Paint Calculator Guide: How Much Paint You Really Need
How to figure out exactly how much paint a room needs, why coverage estimates go wrong, and how coats, primer, and wall texture change the math.
Most paint overbuying (and underbuying) comes down to one thing: skipping the actual coverage math and guessing based on room size alone. A gallon of paint covers roughly 350-400 square feet in a single coat under normal conditions — but "normal conditions" is doing a lot of work in that sentence.
The formula behind the estimate
Wall area = perimeter × ceiling height, minus door and window openings. Divide that by your paint's coverage rate (check the can — it varies by brand and finish), then multiply by the number of coats. Two coats is the standard for a color change or new drywall; one coat can work for a same-color refresh on a wall in good condition.
What changes the real number
- Texture: textured or porous surfaces (stucco, unprimed drywall, brick) absorb more paint — budget 10-20% extra coverage.
- Color change: going from a dark wall to a light color (or vice versa) often needs a tinted primer coat first, which isn't the same product as your finish paint.
- Sheen: flat and matte finishes tend to cover slightly better per gallon than glossier finishes, which are formulated thinner.
Worked example
A 12×12 ft room with 9 ft ceilings has a perimeter of 48 ft, so gross wall area is 48 × 9 = 432 sq ft. Subtracting roughly 40 sq ft for a door and two windows leaves 392 sq ft. At 350 sq ft per gallon per coat and 2 coats, that's (392 ÷ 350) × 2 ≈ 2.24 gallons — round up to 2.5 gallons (a gallon plus a quart) rather than a full 3 gallons.
The Paint Calculator runs this exact formula for your room's dimensions, door and window count, and number of coats, and shows the result in gallons, liters, or exact cans — worth checking before you buy at the store.