How paint gallons are calculated
Paint is sold by the gallon, and every gallon spreads over a set number of square feet per coat. So the job is to find the square feet you are painting, multiply by the number of coats, and divide by the spread rate for your surface.
wall area = 2 × (length + width) × ceiling height − 21 × doors − 15 × windows
ceiling area = length × width
The spread rate depends on the wall surface: 400 ft² per gallon on smooth or previously painted walls, 300 on light texture or orange peel, 250 on heavy texture or knockdown, and 300 on bare drywall. The ceiling uses the same rate as the walls.
wall gallons = wall area × coats ÷ spread rate
ceiling gallons = ceiling area × ceiling coats ÷ spread rate
trim gallons = trim feet × 0.5 × coats ÷ 400
primer gallons = (wall area + ceiling area) ÷ 300
Each line is rounded up to whole gallons on its own, because you can't share a can between two colours. Trim paint is usually a different colour and sheen, so it is shown in quarts and added to the total as whole gallons. Primer is listed on its own line, one coat.
Worked example: 12 × 12 ft room
The calculator opens on a 12 × 12 ft room with 8 ft ceilings, 1 door and 2 windows, smooth, previously painted walls, 2 coats on the walls and 1 on the ceiling.
- Gross wall area: 2 × (12 + 12) × 8 = 384 ft².
- Openings: 21 × 1 + 15 × 2 = 51 ft², so the walls to paint are 384 − 51 = 333 ft².
- Wall paint: 333 × 2 ÷ 400 = 1.67 gallons, so buy 2.
- Ceiling: 12 × 12 = 144 ft²; 144 × 1 ÷ 400 = 0.36 gallons, so buy 1.
- Total: 2 + 1 = 3 gallons of paint, with no trim or primer.
Paint for common room sizes
| Room (ft) | Wall area (ft²) | Wall paint (gal) | Ceiling paint (gal) | Total to buy (gal) |
|---|---|---|---|---|
| 10 × 10 | 269 | 2 | 1 | 3 |
| 12 × 12 | 333 | 2 | 1 | 3 |
| 12 × 14 | 365 | 2 | 1 | 3 |
| 14 × 16 | 429 | 3 | 1 | 4 |
| 16 × 20 | 525 | 3 | 1 | 4 |
| 20 × 20 | 589 | 3 | 1 | 4 |
Spread rates by surface
| Surface | Paint (ft²/gal) |
|---|---|
| Smooth or previously painted | 400 |
| Light texture or orange peel | 300 |
| Heavy texture or knockdown | 250 |
| Bare drywall | 300 |
| Primer, any surface | 300 |
Measuring the room
- Measure the length and width along the floor, wall to wall. Inches, feet-and-inches like 12' 6" and metric like 3.8 m all work.
- Measure the ceiling height from the floor to the ceiling, not to the top of the crown moulding.
- Count every door and window in the walls you are painting. Closets count if you paint their walls; the door deduction is for the opening only, not the door slab.
- For several rooms, use “+ Add another room”. Each room has its own size, openings and ceiling choice; the areas are added up and the gallons rounded once for the whole job. For a room that is not a rectangle, enter an average length and width that gives the same perimeter.
- Trim is lineal feet: add up the baseboard around the room plus the casing around each door and window, both sides if you paint both.
Primer, coats and leftovers
New drywall, patched areas, stains and big colour changes all call for primer. Bare drywall gets a full primer coat; check the primer box and the calculator adds one coat over the walls and ceiling at 300 ft² per gallon. Over water stains or smoke damage, use a stain-blocking primer, or the marks bleed through any number of finish coats.
Texture costs paint: a knockdown wall at 250 ft² per gallon takes 1.6 times as much as a smooth one at 400. Spraying usually uses more paint than rolling because of overspray, so if you spray, plan on extra.
Keep a little of each colour, sealed and labelled with the room, for touch-ups. Paint from a different batch can vary slightly in colour, so if a job needs several gallons of one colour, mix ("box") them together in a bucket before you start.
Sources
- Spread rates: manufacturer technical data sheets (typical values), as tabulated in our paint coverage chart
- Sherwin-Williams ProMar 200 Zero VOC interior latex data sheet: 350–400 ft² per gallon at 4 mils wet
- Drywall primer: Sherwin-Williams ProMar 200 Zero VOC primer (400 ft²/gal), Benjamin Moore Ultra Spec 500 primer (350–400), Zinsser Bulls-Eye 1-2-3 (350 on porous surfaces); this calculator uses a conservative 300
- Opening deductions: trade convention of 21 ft² per door (3 × 7 ft) and 15 ft² per window, the same figures our drywall calculator uses
- Trim: 6 in average profile, 0.5 ft² per lineal foot