In short: Paint jobs die two deaths: three trips back to the store because you short-bought, or a garage shelf of orphan gallons because you panic-bought. Both come from guessing instead of multiplying two numbers.
Updated August 28, 2026, after a fresh review, with updated visuals and links.
Enter total wall width, height, coats, and a count of doors and windows to subtract. The tool returns the exact paintable area and how much to buy, in gallons or liters.
The coverage math, minus the guesswork
Walls are triangles-free rectangles: width x height. A room whose walls sum to 14 m wide at 2.5 m high is 35 m²; minus two openings (1.9 m² each) leaves 31.2 m². Two coats means buying for 62.4 m².
Coverage per unit is roughly 10 m² per liter, or 350 ft² per gallon, per coat. So that room drinks 6.2 liters; the sensible buy is 7 liters or 2 gallons, because touching up with a different batch never matches perfectly.
Texture, dark-to-light color changes, and unprimed new drywall all drink more; the 10-20% margin note under the result is not insurance talk, it is the difference between one trip and two.
Worth remembering
- Buy all paint for a room at once and check the batch numbers match. Same color, different batch, visible line.
- Primer is not optional over new drywall or stains; skipping it costs more in extra finish coats than the primer ever did.
Frequently asked
Do I calculate ceiling paint the same way?
Exactly the same: length x width of the room gives ceiling area. Ceilings usually take one coat of flat ceiling paint unless the color is changing dramatically.
How much paint does one gallon really cover?
The label range is 350-400 ft² on smooth primed walls, one coat. Rough or porous surfaces drop it toward 250 ft², which is where the margin in this calculator earns its keep.
Find every related tool in our Online Calculators: Every Free Tool index.
