Paint Calculator
Paint is sold by the gallon, but rooms are measured in square feet, and the gap between those two numbers is where most DIY painters guess wrong. This calculator closes the gap: enter your room dimensions, the number of doors and windows, your paint finish, and the number of coats, and it tells you exactly how many gallons to buy, with ceiling, trim allowance, and cost built in.
The math follows the same steps professional estimators use. Wall area is the room perimeter times the ceiling height, minus standard allowances for doors (20 sq ft each) and windows (15 sq ft each). That net area is multiplied by the number of coats, adjusted for your chosen finish coverage rate, and rounded up to whole gallons, because stores do not sell fractions of a gallon.
Paint Calculator
Room dimensions in, gallons to buy out. Doors and windows are deducted automatically.
| Paintable wall area | — |
| Ceiling area | — |
| Total area (all coats, with buffer) | — |
| Exact gallons needed | — |
| In quarts (for touch-ups) | — |
Standard allowances: 20 sq ft per door, 15 sq ft per window. Coverage rates follow manufacturer-published ranges (flat 350–400, eggshell/satin 300–350, semi-gloss 250–300 sq ft per gallon); always check your can label.
How to measure a room for paint
Measure the length and width of the room at floor level and the height from floor to ceiling. For a rectangular room, the wall area is 2 × (length + width) × height. Then subtract 20 sq ft for every standard door and 15 sq ft for every standard window. Do not subtract small openings under about 5 sq ft, such as a medicine cabinet; the paint saved there roughly covers the extra needed for the wall around it.
If you are painting the ceiling too, add length × width. For trim and baseboards, most painters allow one extra quart per average room, or enter a 10–15% waste buffer in the calculator, which covers touch-ups as well.
Worked example
A 12 × 12 ft bedroom with 8 ft ceilings, 1 door, 2 windows, 2 coats of eggshell paint (325 sq ft per gallon), and a 10% buffer:
Wall area = 2 × (12 + 12) × 8 − 20 − 30 = 334 sq ft. Total with coats and buffer = 334 × 2 × 1.10 = 735 sq ft. Gallons = 735 ÷ 325 = 2.26, so you buy 3 gallons. Enter these numbers in the calculator to confirm.
Quick reference: gallons by room size
Estimates assume 8 ft ceilings, 2 coats of eggshell, one door, two windows, and a 10% buffer. Use the calculator for your exact room.
| Room | Dimensions | Gallons to buy |
|---|---|---|
| Small bedroom | 10 × 10 ft | 2 |
| Bedroom | 12 × 12 ft | 3 |
| Living room | 14 × 16 ft | 4 |
| Large living room | 20 × 20 ft | 5 |
| Open plan | 20 × 24 ft | 6 |
Primer: how much you need
Primer is a separate purchase from finish paint, and it covers less area per gallon: plan on 250 to 300 sq ft per gallon for one coat on bare drywall, new wood, or patched surfaces (the calculator defaults to 300 and lets you change it). One coat of primer is enough under two finish coats. You can skip primer on previously painted walls in good condition, but dark-to-light color changes, stains, and new drywall all need it, so tick the primer box and the estimate adds the gallons for you.
Pro labor cost estimate
Professional painters in the US typically charge $2 to $6 per square foot of paintable area (HomeAdvisor), with about $2.50 a fair planning average for standard interior work; labor is roughly 85% of a pro job, with paint and supplies making up the rest. Enter a labor rate and the calculator prices your wall area at that rate, then shows a project total of paint plus labor. Leave it blank for a DIY estimate: you will pay only for paint, primer, and supplies.
Painting several rooms at once
Switch to the Multiple rooms tab to estimate a whole floor or a whole house. Add a room for each space with its own dimensions, doors, windows, and ceiling option; the tool lists every room in a table, totals the paintable area, and buys paint once for the combined total, which is cheaper than rounding up per room. Primer and labor apply to the combined area too.
Worked example: two rooms, paint plus primer plus labor
Living room 16 by 14 ft with 9 ft ceilings, 2 doors, 3 windows; bedroom 12 by 12 ft with 8 ft ceilings, 1 door, 2 windows. Eggshell at 325 sq ft per gallon, 2 coats, 10% waste buffer, bare drywall (primer needed), labor at $2.50 per sq ft, paint at $45 per gallon.
- Living room walls: 2 x (16 + 14) x 9 - 2 x 20 - 3 x 15 = 540 - 40 - 45 = 455 sq ft.
- Bedroom walls: 2 x (12 + 12) x 8 - 20 - 30 = 384 - 50 = 334 sq ft.
- Combined: 789 sq ft. Paint: 789 x 2 x 1.10 / 325 = 5.34 gallons, buy 6 ($270).
- Primer: 789 / 300 = 2.63, buy 3 gallons (one coat).
- Labor: 789 x $2.50 = $1,972.50. Project total: $2,242.50.
Paste these exact numbers into the Multiple rooms tab and you will get the same result.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most interior paints cover 350 to 400 sq ft per gallon per coat on smooth, primed walls. Eggshell and satin finishes cover about 300 to 350, and semi-gloss about 250 to 300, because thicker films build at different rates. Always check the coverage printed on your can label, since it overrides any rule of thumb.
Should I subtract doors and windows from the wall area?
Yes. Subtract 20 sq ft per standard door and 15 sq ft per standard window. Skipping this step is the most common reason people overbuy, especially in rooms with several windows.
How many coats of paint do I need?
Two coats is the standard for full, even coverage on walls, and the calculator defaults to it. One coat is usually enough only for a same-color refresh on already-painted walls. New drywall needs a primer coat plus two finish coats.
How much extra paint should I buy?
A 10% buffer covers cutting-in waste, roller absorption, and future touch-ups. Keep the leftover can sealed; it is the only way to get an exact color match later.
Does ceiling paint use the same coverage rate?
Ceiling (flat) paint typically covers 350 to 400 sq ft per gallon. Tick the ceiling option and the calculator adds the ceiling area to the total before dividing by your selected coverage rate.
How much primer do I need for a room?
Divide the wall area by 300 sq ft per gallon for one coat, then round up. A 12 by 12 ft room with 8 ft ceilings has about 334 sq ft of wall after doors and windows, so buy 2 gallons of primer. Previously painted walls in good condition usually need no primer.
How much do painters charge per square foot?
Most US pros charge $2 to $6 per square foot of paintable area, or $20 to $50 per hour (HomeAdvisor). A standard 12 by 12 ft room is about 334 sq ft of wall, so labor alone runs roughly $670 to $2,000 at those rates before paint. The calculator uses $2.50 per sq ft as an editable default.
Can I estimate paint for multiple rooms at once?
Yes. The Multiple rooms tab lets you add up to 12 rooms with separate dimensions, then totals the paintable area and rounds up once for the whole project, which buys less paint than rounding up room by room.