Paint Calculator
Enter the room size, openings and coats. Use the coverage printed on your paint can for the best estimate.
How it works
Wall area = 2 × (length + width) × height. Paintable area = wall area − doors − windows (+ ceiling = length × width, if included). Gallons = paintable area × coats × (1 + allowance) ÷ coverage per gallon, rounded up to whole gallons. 1 U.S. gallon = 3.785 L.
Worked examples
- 12 × 12 ft room, 8 ft walls: 2 × (12 + 12) × 8 = 384 sq ft. Minus one door (21) and one window (15) = 348 sq ft. Two coats = 696 sq ft ÷ 350 = 1.99 gallons, so 2 gallons.
- Same 12 × 12 ft room with the ceiling: 348 sq ft of walls + 144 sq ft of ceiling = 492 sq ft. Two coats = 984 sq ft ÷ 350 = 2.81 gallons, so 3 gallons (10.6 L).
- Larger room: 14 × 16 ft with 9 ft walls, 2 doors and 3 windows. Walls 2 × (14 + 16) × 9 = 540 sq ft, minus 42 + 45 = 87 sq ft of openings = 453 sq ft. Two coats at 400 sq ft per gallon (the top of Sherwin-Williams' range) = 2.27 gallons, so 3 gallons.
How to measure
- Measure the length of each wall along the floor, and the height from floor to ceiling.
- Measure one typical door and window (width × height) and count them. Edit the door and window areas if yours differ from the defaults.
- For the ceiling, use room length × width.
- Read the coverage on your paint can's label and enter that rather than the default.
Common mistakes
- Planning one coat when you need two. New drywall, deep colors and big color changes usually take more than one.
- Using the best-case coverage. Sherwin-Williams gives 350-400 sq ft per gallon; textured or porous surfaces get less.
- Forgetting primer, which is a separate product with its own coverage (about 200-300 sq ft per gallon, per Sherwin-Williams).
- Subtracting small openings that you will paint around anyway. Only subtract doors, windows and other big areas.
- Buying several quarts instead of a gallon. Four quarts make a gallon and often cost more than one.
Assumptions
- Coverage default 350 sq ft per gallon is the low end of the 350-400 sq ft range Sherwin-Williams gives for a gallon. Use the figure on your own can; textured, porous or unprimed surfaces and big color changes need more.
- Door (21 sq ft, about 3 × 7 ft) and window (15 sq ft, about 3 × 5 ft) sizes are rough defaults; edit them to match yours.
- In metric, enter coverage in m² per liter (350 sq ft per gallon ≈ 8.59 m² per liter, using 1 gal = 3.785 L and 1 sq ft = 0.0929 m², NIST). Results still show gallons and liters.
- Rectangular room with walls of equal height. Trim, doors and cabinets are not painted in this estimate.
- Primer is not included; Sherwin-Williams says a gallon of primer covers about 200-300 sq ft.
- The waste allowance is an editable JobsiteCalc default, not an industry standard. Ask your supplier what extra they suggest for your job.
Frequently asked questions
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350-400 square feet, depending on wall texture and desired coverage.
How much does a quart of paint cover?
About 100 square feet, according to Sherwin-Williams, enough for an accent wall or a door.
How much paint for a 12 × 12 room?
With 8 ft walls, one door and one window, the paintable area is 348 sq ft. Two coats at 350 sq ft per gallon need 1.99 gallons, so buy 2 gallons.
Related calculators
- Square footage calculator – for odd-shaped walls or ceilings
- Drywall calculator – if you are hanging new board before painting
- Wallpaper calculator – rolls for the same walls instead of paint
- Siding calculator – for exterior walls and gables
Sources
Formulas and reference values are from these sources. Page last reviewed 2026-10-19.