Paint Calculator

Enter the area, the coverage rate from the product data sheet and the number of coats to estimate the paint needed and the number of cans.

Surface area to paint

Enter the area directly. The tool does not work it out from wall sizes and does not subtract doors or windows: add up the walls and subtract the openings yourself.

How the coverage rate is written (match the product data sheet)

Coverage rate (for one coat)

Enter the rate for a single coat. If the label gives a rate for two coats, convert it to one coat first. Enter the value from the product data sheet; this tool does not suggest a typical rate, because it depends on the product, the surface and how it is applied.

Number of coats (1 to 10; the one-coat rate is multiplied by this)

Allowance for waste (%, added on top of the net amount)

An allowance of 10% means 1.1 times the net amount. It is not the same as a usable-yield percentage, which divides instead. No typical figure is suggested.

Container size (optional)

Enter the size of one container to see how many are needed (rounded up).

Paint needed (including the allowance) —

Details
Coverage rate for one coat	—
Net amount (without the allowance)	—
Amount needed with the allowance	—
Containers needed (rounded up)	—

This is an estimate. The real amount depends on the paint, the surface and how it is applied. Values are shown with 7 significant digits (rounded).

The number copied is the amount needed with the allowance, in US gallons (the first unit shown in the result).

How it is calculated

Coverage for one coat. If the label says how much area 1 gallon covers (for example, ft² per US gal), the amount per unit area is 1 ÷ that number.
Net amount = area × amount per unit area × number of coats. The coverage rate you enter is for one coat, so it is multiplied by the number of coats.
Amount needed = net amount × (1 + allowance). The allowance is added on top of the net amount.
Containers = amount needed ÷ container size, rounded up (only when you enter a container size).

The calculation uses exact fractions. The only rounding is the display, and the container count is rounded up once at the end. Conversions use these definitions: 1 ft = 0.3048 m, 1 US gallon = 231 cubic inches = 3.785411784 L, 1 US quart = 1/4 gallon.

Units

ft² per US gal is the area one US gallon covers, the form many US products use. m² per L is the same idea in metric units, and L per m² is the amount used per square meter.
US gallons and imperial (UK) gallons are different. This tool uses US gallons only. An imperial gallon is 4.54609 L, about 20% larger. If a label uses imperial gallons, convert the rate to m² per L first.
Results are shown in US gallons and liters. Enter a container size in US gallons, US quarts, liters or milliliters.

Example

400 ft² to paint, a coverage rate of 350 ft² per US gal for one coat, 2 coats and a 10% allowance:

Net amount: 400 ÷ 350 × 2 = about 2.2857 US gal
With the allowance: 2.2857 × 1.1 = about 2.5143 US gal
With 1 US gal containers: 2.5143 → 3 containers

What is not included

The area is entered directly. Doors, windows, trim, ceilings and the difference between walls are not worked out for you.
The tool does not know the paint's coverage, drying behavior, primer needs or how porous the surface is. It uses only the numbers you enter.
Powder coatings and paints sold by weight are not covered here; the tool works with paint sold by volume.
The estimate is not a purchasing guarantee. Check the product data sheet and ask the supplier when the amount matters.

Sources

The unit definitions come from NIST SP 811 (Appendix B.8, the foot and the US gallon rows, read word for word) and NIST Handbook 44 Appendix C (1 gallon = 231 cubic inches, 1 quart = 57.75 cubic inches, opened and searched), checked on November 1, 2026. The paint formula itself is a general calculation, not taken from a source.

Frequently asked questions

How much paint do I need for 400 square feet?

It depends on the coverage rate of the product and the number of coats. At 350 ft² per US gallon for one coat, two coats need about 2.29 US gallons before any allowance.

Is the coverage rate per coat or for all coats?

This tool takes the rate for one coat and multiplies by the number of coats. If your label states a rate for two coats, convert it first.

Does it subtract windows and doors?

No. Enter the area you will actually paint.

What allowance should I use?

The tool does not suggest a figure. Decide it from the surface, the paint and your experience.