Paint Calculator
How much paint do you need? Measure any number of rooms, add a waste factor, and get the square footage, quantity and cost to order.
A worked example
A 12 × 15 ft room is 180 square feet. Adding the 5% waste factor this calculator defaults to brings that up to 189 square feet to buy — 1 gallon, at 350 sq ft each. At the mid-range price of $4.00 per square foot that is about $756.00.
| Grade | Price per sq ft | 180 sq ft room, installed |
|---|---|---|
| Budget | $2.00 | $378.00 |
| Mid-range | $4.00 | $756.00 |
| Premium | $6.00 | $1,134.00 |
Price ranges are broad US ballpark figures for material plus typical installation, gathered as placeholders and not yet verified against sourced pricing. Use them to sanity-check a quote, not to replace one. One gallon of interior paint covers roughly 350 sq ft in a single coat. Most jobs need two coats.
To compare quotes on the same basis, see how to calculate price per square foot.
Paint is measured against walls, not floors
The most common mistake in a paint estimate is starting from the floor area. What you are covering is the walls: each wall’s width times the ceiling height, added together. A 12 × 15 ft room with an 8 ft ceiling has (12 + 15) × 2 × 8 = 432 square feet of wall — well over twice its 180 square foot floor.
Add the ceiling separately if you are painting it, using the floor area. Trim, doors and window frames are estimated by the linear foot rather than by area, and usually take a different product anyway.
Coverage and coats
A US gallon covers roughly 350 to 400 square feet per coat on primed, smooth drywall. Bare drywall, new plaster, bare wood and masonry all drink more — 200 to 250 square feet is realistic — and a heavy texture or a popcorn ceiling can halve it again. The figure printed on the tin assumes one coat on a prepared surface, so check it rather than assuming.
Two coats is the working assumption for any colour change, and two is what most warranties require. One coat is only realistic when refreshing the same colour on a sound wall. Going dark to light, or covering a strong red or yellow, usually means a tinted primer plus two topcoats.
Deducting doors and windows
Allow about 21 square feet for a standard door and about 15 for an average window. Most decorators skip the deduction entirely when the openings total under 100 square feet, because cutting in around them uses more paint per square foot than rolling does and the two roughly cancel. On a room with a wall of glass, deduct properly — there the difference is real money.
For the full walk-through — gallons per coat, gable ends and sloped ceilings — see the guide to calculating the square footage of a wall.
Frequently asked questions
How do I calculate how much paint I need?
Measure the wall area, not the floor area: add up each wall's width times the ceiling height, subtract the doors and windows, then divide by the coverage on the tin and multiply by the number of coats. A 12 by 15 ft room with an 8 ft ceiling has 432 square feet of wall; less a door and two windows, call it 380, which is about one gallon per coat at 375 square feet per gallon.
How many square feet does a gallon of paint cover?
Around 350 to 400 square feet per coat on primed, smooth drywall. Expect less on anything porous or textured — bare drywall, new plaster, bare wood and masonry can drop coverage to 200 to 250 square feet, and a heavy texture or popcorn ceiling can halve it again. Check the tin: coverage varies by product, and the figure printed on it assumes one coat on a prepared surface.
Should I subtract doors and windows?
Deduct a standard door at about 21 square feet and an average window at about 15. Most decorators skip the deduction when the openings total under 100 square feet, because the cutting-in around them uses more paint per square foot than rolling does, and the two roughly cancel out. Deduct properly on a room with a wall of glass, where the difference is real.
How many coats of paint will I need?
Two is the working assumption for any colour change, and two is what a warranty usually requires. One coat is realistic only when refreshing the same colour on a sound surface. Going from dark to light, or covering a strong red or yellow, often takes a tinted primer plus two topcoats — budget three coats' worth of material and the primer separately.