How much paint do I need?
The amount of paint depends on four things: the area you paint, the number of coats, how far one gallon or litre spreads on your surface, and how much extra you buy as a buffer. This calculator works through them in that order and only rounds at the very end, when it converts the quantity into whole cans. Rounding earlier, for example per wall or per coat, can make you buy a can you do not need.
Measure the walls, then subtract openings
For a rectangular room, measure the length and width of the floor and the height of the walls. The four walls together cover 2 × (length + width) × height. A 12 × 10 ft room with 8 ft walls has 352 sq ft of wall. Then add up the area of doors, windows and anything else you will not paint, such as built-in cabinets, and enter the total; a standard door is roughly 20 sq ft. Openings are subtracted from the walls only, so a ceiling can never offset them. If your walls are not rectangular, or you are painting a fence or a single feature wall, switch to surface area and enter the area directly.
Tick “include the ceiling” to add length × width to the area. The model uses the same paint and coverage for walls and ceiling; if you use a separate ceiling paint, calculate it separately in surface-area mode.
Coverage per coat comes from the label
Coverage is how much surface one unit of paint covers in a single coat. It varies by product, color and surface: smooth primed drywall takes less paint than fresh plaster, rough masonry or a dark color being covered. Paint makers print a range on the can and data sheet, often a few hundred square feet per US gallon; Behr, for example, estimates per coat and by surface. Use the figure for your product and, when in doubt, the lower end of the range. Enter it per coat; the calculator multiplies by the number of coats once.
The formula
- Net area = 2 × (L + W) × H − openings (+ L × W for the ceiling), or surface area − openings.
- Paint before allowance = net area × coats ÷ coverage per coat.
- Paint to buy = paint before allowance × (1 + allowance %).
- Containers = paint to buy ÷ container size, rounded up to a whole container. Leftover = containers × size − paint to buy.
Worked example
The 12 × 10 × 8 ft room has 352 sq ft of wall. Subtracting 40 sq ft of doors and windows leaves 312 sq ft. Two coats at 350 sq ft per US gallon need 312 × 2 ÷ 350 = 1.783 gallons. A 10% allowance raises that to 1.961 gallons, so you buy 2 one-gallon cans and expect about 0.04 gallons left over. At 35.00 a can, the paint costs 70.00. Adding the 120 sq ft ceiling brings the net area to 432 sq ft and the quantity to 2.715 gallons, which needs 3 cans (105.00). Three coats on the walls alone would need 2.942 gallons, or 3 cans.
Rounding to whole cans
You cannot buy 1.96 cans, so the count is always rounded up. The rounding uses the unrounded quantity: a figure shown as 2.00 could still need 3 cans if the true value is a little above 2. Without any allowance, the example needs 1.783 gallons, still 2 cans. The allowance is a purchase buffer for touch-ups, absorption and spillage, not a measurement of real waste; 10% is a common choice, and you can set it to 0. If several can sizes are sold, compare their price per litre or gallon with the unit price calculator before buying.
Litres versus US gallons
Imperial mode uses the US liquid gallon of exactly 3.785411784 litres and feet of 0.3048 m. The UK (imperial) gallon is about 4.546 litres, so coverage quoted per UK gallon must not be entered as US gallons. Metric mode works in square metres and litres; typical labels quote around 10 to 12 m² per litre. Switching units resets the fields to an example in the new units rather than reinterpreting your numbers.
Estimates versus real coverage
Real coverage depends on application method, surface texture and porosity, and how heavily each coat goes on. The calculator does not model primer, trim and skirting boards, multiple colors, labor or mixing different can sizes. Treat the result as a shopping estimate under your entered coverage. For floors and walls being tiled, the tile calculator does the same job for tiles and boxes, and contractors pricing materials into a job quote can set their margin with the margin and markup calculator.
FAQ
How many coats of paint do I need?
Usually two coats on previously painted walls in a similar color. New plaster, strong color changes or patchy surfaces may need a primer or a third coat.
Should I subtract doors and windows?
Yes, for any large area you will not paint. Small openings matter less, and the allowance covers some of the difference.
Why does the calculator round up to whole cans?
Because cans are sold whole. It rounds the total once, after coats and allowance, so you buy the fewest cans that cover the estimate.
Is a gallon on the label a US or UK gallon?
In the US it is the US gallon of 3.785 litres. The UK gallon is about 4.546 litres. Use metric mode with litres if your label quotes litres.
What allowance should I add, and is primer included?
Around 10% is a common buffer; use 0% if you prefer the bare estimate. Primer is not included: estimate it separately with its own coverage.