Paint Calculator
Enter the length and height of the walls, subtract doors and windows, and choose the number of coats to see how many gallons of paint to buy. Coverage per gallon is editable so you can use the figure on your can.
How to use this paint calculator
- Measure each wall you will paint and add the lengths. Enter the total and the wall height.
- Add the ceiling area if you will paint it with the same paint. Leave it at 0 if not.
- Enter the combined area of doors and windows. Those are not painted with wall paint.
- Choose the number of coats. Two coats is common, but a bare or dark surface may need more.
- Check the coverage per gallon on your paint can and change the default if it differs.
- Add a price per gallon if you want a cost estimate, then press Calculate.
How to calculate how much paint you need
The calculator finds the area to paint, multiplies by the coats and divides by what one gallon covers.
- Area to paint = wall length (ft) x wall height (ft) + ceiling area – area of doors and windows
- Gallons = area to paint x coats / coverage per gallon
- Gallons to buy = gallons, rounded up to a whole gallon
- Cost = gallons to buy x price per gallon
Worked example
You are painting a room with 40 ft of wall, 8 ft high. The wall area is 40 x 8 = 320 sq ft. A door and a window take 40 sq ft, so the area to paint is 280 sq ft. With 2 coats that is 560 sq ft of coverage. At 350 sq ft per gallon you need 560 / 350 = 1.6 gallons, so you buy 2 gallons. At $35 per gallon the paint costs 2 x 35 = $70.00.
If you also paint a 120 sq ft ceiling, the area becomes 400 sq ft, and 800 / 350 = 2.29 gallons, so you would buy 3 gallons.
Assumptions & limits
- The default coverage of 350 sq ft per gallon is an editable placeholder. Real coverage depends on the paint, the surface and how it is applied, so use the figure on your can.
- Rough, porous or heavily textured surfaces soak up more paint than smooth ones.
- The calculator does not add paint for trim, baseboards, doors or windows. Work those out separately.
- Changing color from dark to light may need primer or extra coats that this tool does not predict.
- The price is your own input and the calculator does not look up prices.
Frequently asked questions
How many gallons of paint do I need for a room?
Multiply the wall length by the height, subtract doors and windows, multiply by the number of coats and divide by the coverage per gallon. A room with 280 sq ft of wall and two coats needs about 1.6 gallons at 350 sq ft per gallon.
How much does a gallon of paint cover?
Many cans state a figure such as 350 to 400 sq ft per gallon, but the real number depends on the product and the surface. Enter the number on your can.
How many coats of paint do I need?
Two coats is common. A new, bare or very different color may need more. Ask the paint maker or your supplier about primer.
Should I include the ceiling?
Only if you paint it with the same paint. Enter its area in the ceiling box. Ceiling paint is often sold as a separate product.
Do I subtract windows and doors?
Large openings, yes. The calculator subtracts the area you enter. Small openings are usually ignored.
Can I use the result for exterior walls?
Yes, for the same maths. Enter the exterior wall area and openings, and use the coverage figure from your exterior paint.
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Last updated: November 2026