Paint Calculator
Enter your room size and get the paint you need — walls, optional ceiling, doors and windows subtracted, any number of coats.
Quick answer
One US gallon covers roughly 350–400 sq ft per coat on smooth walls. Measure wall area, subtract doors and windows, multiply by the number of coats and divide by coverage. Re-doing the floor too? See the flooring calculator.
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- Updated October 2026
Paint needed
2.09 gallons
Buy 3 gal (or 9 quarts)
- Paintable area (one coat)
- 365 sq ft
- Total area incl. all coats
- 730 sq ft
Show the working
- Walls = 2 × (14 + 12) × 8 = 416
- Minus 1 door(s) × 21 and 2 window(s) × 15
- 365 × 2 coats ÷ 350 = 2.09 gallons
About the Paint calculator
Paint coverage is printed on the can. A typical US gallon of interior paint covers around 350–400 sq ft per coat on smooth, primed walls; in metric, around 10–12 m² per litre. Rough, porous or very dark-to-light colour changes can cut that significantly.
The calculator subtracts a standard door (about 21 sq ft / 1.9 m²) and window (about 15 sq ft / 1.4 m²) for each one you enter. Large patio doors or picture windows should be counted as two.
How to use it
- Pick feet or metres.
- Enter the room length, width and wall height.
- Count the doors and windows.
- Set the number of coats — two is normal for a colour change.
- Enter your paint’s coverage if it’s on the tin; otherwise the typical value is used.
The formula
Paint = (2 × (L + W) × H − openings) × coats ÷ coverage
- Add L × W if you are painting the ceiling.
- Coverage defaults to 350 sq ft per gallon or 10 m² per litre.
Worked example
A 14 × 12 ft bedroom, 8 ft walls, 2 coats
- Walls: 2 × (14 + 12) × 8 = 416 sq ft.
- Minus one door and two windows: 416 − 21 − 30 = 365 sq ft.
- 365 × 2 coats ÷ 350 = 2.09 gallons → buy 3 gallons, or 2 gallons plus a quart.
Frequently asked questions
How much does a gallon of paint cover?
Usually 350–400 sq ft per coat on smooth walls. Textured surfaces, fresh plaster and bare wood soak up more paint, sometimes reducing coverage to 250 sq ft.
Do I need two coats?
Two coats give even colour and better durability, especially when changing colour. One coat can work when refreshing the same colour with a quality paint.
Do I need primer?
Use primer on new drywall or plaster, bare wood, stains, or when going from a dark to a much lighter colour. Primer has its own coverage rate — calculate it as a separate coat.
Results are estimates for planning — check critical figures with a professional or the official source.
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