How it works
The calculator works out the total wall area, subtracts doors and windows, multiplies by the number of coats and divides by the coverage of your paint.
Wall area = 2 × (Length + Width) × Height
Doors & windows = Σ (Width × Height × Count)
Area to paint = Wall area − Doors & windows
Paint = Area to paint × Coats ÷ Coverage × (1 + Extra ÷ 100)
Cans = ⌈ Paint ÷ Can size ⌉
⌈ ⌉ means rounding up to the next whole number: you can’t buy part of a can. The paint volume itself is not rounded.
Measurements, coverage and can size are converted to one internal unit before calculating, so you can enter the walls in feet and inches and pick gallons or quarts for the cans. One US gallon is 231 cubic inches (NIST Handbook 44) and equals four quarts. The product list uses each manufacturer’s published spread rate – the low end where the data sheet gives a range, so the estimate leans towards more paint – and the coverage used is shown with the result. The sources are linked below.
Worked example
Inputs
- Measure by
- Known wall area
- Wall area
- 700 sq ft
- Number of coats
- 2
- Paint product
- Your own coverage
- Coverage
- 350 sq ft/gal
- Extra paint
- 0 %
- Can size
- 1 gal
Result
- Total wall area
- 700 sq ft
- Doors & windows
- 0 sq ft
- Area to paint
- 700 sq ft
- Coverage used
- 350 sq ft/gal
- Paint needed
- 4 gal (16 qt)
- Cans
- 4
Common mistakes
- Not subtracting doors and windows – or subtracting them twice by entering a net wall area and then listing the openings again.
- Using another paint’s coverage. Spread rates differ between products – pick your product or type the figure from your can – and textured or porous surfaces take more paint than the data sheet says.
- Planning one coat when you are changing color substantially or painting fresh drywall. Follow the coat count on the label.
- Forgetting that primer is a separate product with its own coverage. Run the calculator again for primer using the figure on the primer can.
Assumptions and limits
- In room mode, all four walls are painted to the full wall height you enter. Sloped ceilings, gable ends and half-height walls need the wall area entered directly.
- Doors and windows are treated as rectangles of width × height × count and are subtracted in both modes.
- Coverage is the documented spread rate of the product you pick (the low end of the range on its data sheet) or the figure you type, and it applies to every coat. Data-sheet rates assume a smooth, sealed surface. Real coverage depends on the product, the surface texture and porosity, and how you apply the paint.
- Ceilings, trim, baseboards and doors themselves are not included. Use the ceiling paint calculator for ceilings.
- Cans are rounded up to whole containers of the size you choose; the calculator does not mix gallon and quart cans.
Frequently asked questions
How many square feet does a gallon of paint cover?
There is no single figure – it depends on the product and the surface. Manufacturers’ data sheets for common interior paints list, for example, 350–400 sq ft per gallon (Sherwin-Williams SuperPaint), about 400 (PPG Speedhide, Valspar Signature), 400–450 (Benjamin Moore Regal Select) and 8–12 m² per liter (Tikkurila Harmony). These assume a smooth, sealed surface; rough or porous walls take more paint. Pick your product in the calculator or type the coverage printed on your can.
Should I add extra paint?
The calculation already accounts for every coat. An extra allowance mainly helps with rough or porous surfaces, rollers and trays that hold paint, and having some left for touch-ups. Set it in the “extra paint” field; leave it at 0 if you want the exact figure.
Should I buy gallons or quarts?
One US gallon equals four quarts. If the result is just over a whole number of gallons, compare the price of one more gallon with a few quarts – set the can size to quarts to see how many you’d need.
How do I handle a room with a sloped ceiling?
Switch to “known wall area”. Measure each rectangular wall as width × height, and for a triangular gable add half its width × its height, then add all the pieces together.
Sources
- Regal Select Eggshell (N549) Technical Data Sheet — Benjamin Moore. Regal Select Eggshell interior wall paint: "Spread Rate Per Gallon 400 – 450 Sq. Ft." (preset uses 400).
- Speedhide Interior Latex Flat (6-70 Series) Technical Data — PPG. Walls and ceilings: "Approximately 400 sq. ft. (37 sq. meters) per U.S. Gallon (3.78L) on smooth, nonporous surfaces."
- SuperPaint Interior Latex Flat (A86 Series) Product Data Sheet — Sherwin-Williams. Interior wall paint: "Coverage: 350-400 sq. ft. per gallon" (preset uses 350).
- Harmony Product Data Sheet — Tikkurila. Walls and ceilings: "Theoretical coverage 8-12 m²/l." (preset uses 8).
- Valspar Signature Interior Flat (No. 773955 Series) Product Data Sheet — Valspar. Interior wall paint: "Practical Coverage: 400 sq. ft. per gallon (27-37 m2 per 3.78 L) depending on surface porosity."
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026