How this calculator works
The four walls of a rectangular room have a combined area of twice the length plus width, multiplied by the wall height. Standard openings are deducted, then the net area is multiplied by the number of coats and divided by the label spread rate, and the cans are rounded up because paint is sold whole. Both gallon and litre totals appear because Canadian cans are labelled in litres while US labels and some older plans use gallons.
Formula
A = 2 × (L + W) × H − 21 × doors − 15 × windows (ft²); cans = ceil(A × coats ÷ spread rate). One US gallon is 3.785 L; 1 ft² = 0.09290304 m².
Worked example
A 12 × 10 ft bedroom with an 8 ft ceiling, one door and two windows has 352 − 21 − 30 = 301 ft² of wall. At two coats and 400 ft² per gallon that is 1.5 gallons, so 2 gallons. A 50 m² wall at one coat and 10 m²/L needs 5 L.
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Frequently asked questions
Should I paint the ceiling and trim too?
This page measures walls only. Add the ceiling area separately at about one coat less, and allow roughly 1 L per 10 m² for trim, doors and baseboards.
What spread rate should I use?
Read the can. A typical interior latex covers 350 to 400 ft² per gallon (8 to 10 m²/L) on primed drywall. New or rough surfaces absorb more, so use the lower figure and expect to need a primer coat.
Is a US gallon the same as an imperial gallon?
No. A US gallon is 3.785 L and an imperial gallon is 4.546 L. Canadian paint cans are labelled in litres, and the US label figure is what most spread rates quote, so check which one your can uses.