Paint Coverage & Cost Calculator: Gallons, Cans and Total Cost
Work out how much paint to buy and what it costs: wall area or room size minus doors and windows, number of coats, coverage per gallon, rough or new surfaces, primer, and the cheapest mix of 5-gallon buckets, gallons and quarts.
How it works
Room walls = 2 × (length + width) × height − doors × door area − windows × window area (+ length × width for the ceiling) · Paint (gal) = area × coats ÷ coverage per gallon × surface allowance (1.0, 1.2 rough or 1.3 new) · Cans = round up, or the cheapest mix of 5-gal buckets, gallons and quarts · Cost = cans × price (+ primer: area ÷ primer coverage, rounded up × primer price)
Paint is used per square foot of surface per coat, so the calculator multiplies the area by the number of coats and divides by the coverage printed on the can. A gallon of typical latex wall paint covers about 350–400 sq ft per coat (8.5–10 m² per litre) on a smooth, sealed wall. Rough, porous or never-painted surfaces soak up more; Omni’s guidance is to add up to 20% for porous or rough surfaces and up to 30% for a surface painted for the first time, which is what the Surface setting does. In room mode the walls are the perimeter times the height, minus about 20 sq ft for each door and 12 sq ft for each window (Inch Calculator’s defaults; change them under More options). The ceiling is added only if you tick it. You cannot buy part of a can, so the result rounds up. If you enter a 5-gallon bucket or quart price, the calculator also finds the cheapest mix that still covers the job. Primer is calculated as one coat at its own coverage. Leftover paint is worth keeping for touch-ups.
Worked example
Painting 400 sq ft with 2 coats covers 400 × 2 = 800 sq ft of surface. At 350 sq ft per gallon that is 800 ÷ 350 = 2.29 gallons, so you buy 3 gallon cans; at $38 each the cost is $114, with about 0.71 gal left over. In room mode, a 12 × 12 ft room with 8 ft walls, one door and two windows has 2 × (12 + 12) × 8 − 20 − 2 × 12 = 340 sq ft of wall; two coats need 680 ÷ 350 = 1.94 gallons, so 2 cans.
Sources: Inch Calculator: Paint calculator (350–400 sq ft per gallon; door 20 sq ft, window 12 sq ft) · Omni Calculator: Paint calculator (add up to 20% for porous surfaces, 30% for first-time painting) · GigaCalculator: Paint calculator (worked two-room example)
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, the walls are about 340 sq ft. Two coats at 350 sq ft per gallon need about 1.9 gallons, so 2 gallons. Add a third if the walls are rough, porous or a much darker color, or if you paint the ceiling too.
How many square feet does a gallon of paint cover?
About 350–400 sq ft per coat for typical latex paint on smooth, primed walls (8.5–10 m² per litre). Textured, porous or bare surfaces cover less, which is why the calculator adds 20% for rough surfaces and 30% for never-painted ones.
How many coats of paint do I need?
Two is standard for even color. One may do for a same-color refresh. Covering a dark color with a light one, or painting new drywall, usually needs a primer coat plus two finish coats.
Is a 5-gallon bucket cheaper than single gallons?
Usually, per gallon, once you need about 4 gallons or more. Enter the bucket price under More options and the calculator compares buckets, gallons and quarts and picks the cheapest mix that still covers the job.
Do I subtract doors and windows?
Yes for large ones. Room mode subtracts about 20 sq ft per door and 12 sq ft per window. Small openings are often left in as a margin for touch-ups and waste.
How much primer do I need?
One coat over the same area, divided by the primer’s coverage, often around 300 sq ft per gallon. Prime new drywall, patched areas, stains, bare wood and big color changes.
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This calculator is for educational purposes. Double-check important results before acting on them.