Paint Calculator
Gallons = (wall area − doors and windows) × coats ÷ 350 ft² per gallon. A 12 × 12 ft room, 8 ft walls, one door and two windows: 334 ft² × 2 coats ÷ 350 = 1.91, so buy 2 gallons.
Paint to buy
2 gallons
1.91 gal exactly · 7.2 L
- Paintable area (one coat)
- 334 ft²
- Total area with all coats
- 668 ft²
How it works
Wall area = 2 × (length + width) × wall height
Paintable area = wall area − 20 ft² per door − 15 ft² per window (+ ceiling length × width)
Gallons = paintable area × coats ÷ coverage (ft² per gallon)
Cans: total rounded up to the next quart, shown as gallons + quarts
Assumptions
- Default coverage 350 ft² per gallon, the low end of the 350–400 ft² range Sherwin-Williams gives (Benjamin Moore lists 400–450 ft² for Regal Select). Textured or porous walls take more paint: use the figure on your can.
- A standard door is counted as 20 ft² (about 3 ft × 6 ft 8 in) and a window as 15 ft² (3 ft × 5 ft). Enter large openings such as patio doors as two.
- The ceiling, when included, is painted with the same paint and number of coats.
Worked examples
A 12 × 12 ft bedroom, 8 ft walls, 1 door, 2 windows, 2 coats
- Walls: 2 × (12 + 12) × 8 = 384 ft²
- Openings: 20 + 2 × 15 = 50 ft², so 334 ft² to paint
- 334 × 2 coats ÷ 350 = 1.91 gallons
Result: 1.91 gal: buy 2 gallons.
Checked by the automated test “paint — 12 × 12 × 8 ft room, 1 door, 2 windows, 2 coats = 1.91 gal → 2 gal”.
Same room with the ceiling
- Ceiling: 12 × 12 = 144 ft², so 334 + 144 = 478 ft²
- 478 × 2 ÷ 350 = 2.73 gallons, rounded up to 11 quarts
Result: 2 gallons + 3 quarts (or 3 gallons if quarts are not sold in your color).
Checked by the automated test “paint — adding the ceiling: 478 ft², 2 coats = 2.73 gal → 2 gal + 3 qt”.
Gallons, quarts and leftovers
- The result is rounded up to whole quarts, then shown as gallon cans plus quart cans. If your color is only sold by the gallon, round up to the next gallon.
- Buy all the paint at once and keep a labeled leftover can: it is the easiest way to touch up scuffs later.
- Taller walls add up fast. A 12 × 12 room has 384 ft² of wall at 8 ft and 480 ft² at 10 ft, before you subtract openings.
Common mistakes
- Using the best-case coverage. Sherwin-Williams notes that wall texture and the coverage you want change how far a gallon goes. Start from the low end of the range on the can.
- Forgetting the ceiling or the primer. Tick the ceiling box only if it gets the same paint; estimate primer separately with its own coverage.
- Counting trim and doors with the walls. They usually take a different sheen; estimate them on their own.
Frequently asked questions
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350 to 400 ft². Benjamin Moore's data sheet for Regal Select lists 400 to 450 ft². The exact figure for your paint is printed on the can and on its technical data sheet.
How much does a gallon cover with two coats?
Half as much wall. At 350 ft² per gallon, one gallon covers 175 ft² with two coats. Set the number of coats in the calculator; the product label or data sheet gives the maker's recommendation.
How much paint do I need for a 12 × 12 room?
About 2 gallons for the walls with two coats, if the walls are 8 ft high with one door and two windows (first worked example above). Add 3 quarts if you paint the ceiling with the same paint.
How much primer do I need?
Sherwin-Williams says a gallon of primer covers about 200 to 300 ft², less than paint. Run the calculator with 1 coat and 200 in the coverage box: the 12 × 12 bedroom above needs 1.67 gallons, so 1 gallon and 3 quarts of primer.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.