Paint & Walls
Paint Calculator
Gallons for walls and ceilings by room size, coats and paint coverage — doors and windows deducted.
Inputs
Results
Adjust the inputs to see your result.
About this calculator
A gallon of paint covers roughly 350 sq ft per coat on smooth walls, less on rough or unpainted surfaces. This calculator totals your wall and ceiling area, deducts doors and windows, and converts the result to gallons for the number of coats you plan.
What this number means
Paint is bought in whole gallons, but consumed in fractions — so the useful outputs are both numbers: the exact gallons your walls will drink (which tells you if a quart will finish the last wall) and the whole gallons to put in the cart. The gap between them is your touch-up reserve, and for any colour you love, a little reserve is a feature, not waste.
Openings are deducted at standard sizes — about 21 sq ft for a door and 15 sq ft for a window — because painting around trim is already accounted for in how painters estimate. If your room has a wall of glass or a double door, adjust the counts and the estimate stays honest.
The formula
- Wall area = 2 × (Length + Width) × Ceiling height
- Ceiling area = Length × Width (added only if selected)
- Net area per coat = Wall area + ceiling area − (21 × doors) − (15 × windows), in sq ft
- Gallons = Net area × coats ÷ coverage per gallon; buy the next whole gallon up
Dimensions in inches or metres are converted to feet first.
Worked example
A 12 × 15 ft bedroom, two coats. Ceilings are 8 ft, there is one door and two windows, and the ceiling itself is not being painted.
- Wall area = 2 × (12 + 15) × 8 = 432 sq ft
- Openings = 21 + (2 × 15) = 51 sq ft → net 381 sq ft per coat
- Total coverage = 381 × 2 coats = 762 sq ft → 762 ÷ 350 = 2.18 gallons
- Buy 3 gallons — the part-gallon remainder is your touch-up reserve
Assumptions and limitations
- Coverage of 350 sq ft per gallon assumes smooth, previously painted walls and a quality paint. Fresh drywall, bare wood and heavy texture drink more — drop coverage to 250–300 and add a separate primer coat to your plan.
- Primer is not counted in the gallon figure. Over bare surfaces, budget roughly the same area again in primer (one coat).
- Trim, doors themselves and ceilings in a different finish are separate paints with separate maths — this estimate is for the wall colour.
- Deep and vivid colours can need a third coat over a light base (or a tinted primer). If the colour change is dramatic, set coats to 3.
Frequently asked questions
With 8 ft ceilings the walls total 384 sq ft. Deduct a door and a window and two coats need about 700 sq ft of coverage — at 350 sq ft per gallon that is exactly 2 gallons. Enter your own dimensions above; ceiling height changes the answer more than floor size does.
On smooth, sealed walls with a mid-to-premium paint, yes — manufacturers rate most wall paints at 350–400 sq ft per gallon per coat. Rough texture, bare drywall and cheap paint pull that down toward 250–300, which is why coverage is an input here, not a fixed number.
Count it separately. Primer covers similarly per gallon but is a different, cheaper product, and you only need it on bare drywall, patched areas, stains, or under a dramatic colour change. This calculator counts finish paint; add one primer coat of the same net area if your surface needs it.
Stores sell whole gallons and quarts. The calculator shows the exact consumption (say 2.18 gal) and the purchase (3 gal). The leftover is genuinely useful: labelled and sealed, it is your touch-up paint for the next several years, in the exact same batch and colour.
Count each average window as one in the windows field (15 sq ft). For a picture window or sliding door, count it as two or three windows, or count a double door as two doors. The deductions are averages, and adjusting the counts keeps the estimate accurate enough to buy by.