Paint Calculator

What this paint calculator does

This tool works out the gallons of paint you need to cover a room or wall area, given the wall dimensions, how many doors and windows are in the room, the surface type (smooth, textured, rough, or primer), and how many coats you plan to apply. It outputs the rounded-up gallon count for purchase, a quart suggestion for small jobs, and the paintable square footage the estimate is based on. Everything is arithmetic — no API calls, no account.

How to use the paint calculator

1. Measure your wall length and wall height in feet. For a typical room, the length is one of the four wall lengths and the number of walls is four. For a single accent wall, set walls to one.
2. Count the doors (about 21 sqft each) and windows (about 15 sqft each). Those areas do not get wall paint.
3. Pick the number of coats. Most repaints need two; a deep colour change or very absorbent fresh drywall may need three.
4. Pick the surface type. Smooth modern drywall absorbs the least; rough stucco and masonry absorb the most.
5. Read the gallon count. The calculator rounds up because partial gallons are not sold. Tap Copy summary to save the headline for your trip to the paint store.

The 350-square-feet-per-gallon rule

The single most quoted figure in interior painting is “one gallon covers 350 square feet at one coat.” That number is the industry rule of thumb for smooth, primed drywall painted with a standard mid-priced acrylic interior paint. Manufacturer spec sheets typically list coverage between 250 and 400 sqft per gallon depending on the sheen and product line — flat finishes cover the most because the binders are thinner, while glossy enamels cover the least because they are heavier-bodied.

Real-world coverage depends on a few variables the spec sheet does not always spell out:

• Surface absorption. Bare drywall, fresh patches, and chalky old paint drink more paint than a recently painted wall.
• Texture depth. Knockdown and orange-peel walls increase the true surface area by 5–10 % compared to a smooth wall of the same nominal size; popcorn ceilings can add 20 %.
• Colour transition. Going light-over-dark almost always needs a third coat (or a tinted primer) — and the gallon count climbs.
• Application method. Sprayers waste 10–20 % of the paint to overspray; rollers waste 5 % to the tray; brushes waste the least but cover the slowest.

Coverage rates by surface type

Use the surface dropdown to swap between them — the calculator divides paintable square footage by the coverage figure and multiplies by the number of coats to give the gallon count.

Coats: one, two, or three?

• One coat is enough only for touch-ups in the same colour, or when using a “one-coat coverage” premium paint over a very similar shade. Most professional painters consider one-coat premium paint a marketing claim more than a guarantee.
• Two coats is the industry standard for any repaint where the colour stays similar. It evens out the sheen, hides the previous colour completely, and is the warranty assumption most paint brands make.
• Three coats is needed when going from a dark colour to a light colour, painting over a stain or water mark, or covering bare drywall without a tinted primer.

Primer: when it is worth a separate purchase

Bare drywall, bare wood, fresh patches, glossy surfaces, and dark colours covered by a much lighter one all benefit from a dedicated primer before the topcoat. Primer is cheaper per gallon than paint and seals the surface so the topcoat covers in fewer coats — the math often works out cheaper than skipping the primer and using a third coat of finish paint.

If you are repainting a room the same colour or a similar tone and the existing paint is intact, a paint-and-primer-in-one is fine and the calculator’s smooth-drywall coverage rate applies.

Ceilings and trim

Ceilings need their own calculation — they are usually painted in a dedicated ceiling white flat paint that hides imperfections. For a 12 × 12 ft room, the ceiling is 144 sqft. Two coats at 350 sqft/gal = 0.82 gal — buy one gallon. Trim, doors, and windows are typically painted in semi-gloss or satin for durability — buy quarts of a different sheen, not more wall paint. A quart covers about 87 sqft, which is enough for the trim of a typical small bedroom.

Privacy

This calculator does its arithmetic in JavaScript on your device. There is no fetch call, no analytics on the values you enter, no server-side logging. The page works the same way offline once loaded.