Buy the right amount of paint the first time
Running out mid-wall means another trip to the store and a risk of a slightly different batch. Buying too much leaves half-used cans in the garage. This calculator starts from the room dimensions, subtracts doors and windows, accounts for the number of coats and the coverage printed on the can, and gives you the area, gallons, cans and cost.
How the calculation works
- Wall area = 2 × (length + width) × height.
- Openings: subtract about 21 sq ft per door and 15 sq ft per window.
- Ceiling (optional) = length × width.
- Gallons = area × coats ÷ coverage per gallon.
- Cans = gallons rounded up to whole gallons.
Coverage is printed on every can, in square feet per gallon per coat. Most interior latex paints cover 350–400 sq ft per gallon on smooth, primed drywall.
Worked example: a 14 × 12 ft bedroom
8 ft walls, one door, two windows, two coats, paint covering 350 sq ft per gallon at $45 a gallon.
- Gross wall area: 2 × (14 + 12) × 8 = 416 sq ft.
- Openings: 21 + 2 × 15 = 51 sq ft, so 365 sq ft to paint.
- Paint: 365 × 2 ÷ 350 = 2.09 gallons.
- Cans: 3 gallons for $135, or 2 gallons plus a quart if you are confident about coverage.
To paint the ceiling as well, add 168 sq ft, roughly another gallon of ceiling paint for two coats.
By number of coats
| Coats | Gallons | Gallon cans | Cost |
|---|---|---|---|
| 1 | 1.04 | 2 | $90 |
| 2 | 2.09 | 3 | $135 |
| 3 | 3.13 | 4 | $180 |
Adjust coverage to your surface
- New drywall: prime first with a drywall primer, then use normal coverage for the topcoats.
- Textured, porous or masonry walls: lower coverage to 250–300 sq ft per gallon.
- Big color change: a tinted primer often saves a third finish coat.
- Spraying: airless sprayers use 10–20% more paint than rolling because of overspray.
Choosing a sheen
Flat hides wall imperfections and suits ceilings and low-traffic rooms. Eggshell offers a soft glow and some washability for living rooms and bedrooms. Satin stands up to scrubbing in kitchens, bathrooms, hallways and kids’ rooms. Semi-gloss is the usual choice for trim and doors. The shinier the finish, the more it shows flaws, so prepare walls carefully.
Don’t forget the supplies
Roller covers matched to the sheen, an angled brush for cutting in, a tray, painter’s tape, drop cloths and spackle add $25–50 per room. Keep a little leftover paint and write down the exact color code and sheen for future touch-ups.