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Paint Calculator
This calculator works out the wall area of a rectangular room, deducts standard areas for doors and windows, and converts the result into gallons of paint for the number of coats you plan, using the industry-standard coverage of about 350 square feet per gallon per coat.
Results update as you type. Estimates only — confirm final quantities before ordering.
The formula
Paintable area = Wall area − (21 ft² per door) − (15 ft² per window)
Gallons = Paintable area × Coats ÷ 350 ft² per gallon
The deductions use the estimator's standard sizes: a typical interior door (3 ft × 7 ft) is 21 ft² and a typical window (3 ft × 5 ft) is 15 ft². Coverage of 350 ft² per gallon is the standard figure manufacturers quote for one coat on smooth, primed walls.
Worked example
Example: a 12 ft × 14 ft room with 8 ft ceilings, 2 doors, 3 windows and 2 coats.
- Wall area: 2 × (12 + 14) × 8 = 416 ft²
- Openings: (2 × 21) + (3 × 15) = 87 ft² → paintable area 329 ft²
- Paint: 329 × 2 coats ÷ 350 = 1.88 gal → buy 2 gallons
Reference chart
| Paintable area | 1 coat — exact (buy) | 2 coats — exact (buy) |
|---|---|---|
| 100 ft² | 0.29 (1 gal) | 0.57 (1 gal) |
| 200 ft² | 0.57 (1 gal) | 1.14 (2 gal) |
| 350 ft² | 1.00 (1 gal) | 2.00 (2 gal) |
| 500 ft² | 1.43 (2 gal) | 2.86 (3 gal) |
| 700 ft² | 2.00 (2 gal) | 4.00 (4 gal) |
| 1,000 ft² | 2.86 (3 gal) | 5.71 (6 gal) |
| 1,500 ft² | 4.29 (5 gal) | 8.57 (9 gal) |
Frequently asked questions
How much area does one gallon of paint cover?
About 350–400 square feet per coat on smooth, sealed walls. Rough, porous or heavily textured surfaces — and unprimed new drywall — can drop coverage to 250–300 square feet per gallon, so buy toward the generous side for those.
Is primer counted in this result?
No. This is finish paint only. New drywall, bare wood and stained or glossy surfaces need a coat of primer first; estimate primer with the same area and coverage math, one coat.
Does the calculator include the ceiling?
No — it covers walls only. If you are painting the ceiling too, add the ceiling area (length × width of the room) to the paintable area and divide by 350 per coat.
Why deduct 21 sq ft for a door and 15 for a window?
Those are the long-standing estimator conventions for a standard 3×7 ft door and a 3×5 ft window, including trim. If your openings are much larger or smaller, adjust the counts to match the actual opening area.
Related calculators
- Drywall Calculator — Sheets of 4×8 and 4×12 drywall for your wall and ceiling area.
- Tile Calculator — Tiles and boxes needed, with waste for cuts and breakage.
- Board Foot Calculator — Board feet and lumber cost from thickness, width, length and quantity.
Sources & method: Manufacturer coverage guidance (Behr / Sherwin-Williams: ~350–400 ft² per gallon, one coat, smooth surfaces); door 21 ft² / window 15 ft² deduction convention per Inch Calculator — Paint Calculator.