Paint Calculator
Estimate how many gallons of paint you need for any room or exterior — walls, ceilings, doors and windows included — for one or two coats.
A 12×12 ft room with 8 ft ceilings needs about 2 gallons of paint for two coats (348 sq ft of wall).
How to Use This Paint Calculator
- Enter the room length, width and ceiling height in feet.
- Subtract the area of doors and windows (the calculator applies standard sizes if you leave them out).
- Choose the number of coats — 2 is the norm when changing color.
- Add the ceiling if you are painting it too.
- Read the gallons needed, rounded up to the nearest gallon.
The Formula
Wall area = perimeter × height, minus openings. Gallons = wall area ÷ coverage per gallon, times the number of coats. Most interior latex paints cover roughly 350–400 sq ft per gallon on smooth, primed surfaces.
Walls = (2L + 2W) × HWall area in square feetNet = Walls − openingsStandard: window 15 ft², door 21 ft²Gallons = Net ÷ 350–400Coverage per gallon, per coatTotal = Gallons × coatsFinal estimate, rounded upWorked Example: 12×12 Room
A 12×12 ft room with 8 ft ceilings: perimeter 48 ft × 8 ft = 384 sq ft of wall. Minus one door (21) and one window (15) = 348 sq ft. At 350 sq ft per gallon: 1 gallon per coat, so 2 gallons for two coats.
Gallons Needed by Room Size (2 coats, 8 ft ceilings)
| Room | Wall area | Gallons (2 coats) |
|---|---|---|
| 10 × 10 ft | 284 sq ft | 2 |
| 12 × 12 ft | 348 sq ft | 2 |
| 12 × 14 ft | 380 sq ft | 3 |
| 14 × 16 ft | 444 sq ft | 3 |
| 20 × 20 ft | 604 sq ft | 4 |
Assumes one door + one window (36 sq ft) and 350 sq ft per gallon per coat.
Interior vs Exterior
Interior walls are the most common job, but the same math works for ceilings and exteriors. Exteriors need more paint: siding textures and wood absorb more, so coverage drops toward 250–300 sq ft per gallon on rough surfaces. Always check the coverage figure on the can — manufacturers state it for their specific product.
Quick Tips
- Two coats are the norm when changing color; one may do for touch-ups.
- Prime bare drywall or patched areas before the first coat.
- One gallon covers about 350–400 sq ft of smooth interior wall.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls (348 sq ft of wall at ~350 sq ft per gallon).
How many square feet does a gallon of paint cover?
Roughly 350–400 sq ft per coat on smooth interior walls; less on textured or exterior surfaces.
How many coats of paint do I need?
Two coats are typical when changing color. One coat may cover a repaint of the same color.
Do I need primer before painting?
Primer is recommended on bare drywall, patched areas, stains and drastic color changes.