How much paint do you need?
Measure the length, width and height of a rectangular room in feet. The calculator adds all four walls, subtracts the total area of doors and windows, then applies your number of coats and coverage per gallon. Ceiling area is optional and uses the same paint settings.
Gallons = [(2 × (length + width) × height) − openings + optional ceiling area] × coats ÷ coverage × (1 + allowance ÷ 100)
A 12 × 10 room: worked example
With an 8-foot ceiling, the walls total 352 square feet. Subtract 42 square feet of openings to get 310 square feet. Two coats at 350 square feet per gallon, plus a 10% allowance, require about 1.95 gallons. Rounded up to whole cans, buy 2 gallons. At the sample price of $40 per gallon, paint costs $80 before tax.
Choosing coverage and allowance
Read the label for the paint you plan to use. BEHR describes coverage of 250–400 square feet per gallon for certain interior paints; this range is product-specific. Our editable 350-square-foot starting value is an assumption, not a promise of coverage. Porous or textured surfaces and application technique can change consumption.
The 10% allowance is an adjustable planning assumption for losses and touch-ups. It is applied after the coat calculation. Primer, trim and paint for a ceiling with different coverage should be estimated separately.
Source: BEHR: determining how much interior paint to buy.
Paint calculator FAQs
Should I subtract doors and windows?
Enter their combined area if you will not paint them. For example, a 3 × 7-foot door is 21 square feet. Measure your actual openings instead of assuming every door is the same size.
Does paint-and-primer mean I only need one coat?
No coat count is assumed from the product name. Follow the product instructions and assess your surface and color change. This tool defaults to two coats, which you can change.
Can I calculate several rooms?
Calculate each room separately and download each estimate. If all rooms use exactly the same product and color, add their unrounded gallon requirements before rounding your final purchase; rounding each room separately can overestimate the combined order.