The Paint Calculator answers “how much paint do I need?” in gallons and liters. Enter a room’s length, width and wall height with the number of doors and windows, or simply enter a known area. It works out the paintable area, multiplies by the number of coats and divides by the coverage on your paint can.
Paint Calculator
How much paint you need in gallons or litres — for a room’s walls and ceiling, or any area — with coats, doors and windows.
You also get the number of gallon cans to buy. Specialized calculators for single walls, ceilings, exteriors, decks, trim and painting cost are linked further down.
Formula
Paint = paintable area × coats ÷ coverage
Paintable area = 2(L + W) × H − 21 ft² × doors − 15 ft² × windows (+ L × W for the ceiling)
Where L, W = room length and width, H = wall height, and coverage = ft² per gallon per coat from the can. The 21 ft² and 15 ft² are the calculator’s allowances for one door and one window.
How the Calculation Works
Step 1: Wall area = perimeter × wall height.
Step 2: Subtract doors and windows; add the ceiling if you are painting it with the same paint.
Step 3: Multiply by coats and divide by coverage.
Step 4: Round up to whole cans.
Variables Explained
| Input | Meaning |
|---|---|
| Coats | Number of finish coats; two is common |
| Coverage per gallon | Listed on the can — many wall paints state roughly 350–400 ft² per gallon |
| Doors, windows | Openings not painted |
Example Calculation
Given: 14 × 12 ft room, 8 ft walls, 1 door, 2 windows, walls only, 2 coats, 350 ft²/gal
- Walls = 2 × 26 × 8 = 416 ft²
- Paintable = 416 − 21 − 30 = 365 ft²
- Paint = 365 × 2 ÷ 350 ≈ 2.09 gal (≈ 7.9 L)
- Buy 3 gallon cans — or 2 gallons plus a quart
Paint Coverage Reference
| Coverage on can | Gallons for 1,000 ft², 2 coats |
|---|---|
| 250 ft²/gal | 8.0 |
| 300 ft²/gal | 6.7 |
| 350 ft²/gal | 5.7 |
| 400 ft²/gal | 5.0 |
Material Estimation Notes
- Rough or porous surfaces, fresh drywall and big color changes can use more paint or need primer.
- Keep a little extra for touch-ups.
- Ceilings and trim are often a different paint — calculate them separately.
Frequently Asked Questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window: (384 − 36) × 2 ÷ 350 ≈ 2 gallons for two coats.
How many square feet does a gallon of paint cover?
Check the can; many wall paints list about 350–400 ft² per coat.
How do I convert gallons to liters?
1 US gallon = 3.785 L.
Do I subtract doors and windows?
Yes, the calculator subtracts standard allowances; for large openings enter the known area instead.
Do I need two coats of paint?
Two coats give more even color in most cases; follow the paint manufacturer’s guidance.
Should I include the ceiling?
Only if it will be painted with the same paint.