Estimate how many gallons (or liters) of paint you need for your walls, accounting for doors, windows, and number of coats.
Paint Calculator
Construction & DIY
Inputs
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Measure the total length of all walls you plan to paint.
Enter the wall height in feet.
Set the number of coats (2 is standard for full coverage).
Enter the number of doors and windows to subtract their area.
Adjust paint coverage if your paint specifies a different sq ft per gallon.
Click Calculate for gallons and liters needed.
Paint needed = (wall area − doors & windows) × coats ÷ coverage. Most interior wall paints cover about 350 sq ft per gallon, but check the can — primer and some premium paints differ. Always round up to a full gallon since partial coverage wastes a trip back to the store.
A 12×12 ft room with 8 ft ceilings has ~384 sq ft of wall. At 2 coats and 350 sq ft/gal, you need about 2–3 gallons after subtracting a door and window.