of glazing the difference is close to a quart.
Worked example
A 13 x 10 ft room with an 8 ft ceiling, one door, one window, two coats and paint covering 350 sq ft per gallon:
- Perimeter: 2 x (13 + 10) = 46 ft
- Walls: 46 x 8 = 368 sq ft
- Minus door and window: 368 - 17 - 16 = 335 sq ft
- Two coats: 335 x 2 = 670 sq ft of painting
- Paint: 670 / 350 = 1.91 gal, plus 10 % waste = 2.1 gallons
In practice that is two gallons plus a quart. The leftover is exactly what you want in the garage for touch-ups.
Good to know
- The ceiling is separate. Tick the box and the length times width is added. Ceilings usually drink more per square foot because they are harder to roll evenly.
- The first coat on new drywall soaks in. Over fresh board or plaster, budget for a primer or a thinned first coat.
- Big colour changes. Going from red to white can take three coats. Put it in the field: working it out is cheaper than finding out halfway.
- Buy the same batch. Two tins of the same colour from different batches are not always an exact match.
Frequently asked questions
How much paint do I need for a 12x12 room?
With an 8 ft ceiling, one door, one window and two coats, about 2.4 gallons. That comes from roughly 351 sq ft of net wall area painted twice, with paint covering 350 sq ft per gallon plus 10 % waste.
How much does a gallon of paint cover?
Between 350 and 400 square feet per coat for interior emulsion on a prepared surface. On porous, textured or unprimed walls it can drop to 250 sq ft. The figure on your tin is the one to use.
Do you subtract doors and windows?
Yes. The defaults are 17 sq ft per door and 16 sq ft per window, and both are editable. In a small room with large glazing the deduction genuinely matters.
How many coats of paint do I need?
Two is standard over a wall already painted a similar colour. One coat usually leaves the colour patchy. Three are needed when covering a dark colour with a light one, or painting unprimed new plasterboard.
Why is this different from the store's estimate?
Nearly always the coverage figure. Many retail charts quote 400 sq ft per gallon with no waste and a single coat. Here you can enter the real coverage of your tin and the coats you will actually apply.
How much waste should I allow?
10 % covers what stays in the tray, the roller and the drips. If this is your first time painting, use 15 %: having a spare quart beats running out mid-wall.