Paint Calculator
Enter room dimensions and coats to get paintable area, gallons needed and gallons to buy — with the spread-rate reality of rough or unprimed surfaces.
Last updated: 2026-09-28
How the calculation works
- Wall area is the room perimeter × height; the ceiling adds L × W when included.
- Gallons multiply area by coats and divide by the spread rate — the single biggest uncertainty in the estimate.
- Openings are deliberately not deducted: cut-in labor around doors and windows consumes what the area deduction would save.
- Primer spreads slower (~250 ft²/gal) and is computed at one coat for bare drywall.
Formula
wall area = 2 × (L + W) × H gallons = area × coats ÷ spread rate
| Symbol | Meaning | Unit |
|---|---|---|
A | Paintable area | ft² |
spread | Coverage per gallon per coat | ft²/gal |
Worked example
Interpreting the result
The spread rate is where estimates die: the 375 ft²/gal on the can assumes smooth primed drywall, ideal application and no waste. Dark-over-light color changes, textured walls and thirsty unprimed board can halve it. Buy the full rounded gallon count — paint is batch-tinted and 'close enough' matching across visits rarely is. One gallon covers a small bedroom's walls once; most rooms with two coats need 2-3 gallons.
Assumptions
- Rectangular room, standard spray or roller application.
- Openings (doors/windows) not deducted — waste and cut-in absorb them.
- 375 ft²/gal default spread for interior latex on primed drywall.
Limitations
- Trim, baseboards and doors are separate quantities with different products.
- Textured surfaces, masonry and previously dark walls need adjusted spread rates.
Frequently asked questions
How much paint for a 12×12 room?
Walls 384 ft²; two coats at 375 ft²/gal = 2.05 gal → buy 3 gallons (2 with careful rolling and no color change). Including the ceiling adds ~0.6 gal per coat-pair.
Do I need two coats?
Two coats is the standard for uniform color and washability; quality paints with good hide sometimes manage one on like-color repaints. Color changes and new drywall always need primer plus two finish coats.