Paint looks simple — one can, one wall — until you are halfway through a second coat on a Sunday evening with an empty can. This calculator works out the wall area of a rectangular room, subtracts the doors and windows you will not paint, and converts the result into gallons using the standard coverage rate and the number of coats you plan.
Enter the room’s length, width, and ceiling height. If your walls are not a simple rectangle, add up each wall (length × height) yourself and divide by the coverage the same way — the formula below shows exactly how the tool does it.
Paint Calculator
Worked example: a 12 × 10 ft bedroom
A bedroom measures 12 ft by 10 ft with an 8 ft ceiling. It has one standard door and one window, and you plan two coats.
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft (the perimeter times the ceiling height)
- Openings: a 3 × 7 ft door is 21 sq ft and a 3 × 5 ft window is 15 sq ft, so subtract 36 sq ft
- Paintable area: 352 − 36 = 316 sq ft
- Paint per coat: 316 ÷ 350 = 0.90 gallons
- Two coats: 0.90 × 2 = 1.81 gallons, rounded up to 2 gallons to buy
Two gallons also covers your touch-up reserve. Keep the leftover in a sealed, labeled container and future scuff repairs will match.
How the calculation works
Gallons = (wall area − openings) ÷ coverage × coats, rounded up to whole gallons.
The default coverage is 350 sq ft per gallon, the figure major US paint manufacturers publish for one coat on a smooth, primed surface. Real coverage drops on rough, textured, or unprimed surfaces — sometimes to 250–300 sq ft per gallon — which is why the calculator lets you change the rate. A standard interior door is about 21 sq ft and an average window about 15 sq ft, handy defaults when you subtract openings.
Typical paint costs
Typical US prices, checked November 2026. Materials only; prices vary by region, store, and any current promotions.
| Product | Typical cost per gallon |
|---|---|
| Standard interior latex paint | $25 – $45 |
| Premium interior paint | $45 – $70 |
| Primer | $15 – $30 |
| Ceiling paint | $20 – $35 |
The worked example needs 2 gallons, so roughly $60 – $90 in standard interior paint — plus a gallon of primer ($15 – $30) if you are painting over new drywall or a strong color change.
Before you buy
- Measure the room and compute wall area as perimeter × ceiling height; for the floor area method, see the Square Footage Calculator page.
- Subtract doors (about 21 sq ft each) and windows (about 15 sq ft each) that you will not paint.
- Decide coats honestly: two over primer for bare drywall or big color changes; one may be enough for a same-color refresh in good condition.
- Check the coverage printed on the can you are actually buying, and lower it toward 300 sq ft if your walls are textured or previously unpainted.
- Buy all the paint for one room in a single batch, and mix the gallons together (“boxing”) before the final coat for an even color.
FAQ
How much paint do I need for a 12 × 12 ft room?
With 8 ft ceilings, the walls total 384 sq ft. Subtract about 36 sq ft for a door and window, and two coats at 350 sq ft per gallon comes to 1.99 gallons — so buy 2 gallons.
Does the 350 sq ft coverage figure include two coats?
No. Coverage is per coat. Two coats need roughly double the paint of one, which is why the calculator multiplies by the number of coats before rounding up.
Should I subtract doors and windows?
Yes for standard-size openings — it is typically 30–40 sq ft per room, close to half a gallon over two coats. Small windows can be left in as a safety margin if you prefer a little extra paint.
Do I need primer as well as paint?
On new drywall, patched areas, stains, or when changing from a dark or saturated color to a light one, yes — primer is what makes the finish coats even. Paint-and-primer-in-one products handle mild cases, but bare drywall still drinks a dedicated primer coat. If the walls were just built, the Drywall Calculator page covers the sheet estimate for that stage of the project.
How do I calculate paint for a ceiling?
The ceiling is just another rectangle: length × width. Add it as a separate calculation with ceiling paint, usually one or two coats, and do not subtract the light fixture — it is negligible.
Related calculators
The Square Footage Calculator gives you the floor and wall areas that feed this estimate. Prefer paper on the walls? The Wallpaper Calculator works out rolls with the pattern-matching waste included. And if the same room is getting a new floor, the Flooring Calculator estimates boxes of LVP, laminate, or hardwood.
Estimates only: results are based on standard coverage rates and the measurements you enter, and actual coverage varies with surface condition and product. Confirm the coverage printed on your paint can before buying. These tools do not cover structural, load-bearing, electrical, gas, or plumbing design — for safety-critical work, hire a licensed professional.