How many gallons of paint do you need?

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.

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.

The Bottom Line

Paint is one of the easier materials to estimate, but most people still buy too little on the first trip and too much on the second. Measure the room, subtract doors and windows, multiply by the number of coats, and divide by coverage per gallon. This calculator does that math for you. Buy it all at once from the same batch for color consistency. Most paint stores will take back unopened gallons, so rounding up to the next gallon is safe. Keep at least a quart of leftover paint for touch-ups — label it with the room and date. Dried-out paint from three years ago is useless when you need to patch a scuff.