Paint Calculator

How to estimate paint

Paint is bought by volume but used by area: each can covers a certain number of square feet or square meters at the film thickness the maker intends. So the job is to find the area you will paint, multiply by the number of coats, and divide by the coverage.

1. Choose Whole room for a rectangular room (enter its length, width and wall height) or Wall by wall to list only the walls you are painting, such as a single accent wall.
2. Enter how many doors and windows there are. Their sizes start at typical values (a 36 × 80 in door and a 36 × 48 in window, or 90 × 210 cm and 120 × 120 cm); open Door and window sizes to change them.
3. Set the number of coats and the coverage printed on the can. The starting value, 350 ft² per gallon or 9 m² per liter, is an assumption, not a product figure.
4. Read the paint to buy, rounded up to whole gallons or liters, and a set of cans that covers the need with the least paint left over.

Paint formula

L, W, H

room length, room width and wall height

doors, windows

number of each × width × height of one

ceiling

L × W when the ceiling is included, otherwise 0

coverage

area one gallon (ft²/gal) or one liter (m²/L) covers in one coat

Coverage converts exactly between the two systems: 1 ft²/gal = 0.09290304 m² ÷ 3.785411784 L ≈ 0.02454 m²/L, so 350 ft²/gal is about 8.59 m²/L. The cans are chosen by testing every combination of the available sizes: the smallest total that is at least the paint needed, then the fewest cans for that total.

Worked example

A 12 × 14 ft room with 8 ft walls, one door, two windows, two coats, and paint that covers 350 ft² per gallon:

1. Walls: 2 × (12 + 14) × 8 = 416 ft²
2. Openings: one door 3 × 6.67 ft = 20 ft², two windows 2 × 3 × 4 ft = 24 ft², 44 ft² in all
3. Area to paint: 416 − 44 = 372 ft²; two coats: 744 ft²
4. Paint: 744 ÷ 350 = 2.13 gal (8.05 L), so 3 gallons, or with less left over, two 1-gallon cans and one quart (2.25 gal).

Painting the ceiling as well adds 12 × 14 = 168 ft²: 540 ft² × 2 coats ÷ 350 = 3.09 gal, so 4 gallons, or 3 gallons and a quart.

In metric, a 4 × 3.5 m room with 2.5 m walls has 37.5 m² of wall. Less a 0.9 × 2.1 m door (1.89 m²) and two 1.2 × 1.2 m windows (2.88 m²) that leaves 32.73 m²; two coats at 9 m²/L need 65.46 ÷ 9 = 7.27 L, which is 8 L rounded to whole liters or a 5 L and a 2.5 L can.

Coverage, coats and primer

• Coverage is on the label. Manufacturers print a spread rate on the can and the data sheet, and it depends on the surface. Benjamin Moore’s data sheet for Regal Select interior semi-gloss, for example, gives 400–450 ft² per gallon, depending on surface texture and porosity. Textured walls, bare plaster, new drywall and masonry absorb more and cover less.
• Coats. Two coats is the usual plan. A big color change, a light color over a dark one, or patched walls may need a third, or a primer first.
• Primer is a separate product with its own coverage. Run the calculator again for it, with one coat and the primer’s spread rate.
• Trim, doors and ceilings are often painted in a different sheen or product. Estimate each product on its own.

What the estimate leaves out

• Loss in the tray and roller. Some paint stays in the tray, roller and brush. The whole-unit rounding usually covers it for a single room.
• Odd shapes. Gable ends, stairwells and sloped ceilings are not rectangles. Split them into rectangles and triangles, or find their area with the square footage calculator and enter it as a wall of that area (for example 1 ft high).
• Can sizes. US stores sell quarts, gallons and 5-gallon pails. Metric sizes vary: 10, 5, 2.5 and 1 L are used here, but you may find 0.75 L, 4 L or 15 L cans. For a floor or tiling job instead, see the tile calculator.