Paint Calculator

Paint calculator: gallons per coat

A paint calculator divides the wall area by how far a gallon of paint spreads. This one uses 350 sq ft per gallon, a common figure for one coat on a smooth wall. It then multiplies the gallons by your price per gallon, which starts at $35.

Gallons = wall area ÷ 350. Cost = gallons × price per gallon.

Wall area is the total length of the walls times their height. The calculator calls them length and width, but think of them as total wall length and ceiling height.

Getting the wall area

Walk around the room and add up the length of every wall you will paint. For a rectangular room, that is 2 × (length + width). Multiply by the ceiling height. A 12 ft by 14 ft room has 2 × (12 + 14) = 52 ft of wall. At 8 ft high that is 52 × 8 = 416 sq ft.

Worked examples

Example 1, the default wall run. 40 ft of wall, 8 ft high.

• Area: 40 × 8 = 320 sq ft
• Gallons: 320 ÷ 350 = 0.91
• Cost: 0.914 × $35 = $32.00

Example 2, a full room with openings removed. 12 × 14 ft room, 8 ft ceilings, one door and one window.

• Wall area: 52 × 8 = 416 sq ft
• Door: 3 × 7 = 21 sq ft
• Window: 3 × 5 = 15 sq ft
• Net area: 416 − 36 = 380 sq ft
• Enter 380 as the length and 1 as the width: 380 ÷ 350 = 1.09 gallons
• Cost: $38.00 for one coat

For two coats, double it: 2.17 gallons. You would buy a gallon and a half-gallon, or two gallons plus a quart, depending on what the store sells.

Gallons by room size, one coat

All rooms below have 8 ft ceilings and no deductions.

Room	Wall length	Wall area	Gallons	Cost at $35
10 × 10 ft	40 ft	320 sq ft	0.91	$32.00
10 × 12 ft	44 ft	352 sq ft	1.01	$35.20
12 × 14 ft	52 ft	416 sq ft	1.19	$41.60
14 × 16 ft	60 ft	480 sq ft	1.37	$48.00

A ceiling is a separate job. A 12 × 14 ft ceiling is 168 sq ft, which comes to 0.48 gallons.

What changes the coverage

• Porous or new drywall soaks up the first coat, so it spreads less. Prime it first.
• Textured walls have more surface than a flat measurement shows.
• A dark color over a light one, or the reverse, may need extra coats.
• Rolling thick with a heavy nap uses more paint than a thin, even pass.
• Spraying wastes more paint than rolling.

Rounding up

The tool shows gallons to two decimals and does not round. In the store you buy cans, so round up to the next size that covers your total. It is wise to keep a little leftover for touch-ups, and to buy all the paint of one color at once so the batch matches.

Common mistakes

• Entering the room’s floor dimensions instead of the wall length.
• Forgetting a second coat.
• Using the ceiling height as the length.
• Subtracting a window but forgetting trim, which has its own paint.
• Skipping the surface prep, which matters more than the amount of paint.

Safety note

If the house was built before 1978, test old paint for lead before sanding or scraping, and follow safe work practices or hire a certified professional.

Trim, ceilings, and extras

Trim, doors, and the ceiling usually get their own paint, often in a different sheen. Measure those separately instead of folding them into the wall figure. As a rough check, many people paint walls in a flat or eggshell finish and trim in semi-gloss, which means two cans on one project. Also buy a tray liner, tape, and a few roller covers, since cheap covers shed lint and ruin a good coat.

Related calculators

The primer calculator works out the first-coat product, and the drywall calculator counts sheets if you are hanging board before painting. If you are adding paneling, the wainscoting calculator covers the lower wall.