Buying paint is a balance between running out halfway through the second coat and stacking leftover cans in the garage. This paint calculator works out the wall area of a room from its length, width and height, subtracts doors and windows, multiplies by the number of coats and divides by the coverage printed on the can. It tells you how many gallons and quarts to buy for the walls and, separately, for the ceiling, since ceilings usually get a different flat white.
The default coverage is 350 ft² per gallon, the low end of the 350–400 ft² range that Sherwin-Williams gives for a typical gallon of wall paint. Using the low end builds in a margin for texture and touch-ups. Primer covers less, roughly 200–300 ft² per gallon according to the same source, so run it separately by changing the coverage. Hanging new walls first? The drywall calculator counts sheets from the same room dimensions.
How to use the paint calculator
- Enter the room length, width and wall height. For rooms with sloped or vaulted walls, use the square footage calculator to add up wall sections and enter the total as length × 1 with the height set to the area.
- Enter the number of doors and windows and their typical size. A standard 36 × 80 inch door is 20 ft²; a 3 × 5 ft window is 15 ft². Ignore small openings if you prefer a margin.
- Choose the number of wall coats: usually two for a color change, sometimes one for a refresh in the same color.
- Choose whether you are painting the ceiling and how many coats.
- Check the coverage on the product label and enter it. Rough, porous or unprimed surfaces use more paint.
Formula
The result is rounded into whole gallons plus quarts (4 quarts per gallon). If you would need 3 or more quarts, it rounds up to another gallon, which is usually cheaper than three quarts.
Worked example
A 14 × 12 ft bedroom with 8 ft walls, one door, two windows, two coats on the walls and one coat on the ceiling, at 350 ft² per gallon.
- Walls: 2 × (14 + 12) × 8 = 416 ft².
- Openings: 1 × 20 + 2 × 15 = 50 ft², leaving 366 ft² to paint.
- Wall paint: 366 × 2 ÷ 350 = 2.09 gal, so buy 2 gallons + 1 quart.
- Ceiling: 14 × 12 = 168 ft² × 1 coat ÷ 350 = 0.48 gal, so 2 quarts (or one gallon if you want spare for a second coat).
- Total: 2.57 gal, about 9.7 litres.
How many coats do you need?
Two coats are the norm when changing color, painting over patched drywall, or using a lighter color over a dark one. A single coat can be enough when repainting the same color over sound paint. New drywall should be primed first (or use a self-priming paint as directed on the label). Dramatic color changes often benefit from a tinted primer, which can save a third topcoat.
Converting coverage to metric
350 ft² per US gallon is about 8.6 m² per litre (1 gallon = 3.785 L, 1 m² = 10.764 ft²). If you measured in metres, the calculator converts automatically; the coverage field stays in ft² per gallon.
Paint for common room sizes
| Room (8 ft walls) | Wall area | Two coats at 350 ft²/gal |
|---|---|---|
| 10 × 10 ft | 320 ft² | 1.83 gal |
| 12 × 12 ft | 384 ft² | 2.19 gal |
| 12 × 14 ft | 416 ft² | 2.38 gal |
| 14 × 16 ft | 480 ft² | 2.74 gal |
| 16 × 20 ft | 576 ft² | 3.29 gal |
Before subtracting doors and windows. Values are simple arithmetic from the formula above.
Caveats and assumptions
- Coverage on the label assumes a smooth, primed surface and the recommended film thickness. Textured walls, bare drywall and masonry can absorb far more.
- Trim, doors and cabinets are usually painted in a different sheen and are not included. Estimate them separately.
- Buy all of a custom color at once if you can; separate batches can differ slightly.
- Spraying uses more paint than rolling because of overspray.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls the wall area is 384 ft². Two coats at 350 ft² per gallon need about 2.2 gallons before subtracting doors and windows, so two gallons plus a quart usually does it.
How many square feet does a gallon of paint cover?
Sherwin-Williams gives about 350–400 ft² per gallon for typical paint on a smooth surface. Primer covers about 200–300 ft². Always check the product label.
Should I subtract windows and doors?
For a tight estimate, yes: about 20 ft² per door and 15 ft² per average window. Skipping the subtraction builds in a small margin for touch-ups.
How much paint for a ceiling?
Multiply length × width for the ceiling area and divide by coverage. A 12 × 14 ft ceiling is 168 ft², about half a gallon per coat.
Is it cheaper to buy quarts or a gallon?
Paint is usually cheaper per ounce by the gallon, which is why this calculator rounds three quarts up to another gallon.
Sources
- Sherwin-Williams — Paint Calculator FAQs — 350–400 ft² per gallon of paint; 200–300 ft² per gallon of primer; ~100 ft² per quart
- Sherwin-Williams — SuperPaint Interior product data sheet — example label coverage at a stated film thickness
- NIST — Unit Conversion — gallon to litre, ft² to m²
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.