Paint Calculator

How the math works

Paint coverage is an area problem. The calculator finds your paintable wall area — for a room, that's the perimeter times the ceiling height — subtracts the openings you won't paint (about 21 square feet per door, 15 per window), and optionally adds the ceiling.

Then it divides by the coverage rate for your surface and multiplies by the number of coats. Smooth, previously painted drywall covers at roughly 400 square feet per gallon per coat. Texture and masonry cover less because the rough surface has more actual area — heavy texture can drop to 250. Round up to whole gallons, because you can't buy 1.7 gallons, and you always want a little left for touch-ups.

The formula

The whole calculation chain fits on four lines, so you can check any estimate — ours or a painter's — by hand:

Wall area = 2 × (length + width) × ceiling height
Paintable area = wall area − (doors × 21) − (windows × 15), plus length × width if you're painting the ceiling
Paint gallons = paintable area × coats ÷ coverage rate, rounded up (never less than 1)
Primer gallons = paintable area ÷ 300, rounded up — primer is one coat

The 21 sq ft per door is a standard 3-by-7 opening; the 15 per window a typical 3-by-5. Primer's lower rate (300) reflects the porous, unsealed surfaces it usually covers.

Worked example: a 14 × 16 living room

Say you're painting a 14 × 16 living room with 9-foot ceilings — walls and ceiling, two coats over orange-peel texture, primer because you're going from dark navy to off-white:

• Wall area: 2 × (14 + 16) × 9 = 540 sq ft
• Openings: 2 doors × 21 + 3 windows × 15 = 87 sq ft, leaving 453 sq ft of wall
• Ceiling: 14 × 16 = 224 sq ft, for 677 sq ft paintable in total
• Paint: 677 × 2 coats ÷ 350 (light texture) = 3.87 → 4 gallons
• Primer: 677 ÷ 300 = 2.26 → 3 gallons

At $45 per gallon, that's 7 gallons ≈ $315 in material. The raw math says 3.87 gallons, but the fourth can isn't optional — running dry mid-wall leaves a visible lap line, and the leftover fraction becomes your touch-up supply.

Coverage rates by surface

The per-coat rates the calculator uses, what they mean for the default 12 × 12 room (348 sq ft of wall after openings, two coats), and the roller nap that reaches into each surface:

Nap matters: a 3/8-inch roller on heavy texture skips the valleys, so the wall looks covered until light rakes across it. A longer nap holds more paint — part of why texture consumes more gallons.

Buying in the right cans

Above four gallons, a five-gallon pail is cheaper per gallon than five separate cans — and it keeps the color consistent (every can is a slightly different batch). The calculator suggests the cheapest pail-plus-can mix. It also keeps a minimum of one gallon: even a tiny accent wall needs a full can.

Common mistakes

• Buying for one coat. Almost every repaint needs two. Estimating one coat is the most common way to run short halfway through the second.
• Forgetting texture. The same room takes 60% more paint on heavy texture than on smooth drywall. Match the surface setting to your walls.
• Skipping primer on bare drywall. Unprimed drywall soaks up the first finish coat unevenly. A gallon of primer is cheaper than a third coat of finish.
• Not deducting big openings. A wall of windows or a pair of double doors is real square footage you're not painting.
• Not boxing multiple cans. Tint varies slightly can to can, even in the same color. Pros pour every can for a room into one bucket and mix ("boxing") so a mid-wall can change never shows; a 5-gallon pail is one batch and sidesteps it.

When this calculator is the wrong tool

Use a different reference for: exterior siding (weather and substrate change coverage dramatically), cabinets and trim (sold and measured by the linear foot, with different products), and specialty finishes like Venetian plaster or epoxy floor coatings. This tool covers interior wall and ceiling paint.

Sources & how we keep this current

Every constant in this calculator traces to a public source:

• Paint manufacturer data sheets (Sherwin-Williams, Behr, Benjamin Moore) — the coverage rates. Interior wall paints consistently list 350–400 sq ft per gallon per coat on smooth, sealed surfaces; Behr's guidance drops to roughly 250 sq ft on rough or porous substrates — our heavy-texture rate.
• Primer data sheets (KILZ, Zinsser) — listed at 300–400 sq ft per gallon; the calculator uses the conservative 300 because primer usually goes over bare drywall, patches, or stains that absorb more.
• Standard estimating practice — the 21 sq ft door (3 × 7) and 15 sq ft window (3 × 5) deductions are the bidding conventions in painting-trade estimating guides such as the PCA's (Painting Contractors Association).
• Roller manufacturer guidance (Purdy, Wooster) — the nap recommendations in the table above.

The coverage figures live in a versioned data file with a last-verified date and get re-checked against current manufacturer spec sheets. Coverage is printed on every can — if your product lists a different rate, trust your label over any average.