Paint Calculator
Enter the room measurements, the number of doors and windows and how many coats. We tell you how much paint to buy.
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The formula
Paintable area = 2 x (length + width) x height - doors x 20 sq ft - windows x 15 sq ft. Paint = area x coats / coverage per gallon.
How the estimate works
We add up the area of all four walls, subtract a standard allowance for each door (20 sq ft) and window (15 sq ft), multiply by the number of coats and divide by what a gallon covers. Most paint covers about 350 square feet per gallon, and the tin tells you the exact figure.
An example
A 12 by 14 foot room with 8 foot walls has 416 sq ft of wall. With one door and two windows, that leaves 416 - 20 - 30 = 366 sq ft. Two coats is 732 sq ft, which needs about 2.1 gallons, so you would buy 3 one-gallon cans or one 2-gallon and a quart.
Tips
- Dark colours and fresh plaster often need an extra coat or a primer.
- Rough surfaces use more paint than smooth walls.
- Buy from one batch so the colour matches, and keep a little for touch-ups.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Room length (ft): 12; Room width (ft): 14; Wall height (ft): 8; Doors: 1; Windows: 2; Coats: 2; Coverage per gallon (sq ft): 350 | Paint needed: 2.25 gallons |
| Room length (ft): 10; Room width (ft): 10; Wall height (ft): 9; Doors: 1; Windows: 1; Coats: 2; Coverage per gallon (sq ft): 350 | Paint needed: 2 gallons |
| Room length (ft): 16; Room width (ft): 20; Wall height (ft): 8; Doors: 2; Windows: 4; Coats: 1; Coverage per gallon (sq ft): 400 | Paint needed: 1.25 gallons |
Paint Calculator: frequently asked questions
Do I include the ceiling?
We show the ceiling separately as one coat. Add it if you plan to paint it too.
How much does a gallon cover?
Typically 350 to 400 square feet on smooth walls. Check the can.
How accurate is this estimate?
It is as accurate as the numbers you give it. Real-world amounts such as paint, concrete or fuel vary, so allow a margin.
Is what I enter in this tool saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this tool free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026