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Paint Calculator
Enter your room, subtract the doors and windows, choose how many coats, and this calculator tells you the paintable area, the exact gallons required and — rounded up, because paint is sold by the gallon — how many gallons to buy.
Coverage of 350 sq ft per gallon is the typical figure printed on interior paint; textured walls, dark-over-light colour changes and unprimed drywall can need 20–30% more. The default door (21 sq ft, a standard 3 × 7 ft door) and window (15 sq ft, 3 × 5 ft) sizes are editable — measure yours if they differ. Quantities are estimates for planning; buy one extra gallon for touch-ups on large jobs.
How it works
Wall area = 2 × (length + width) × ceiling height — the perimeter of the room times its height. Add the ceiling (length × width) if selected, then subtract every opening: doors × area per door plus windows × area per window. Gallons exact = paintable area × coats ÷ coverage per gallon; gallons to buy is that figure rounded up to the next whole gallon. Worked example: a 12 ft × 15 ft room with 8 ft ceilings has wall area 2 × (12 + 15) × 8 = 432 sq ft. Subtract 1 door (21 sq ft) and 2 windows (2 × 15 = 30 sq ft): 432 − 51 = 381 sq ft paintable. Two coats at 350 sq ft per gallon: 381 × 2 ÷ 350 = 2.18 gallons exact, so buy 3 gallons — at $45 a gallon, $135.
Frequently asked questions
Why round up to whole gallons?
Paint is sold by the gallon (and quart), and running out mid-wall leaves a visible seam where the new batch starts. Rounding up also leaves paint for touch-ups, which every room eventually needs.
Do I need primer too?
On new drywall, a drastic colour change, or stained walls, yes — and primer is usually calculated the same way but at one coat. This calculator covers finish paint; for primer, run it once with 1 coat and your primer's coverage rate.
What if my room is not a rectangle?
Break it into rectangles: calculate each wall section's area (length × height) separately and add them, or run the calculator once per section. Bay windows and alcoves are the usual reason the rectangle figure comes up short.
Does the ceiling use the same paint?
Usually ceiling paint is a separate, flatter product, so if you include the ceiling here, treat that gallon count as ceiling paint, not extra wall paint. Many painters buy the ceiling quantity separately.
Why did the estimate come up short on textured walls?
Texture adds surface area that a flat measurement misses, and thirsty surfaces (unprimed drywall, fresh plaster) absorb the first coat. On either, budget 20–30% more than the flat figure — or drop the coverage rate from 350 to about 275 sq ft per gallon.
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