Paint Calculator: How Many Gallons for a Room

The formula

1. Wall area: 2 × (length + width) × wall height. A 12 × 12 foot room with 8 foot walls has 2 × 24 × 8 = 384 square feet of wall.
2. Subtract openings: the calculator uses 21 square feet per door (about 3 × 7 feet) and 15 per window by default. Change them under "Door and window sizes" to match yours. One door and two windows bring the example to 333 square feet.
3. Multiply by coats: 333 × 2 = 666 square feet of painting.
4. Divide by coverage: 666 ÷ 350 = 1.9 gallons.

The ceiling, if included, is length × width, worked out separately because ceilings are usually painted with a different product.

Rounding to cans

Paint is sold in gallons and quarts (4 quarts per gallon). The calculator rounds the leftover part of a gallon up to whole quarts, and switches to another gallon when you would need three or more quarts. Compare prices at your store, since a gallon may cost about the same as three quarts; for large jobs, 5 gallon pails may be cheaper.

Where coverage comes from

Every paint can and technical data sheet lists a spread rate, usually as a range of square feet per gallon. That figure assumes a smooth, sealed surface and a normal film thickness. You will get less than the label says when:

• the surface is textured, porous or freshly patched;
• you are painting new drywall or bare wood that needs primer first;
• you are covering a much darker or brighter colour;
• you are rolling with a thick-nap roller, which holds more paint.

If in doubt, use the lower end of the range on the label.

Measuring the room

Measure wall lengths along the floor and the height from floor to ceiling. For rooms that are not rectangles, add up the length of every wall to get the perimeter and use it in place of 2 × (length + width): enter half the perimeter as the length and 0 as the width. For sloped ceilings and gable walls, see the triangle formula in our measuring guide.

Older homes and lead paint

The U.S. EPA notes that homes built before 1978 may contain lead-based paint. Sanding, scraping or removing old paint can create hazardous lead dust. Before disturbing old paint, read the EPA's Renovate Right guidance, and hire a certified lead-safe contractor for paid renovation work in pre-1978 homes, as federal rules require.

Trim, doors and cabinets

This calculator is for walls and ceilings. Trim, doors and cabinets use much less paint, usually a different sheen, and are easiest to estimate from the coverage on a quart: measure the face area, multiply by coats and divide by the label rate.

Common questions

How much paint for a 12 by 12 room?

With 8 foot walls, one door and two windows, the walls are about 333 square feet after openings. Two coats at 350 square feet per gallon is 1.9 gallons, so buy 2 gallons. The ceiling adds about 0.8 gallons for two coats.

Why does the calculator ask for coverage?

Paint products list different spread rates, and the same paint covers less on rough or porous surfaces. The figure on your can is a better guide than any general number. 350 square feet per gallon is only a starting value; replace it with yours.

Do I need two coats?

Two coats are common for an even finish, especially with a colour change. Some paints claim one-coat coverage over similar colours. The label states the recommended number of coats.

Should I buy extra for touch-ups?

Keeping a little of the same batch helps with touch-ups later, because a new can may not match exactly. Rounding up to the next quart usually covers it.