How to Calculate Paint for a Room
Paint is measured by the gallon, and a gallon covers roughly 350 to 400 square feet with one coat on a smooth, primed surface. This calculator uses 350 — the conservative end of the published range — which accounts for slight absorption, uneven walls, and roller waste. Buying one more gallon than you think you need is always cheaper than a second trip.
Net area = Wall area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
Gallons = ceil(Net area × Coats ÷ 350)
One Coat or Two?
Two coats is the professional standard for any finished interior room. One coat is acceptable for touch-ups, for painting over a nearly identical colour, or for a preliminary prime coat before the finish colour goes on. Three coats are occasionally needed when covering a very dark colour with a light one — in that case, consider a tinted primer as the first coat to reduce the work the finish coat has to do.
What Affects Paint Coverage
- Surface texture — Textured walls like orange peel or knockdown absorb more paint. Add 10–15% to your gallon estimate for heavily textured surfaces.
- Porosity — New drywall without primer is highly absorbent; the first coat disappears faster. Always prime new drywall before painting.
- Colour change — Going from dark to light is harder than dark to dark or light to light. Tinted primer reduces coats needed.
- Sheen level — Flat paint tends to absorb more; semi-gloss is more self-levelling and covers slightly better. The difference is minor for estimation purposes.
- Application method — Rollers waste less than brushes for large surfaces; sprayers waste more (overspray).
Primer: Separate from Paint
If you're priming, calculate primer as a separate layer. New construction or bare drywall needs one coat of primer. Repaint over existing paint generally doesn't need primer unless you're making a dramatic colour change. Primer coverage is similar to paint — 300 to 400 sq ft per gallon depending on brand. Most primers are available in 5-gallon buckets for large projects, at roughly 30% savings over individual gallons.
Buying Strategically
Always buy from the same dye lot for the main colour — different batches can have slight variations visible when dry. For touchups later, keep the paint code (on the lid or a sticker) and buy from the same manufacturer. If the store can't match an existing paint, a spectrophotometer scan of a paint chip can get close.