How to use the paint calculator
- Measure the floor length and width in feet, and the wall height from floor to ceiling. For an L-shaped or irregular room, add up the lengths of all the walls, then enter half of that total as the length and 0 as the width, since the tool only uses the perimeter, 2 × (length + width). (Leave the ceiling box unticked in that case.)
- Count the doors and windows you won't paint. Each standard door takes off about 21 sq ft (3 × 7 ft) and each window about 15 sq ft (3 × 5 ft).
- Choose the number of coats. Two is typical when changing color; one may be enough when refreshing the same color.
- Check the label on your paint for its coverage and enter it. Rough or porous surfaces use more paint, so lower the number for textured walls or bare drywall.
How it's calculated
Gallons = wall area × coats ÷ coverage per gallon
Ceiling area = length × width
The tool then suggests what to buy: whole gallons plus any quarts needed to cover the remainder. If you'd need three or more quarts, it rounds up to another gallon, because a gallon usually costs less than three quarts. Ceiling paint is calculated separately because ceilings usually get a different flat white paint.
Worked example
A 14 × 12 ft bedroom with 8 ft walls, one door and two windows, painted with two coats of paint that covers 350 sq ft per gallon:
- Gross wall area: 2 × (14 + 12) × 8 = 416 sq ft
- Minus openings: 416 − 21 − 30 = 365 sq ft
- Paint: 365 × 2 ÷ 350 = 2.09 gallons, so buy 2 gallons + 1 quart
A 10 × 10 ft room with 8 ft walls, no openings counted, one coat at 400 sq ft per gallon, needs 320 ÷ 400 = 0.8 gallons, so a single gallon covers it.
FAQ
How much area does a gallon of paint cover?
Most interior wall paints list about 350 to 400 square feet per gallon for one coat on smooth, primed walls. Textured or unprimed surfaces can cut that noticeably.
Do I need two coats of paint?
Two coats give more even color and are standard when changing colors or painting over patches. One coat can be enough when repainting the same color over a sound surface.
Should I subtract doors and windows?
Yes, for a more accurate estimate. This tool removes about 21 sq ft per door and 15 sq ft per window. Keep a little extra paint for touch-ups anyway.
Does primer count as a coat?
Primer is a separate product with its own coverage rate. Run the calculator once for primer with one coat and its coverage, then again for the topcoat.