Paint Calculator
Enter your room size, doors, windows and number of coats to see how many gallons of paint to buy — and whether a quart or a 5-gallon bucket makes more sense.
Enter the room length, width and wall height.
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How to use the paint calculator
- Measure the room at floor level and the height of the walls from floor to ceiling.
- Count doors and windows. The calculator subtracts about 21 sq ft per door and 15 sq ft per window. Skip them for very small openings — the extra paint helps with touch-ups.
- Set coats and coverage. Two coats is standard. The coverage is printed on the can; 350 sq ft per gallon is a common middle value.
- Read the result in gallons and as a practical shopping list of 5-gallon buckets, gallons and quarts.
How to calculate paint
Worked example: a 12 × 10 ft bedroom with 8 ft walls
- Walls: 2 × (12 + 10) × 8 = 352 sq ft; minus 1 door and 2 windows (21 + 30) = 301 sq ft
- Two coats at 350 sq ft/gal: 301 × 2 ÷ 350 = 1.72 gallons
- Buy 2 gallons (1 gallon + 3 quarts costs more than 2 gallons in most stores)
How much paint for common rooms?
| Room (8 ft walls) | Wall area* | Gallons, 2 coats (350 sq ft/gal) |
|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.5 → buy 2 |
| 12 × 12 ft | 333 sq ft | 1.9 → buy 2 |
| 12 × 16 ft | 397 sq ft | 2.3 → buy 2 gal + 2 qt |
| 15 × 20 ft | 509 sq ft | 2.9 → buy 3 |
*Less one door and two windows.
What changes how much paint you need
- Surface: new drywall, bare wood and masonry soak up paint. Prime first — primer is cheaper and seals the surface so the top coat covers as rated.
- Color change: going from dark to light (or to a deep, saturated color) often needs a tinted primer or a third coat.
- Texture: knockdown, popcorn and rough plaster increase the real surface area; reduce the coverage figure by 10–20%.
- Application: rollers with thicker naps and sprayers use more paint than the label assumes.
Keep a little extra. A leftover quart from the same batch is the easiest way to touch up scuffs later without a visible patch.
Frequently asked questions
How many square feet does a gallon of paint cover?
Manufacturers typically quote about 250 to 400 square feet per gallon per coat, depending on the product and surface. Use the figure on your can.
How much paint for a 12 × 12 room?
With 8 ft walls, one door and two windows, about 333 sq ft of wall — roughly 1.9 gallons for two coats. Buy 2 gallons; add about 0.8 gallons if you also paint the ceiling with two coats.
Do I need two coats?
Usually. Even "one-coat" paints often need a second coat for an even finish, especially over patches, fresh drywall or a color change.
Should I subtract doors and windows?
For accuracy, yes — each door is around 20 sq ft. For small rooms you can skip it and treat the difference as your extra for touch-ups.
Sources
- Behr, How much paint do I need? — coverage of about 250–400 sq ft per gallon per coat.