Paint Calculator

This calculator turns room measurements into the amount of paint to buy. Enter the length, width and wall height of up to four rooms, the number of doors and windows, how many coats you plan, and the coverage printed on the can. It works out the paintable wall area, adds ceilings, baseboard trim and a primer coat if you tick them, and rounds each product up to what you can actually buy: whole quarts in the US, half liters in metric.

Wall paint, ceiling paint, trim enamel and primer are usually different products, so each is estimated and rounded separately. The diagram unfolds each room’s four walls into one strip so you can check that the dimensions you typed look right.

wall area = 2 × (L + W) × H − doors × door area − windows × window area − other

paint = area × coats × (1 + extra ÷ 100) ÷ coverage

L and W are the floor length and width, and H is the wall height, so 2 × (L + W) is the distance around the room. Door and window areas default to 20 square feet (a 36 by 80 inch door) and 15 square feet (a 3 by 5 foot window), or 1.9 m² and 1.4 m² in metric; change them to match your openings. Coverage is the spread rate per coat from the can label, in square feet per US gallon or square meters per liter. The extra percentage allows for rollers, trays, touch-ups and rough surfaces.

Ceilings use the floor area, L × W, with the same number of coats. Trim is treated as a band of the height you choose running around the room, and that band comes off the wall area. Primer is one coat over everything being painted, at its own coverage rate. A spread rate of 350 ft² per gallon is about 8.6 m² per liter.

A 12 by 14 foot bedroom

With 8 foot walls the room has 2 × (12 + 14) × 8 = 416 ft² of wall. Taking off one door and two windows leaves 366 ft². Two coats at 350 ft² per gallon with 10% extra need 2.3 gallons, so you would buy 2 gal + 2 qt. One coat would need 1.15 gallons (1 gal + 1 qt).

Adding the ceiling and a primer coat

The ceiling is 12 × 14 = 168 ft²; two coats need 1 gal + 1 qt. One coat of primer at 300 ft² per gallon over walls and ceiling (534 ft²) needs 1.96 gallons, so 2 gal.

A metric room

A 4 m by 3.5 m room with 2.4 m walls has 36 m² of wall, or 32.7 m² after one door and one window. At 10 m² per liter, two coats with 10% extra need 7.19 liters, rounded up to 7.5 L.

Gallons of wall paint for two coats at 350 ft² per gallon with 10% extra, one door and two windows per room. Ceilings are not included. Every figure comes from the calculator’s engine.

The coverage on a label assumes a smooth, sealed surface and the recommended film thickness. Textured walls, bare drywall, fresh plaster and patched areas soak up more paint, so the real spread rate is lower; raise the extra percentage or lower the coverage for those surfaces. Many wall paints list a range on the can, and using the low end of the range gives a safer estimate.

Two coats is the usual plan for an even finish, especially when changing color. Primer seals porous surfaces and blocks stains so the finish coats cover as stated. Paint and primer manufacturers publish the spread rate and recommended number of coats on each product’s data sheet, and those figures should take priority over the defaults here.

• Room shape. The wall formula assumes a rectangular room with a flat ceiling. For a vaulted ceiling, gable wall or stairwell, measure those surfaces separately and enter them as an extra room, or add their area by hand.
• Openings. Doors and windows are subtracted at the sizes you set. Large patio doors, archways and built-in shelving should go in the other unpainted box.
• Door and window trim. The trim option covers baseboards only. Casings, crown molding and doors themselves need their own estimate.
• Batch color. Tinted paint can vary slightly between mixes, so buying the full amount at once, and mixing cans together before you start, avoids a visible change partway along a wall.