Paint Calculator

Works out the wall area of a room minus doors and windows, and how much paint it takes for your number of coats and the coverage of your paint.

How to use

1. Under "Measure by", choose "Whole room" and enter "Room length", "Room width" and "Wall height". If the walls differ, choose "Wall by wall".
2. Under "Doors and windows (not painted)", set the number of "Doors" and "Windows" and their sizes. Switch on "Paint the ceiling too" if needed.
3. Set "Coats" (2 by default) and replace "Coverage per coat" with the value on your can to see "Paint needed" and "Cans to buy".
4. For new walls or a big color change, switch on "Also calculate primer" to get the primer separately.

How it works

• Area to paint = room perimeter × wall height − doors and windows (+ ceiling). Paint needed = area × coats ÷ coverage.
• The default coverage, 375 sq ft per gallon (about 9.2 m² per litre), is the middle of the typical 350 to 400 sq ft that Sherwin-Williams gives (checked October 2026). Rough, porous or dark surfaces take more.
• Cans are counted in quarts, gallons and 5-gallon pails (1, 4 and 18 L on metric pages), with a table for buying only one size.

Questions and answers

How much paint do I need for a 12x12 room?

With 8 ft walls the wall area is 384 sq ft, or about 352 sq ft after one door and one window. Two coats at 375 sq ft per gallon need about 1.9 gallons.

How many coats of paint do I need?

The default is 2 coats. The right number depends on the paint and the color underneath, so follow the can and change "Coats".

How many square feet does a gallon of paint cover?

It depends on the product. The default is 375 square feet per coat, so two coats cover about 187 square feet. Use the value on your can.

Should I subtract doors and windows?

Yes, the tool subtracts them. The starting sizes are a 3 ft × 6 ft 8 in door and a 3 × 4 ft window; change them to match yours.