Paint Coverage Calculator
Calculate exactly how many gallons of paint to buy for any room — accounting for doors, windows, coats, and a 10% waste buffer.
Buying too little means a mid-project store run. Buying too much wastes money. One accurate measurement takes 30 seconds.
350 sq ft
Coverage per gallon of standard interior latex paint — the professional industry benchmark
2 coats
Standard application for full coverage — especially on colour changes or previously dark walls
10%
Buffer to add to your purchase — covers spills, touch-ups, and minor measurement errors
What most paint calculators get wrong
Simple area calculators miss doors, windows, and coating factors — this one doesn't.
Measure walls, not floors
A common mistake is confusing floor area with wall area. A 12 × 14 ft room has 168 sq ft of floor — but over 450 sq ft of wall if ceilings are 9 ft. Wall area is always larger than floor area in any standard room. Always measure from the walls up.
Doors and windows mean less paint needed
A standard interior door is roughly 21 sq ft (7 ft × 3 ft). A standard window is roughly 15 sq ft (3 ft × 5 ft). On a room with 2 doors and 3 windows, that's 87 sq ft you don't need to paint — potentially saving a full gallon on a large project.
Buy in the right can size
Paint typically comes in quart (0.25 gal), one-gallon, and five-gallon containers. Five-gallon buckets are cheapest per gallon but wasteful for small rooms. If your estimate lands between sizes, round up one size — leftover paint is useful for future touch-ups.
How the Paint Coverage Calculator Works
Formula
Wall Area = 2 × (Length + Width) × Height
Net Area = Wall Area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
Gallons Needed = (Net Area × Coats) / 350
Recommended Purchase = Gallons Needed × 1.10 (10% buffer)Enter room dimensions
Length, width, and ceiling height in feet. Standard US ceilings are 8–9 ft.
Count doors and windows
Each door subtracts ~21 sq ft · Each window subtracts ~15 sq ft from the paintable area.
Choose number of coats
2 coats is standard for most repaints · 1 coat for same-colour refresh · 3 coats for dark-to-light colour changes.
See gallons + buffer
The result shows exact gallons needed and the recommended purchase with a 10% safety buffer.
The most common painting mistake is buying paint based on floor area rather than wall area. In a standard 12×14 ft room with 9 ft ceilings, the floor is 168 sq ft but the walls are over 450 sq ft — nearly three times more. Always calculate wall area, not floor space.
This calculator uses the 350 sq ft/gallon industry standard, which gives a slightly conservative estimate on smooth surfaces. Premium paints may cover up to 400 sq ft/gallon, so if you're using a high-quality product, you may have a little extra — useful for touch-ups.
Frequently Asked Questions
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