Paint Calculator
Find out exactly how many gallons — and quarts — to buy for a room, a single wall, or a ceiling, with doors, windows, and coats accounted for.
8″ is standard; newer homes often 9″.
Counted at 20 sq ft each.
Counted at 15 sq ft each.
Check your paint can — manufacturers print coverage per gallon on the label.
You need
Enter dimensions
| Detail | Amount |
|---|---|
| Paintable area | — |
| Area × coats | — |
| Buy as | — |
Mid-range interior latex commonly runs $30–$60 per gallon.
How to Calculate Paint
Paint is bought by coverage — how many square feet one gallon covers per coat. The math is three steps for any surface:
- Find the paintable area. For a whole room, that's the perimeter × ceiling height; for one wall, width × height; for a ceiling, length × width. Subtract about 20 sq ft per door and 15 sq ft per window.
- Multiply by coats. Two coats means the paint has to cover the area twice.
- Divide by coverage. Typical interior latex covers about 350 sq ft per gallon on a normal wall — less on rough or bare surfaces.
Paint formulas
Room walls: area = 2 × (length + width) × height − openings
Gallons = (area × coats) ÷ coverage per gallon
Worked example — 12 × 12 room, 8″ ceilings, 1 door, 1 window, 2 coats: perimeter 48 ft × 8 ft = 384 sq ft − 35 sq ft of openings = 349 sq ft. × 2 coats = 698 sq ft ÷ 350 = 2.0 gallons.
Paint Needed for Common Room Sizes
Two coats on walls, 8 ft ceilings, one door and one window subtracted, 350 sq ft/gal coverage:
| Room size | Wall area | Gallons (2 coats) | Buy |
|---|---|---|---|
| 10 × 10 ft | 285 sq ft | 1.6 | 1 gal + 3 qt |
| 12 × 12 ft | 349 sq ft | 2.0 | 2 gallons |
| 12 × 16 ft | 413 sq ft | 2.4 | 2 gal + 2 qt |
| 16 × 20 ft | 541 sq ft | 3.1 | 3 gal + 1 qt |
| 20 × 20 ft | 605 sq ft | 3.5 | 3 gal + 2 qt |
Tips Before You Buy
- Buy all the paint at once. Cans tinted in separate batches can vary slightly — or "box" them by mixing cans together before starting.
- Dark over light needs primer. A tinted primer coat is cheaper than a third coat of finish paint.
- Keep the leftover. Sealed in the original can, leftover paint is your color-matched touch-up supply for years.
- Ceilings use flat, walls use eggshell or satin. Different sheens hide and wash differently — the calculator works for any of them.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls (349 sq ft after subtracting a door and window at 350 sq ft/gal coverage). Add a gallon of ceiling paint if the ceiling is included.
How many square feet does a gallon cover?
350–400 sq ft per coat on smooth, previously painted surfaces; 250–300 on rough, textured, or bare surfaces. The can's label states the manufacturer's figure.
Do I really need two coats?
For uniform color and warranty coverage, yes. One coat only works when repainting the same color and sheen. Drastic changes may need primer plus two coats.
Should I subtract doors and windows?
Yes — about 20 sq ft per door and 15 per window. For a single small window, many painters skip the subtraction and keep the extra as touch-up.
How much ceiling paint for a 12×12 room?
144 sq ft — one coat takes about 0.4 gallons, so a single gallon handles two coats.
Quart or round up to a gallon?
A gallon costs about twice a quart's price for four times the paint. Within a quart of the next gallon? Round up and keep the remainder for touch-ups.
References
- Sherwin-Williams — paint coverage and estimating guidance: sherwin-williams.com
- Benjamin Moore — product data sheets (spread rate per gallon): benjaminmoore.com
- PPG — how much paint do I need: ppgpaints.com