Paint calculator: how much paint do I need

1. Measure the walls. Add the room's length and width, double it, and multiply by the wall height. A 14 × 12 ft room with 8 ft walls: 2 × (14 + 12) × 8 = 416 sq ft.
2. Take off the doors and windows. A 3 × 7 ft door is 21 sq ft and a 3 × 5 ft window 15 sq ft. One door and two windows: 416 − 21 − 30 = 365 sq ft.
3. Multiply by the coats. Two coats is the usual plan for a color change: 365 × 2 = 730 sq ft of coverage.
4. Divide by the coverage per gallon from the can or the data sheet: 730 ÷ 350 = 2.09 gal.
5. Round to what you can buy. 2.09 gal is 2 gallons and 1 quart.

The calculator does each step and shows it, and does the same for the ceiling, the doors, the baseboard and a coat of primer if you switch them on.

Coverage is printed on every can and in every product data sheet. It is the area one gallon covers in one coat at the film thickness the maker tested. A few current figures, from the makers' own pages:

Those numbers are for a smooth, sealed surface, brushed or rolled. Real walls take more paint when they are:

• Porous: new drywall, bare plaster, unpainted wood and masonry soak up the first coat. Prime them first.
• Rough or textured: knockdown, orange peel and stucco have more surface than their flat area.
• Sprayed: overspray is lost, and the data sheet figures do not include it.
• A big color change: deep colors and light-over-dark can need a third coat.

That is why the calculator starts at 350 sq ft a gallon, the low end of the figures above. If you know your product and your surface, enter your own coverage.

Worked with the calculator at 350 sq ft a gallon, two coats, one door and the windows shown. The ceiling is a separate product.

Three quarts or more is rounded to a gallon, and five gallons or more is shown as a 5 gallon pail. If a quart and a gallon of the same color are mixed at different times, box them together before you start so the color is even across the walls.

A bedroom repaint, walls only. 14 × 12 ft, 8 ft walls, one door, two windows, two coats at 350 sq ft a gallon: 365 sq ft × 2 ÷ 350 = 2.09 gal. Buy 2 gallons and a quart.

The same room, everything. Add the ceiling (14 × 12 = 168 sq ft, two coats, 0.96 gal of ceiling paint), both sides of the door (21 × 2 = 42 sq ft) and 4 in baseboard around the room less the door (49 ft × 4 ÷ 12 = 16.3 sq ft). Trim and door paint: 58.3 × 2 ÷ 350 = 0.33 gal, a quart and a half, so 2 quarts. One coat of primer over all of it: (365 + 168 + 42 + 16.3) ÷ 350 = 1.69 gal, so 1 gallon and 3 quarts, which rounds to 2 gallons.

A large open area. 1,000 sq ft of wall, one coat. At Sherwin-Williams' 400 sq ft it is 2.5 gal; at the 350 low end it is 2.86 gal. Planning at the low end is the difference between finishing and a trip back to the store.

Primer seals the surface so the finish coats cover evenly and stick. Prime:

• New drywall and fresh joint compound, which absorb paint unevenly and can show the joints through the finish.
• Stains, patches, and bare wood or masonry.
• Glossy old paint, after a scuff sand, and oil paint you are covering with latex.
• A big change from dark to light.

A paint-and-primer product can save a coat over sound, previously painted walls, but it does not replace a real primer on new drywall or stains. One coat of primer is usual. Benjamin Moore lists its Ultra Spec 500 interior primer at 350 to 400 sq ft a gallon, the same range as many finish paints.

A room is usually three products, not one, and each is counted on its own line:

• Walls take the main color, often in an eggshell or satin sheen that can be wiped clean.
• Ceilings usually take a flat ceiling white, which hides roller marks and the small flaws a raking light shows. Measure the ceiling as the room's length times its width. On a sloped or vaulted ceiling, measure along the slope, or use the square footage calculator with triangles for the gable walls.
• Trim and doors take a harder semi-gloss or enamel. A flush door is about 21 sq ft a side; a six-panel door has more surface in its profiles, so allow a little more. Baseboard is its length times its height, and door and window casing is worked out the same way.

Exterior walls follow the same math with their own product's coverage. Work out each wall as a rectangle, add the gable triangles (base × height ÷ 2), take off the doors, windows and garage door, and divide by the coverage on the exterior product's data sheet. Siding with deep laps, rough-sawn wood, stucco and brick take noticeably more paint than smooth panels, and exterior jobs are often sprayed, so start with the low end of the coverage range and enter the total wall area in the calculator.

Keep the leftover paint, labeled with the room and the date. A quart of the exact color makes touch-ups invisible, which is worth more than the return you would get for it.

Paint is usually the smaller part of a painting job; labor and prep are the larger. Once you know the gallons, price the job by the square foot or by the hour, and add the prep, patching, masking and cleanup.

For the full job price with labor, see the painting cost calculator. "Save as an estimate in Koira" carries the paint by the can and a wall-painting line by the square foot into an estimate, where you add your prices and send it from your phone. For walls you are also hanging, the drywall calculator covers sheets and finishing.