Paint Calculator
Buying Paint Is a Coverage Problem, Not a Guessing Game
Paint cans list coverage per gallon, but translating that into "how many cans do I actually need" means netting out every door and window and deciding how many coats the color and surface really require. Buy short and you're mixing a second batch mid-wall, which almost never matches the first exactly. Buy long and a half-used gallon sits in the garage for years.
The Formula
Gallons = (Paintable Area × Coats) ÷ Coverage per Gallon
The calculator deducts 21 sq ft per standard door (roughly 3 ft × 7 ft) and 15 sq ft per standard window (roughly 3 ft × 5 ft), then divides the paintable area — multiplied by the number of coats — by a standard coverage assumption of 350 sq ft per gallon, before adding a 10% overage and rounding up to a whole gallon.
Where This Calculation Matters
- Interior rooms with lots of trim — a bedroom with two windows and a closet door loses real wall area to those openings; skipping the deduction overstates how much paint you need.
- Two-coat jobs on new drywall — unprimed or color-changing walls typically need two full coats, which roughly doubles the gallons required compared to a single-coat touch-up.
- Exterior siding — the same formula applies to exterior walls, just with the wall's actual length and height and its own door/window count.
Worked Example
A 40 ft wall, 9 ft high, with 2 doors and 3 windows, painted in 2 coats:
| Step | Calculation | Result |
|---|---|---|
| Gross area | 40 × 9 | 360 sq ft |
| Deductions | (2 × 21) + (3 × 15) | 87 sq ft |
| Paintable area | 360 − 87 | 273 sq ft |
| Total area (2 coats) | 273 × 2 | 546 sq ft |
| Gallons (incl. 10% waste) | (546 ÷ 350) × 1.10 | 2 gallons |
How to Use This Calculator
- Enter the Wall Length and Wall Height in feet.
- Enter the Number of Doors and Number of Windows to be deducted from the paintable area.
- Enter the Number of Coats — leave blank to default to 2, the standard for most interior repaints.
- Select Calculate to get the number of gallons, rounded up and including a 10% overage for touch-ups.
Related Calculations
Painting drywall you just hung? The Drywall Calculator sizes the sheets before you get to the paint, and the Insulation Calculator covers what typically goes in the wall cavity first.