Paint Calculator: How Many Gallons Do I Need?
Enter your room dimensions and the calculator subtracts doors and windows, then tells you how many gallons of wall paint, ceiling paint, trim paint and primer to buy.
A 12 × 12 ft room with 8-foot ceilings needs about 2 gallons of paint for two coats on the walls (about 333 sq ft after one door and two windows, at 350 sq ft per gallon), plus just under 1 more gallon if you paint the ceiling.
Your numbers
Result
How to calculate how much paint you need
Paint is sold by the gallon, and one gallon typically covers 300–400 square feet per coat. To estimate how much to buy, find the wall area, subtract the openings you won't paint, then multiply by the number of coats.
paintable area = wall area − doors (21 sq ft each) − windows (15 sq ft each)
gallons = paintable area × coats ÷ coverage per gallon
If you are painting the ceiling, add length × width to the paintable area. Ceilings normally use a separate flat paint, so buy that in its own can.
Gallons of paint by room size (2 coats, 8 ft ceiling, 1 door, 2 windows)
| Room size | Wall area | Gallons (walls) | Add for ceiling |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.5 | +0.6 |
| 10 × 12 ft | 301 sq ft | 1.7 | +0.7 |
| 12 × 12 ft | 333 sq ft | 1.9 | +0.8 |
| 12 × 15 ft | 381 sq ft | 2.2 | +1.0 |
| 14 × 16 ft | 429 sq ft | 2.5 | +1.3 |
| 15 × 20 ft | 509 sq ft | 2.9 | +1.7 |
| 20 × 20 ft | 589 sq ft | 3.4 | +2.3 |
How many coats?
Two coats of quality paint give an even, durable finish. One coat is only enough when repainting a similar color with a paint-and-primer product. Switching from dark to light, or painting new drywall, usually needs a primer first.
Choosing a sheen
| Sheen | Best for |
|---|---|
| Flat / matte | Ceilings, low-traffic walls (hides flaws) |
| Eggshell / satin | Living rooms, bedrooms, hallways |
| Semi-gloss | Kitchens, bathrooms, trim and doors |
Tips for buying paint
- Buy all the paint for a room at once and mix cans together ("boxing") so color is consistent.
- Keep leftover paint for touch-ups; label the can with the room and date.
- Rough or textured surfaces absorb up to 20% more paint, so lower the coverage number to about 250–300.
- For exterior work, use the siding area and a lower coverage figure, and see the siding calculator for wall area.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area before openings. Subtracting one door and two windows and using two coats, you need about 2 gallons for the walls.
How many square feet does a gallon of paint cover?
A gallon of paint usually covers 300–400 square feet per coat; 350 is a common planning figure.
How much paint do I need for a ceiling?
A 12×12 ceiling is 144 sq ft; two coats need 288 sq ft of coverage — a little under one gallon.
Do I subtract windows and doors?
Yes, subtracting them avoids overbuying. This calculator uses 21 sq ft per door and 15 sq ft per window. Skip the deduction if you plan to paint the trim in the same color.
How many gallons of paint to paint a house interior?
A typical 1,500 sq ft home with about 4,000 sq ft of wall area needs roughly 20–25 gallons for two coats, plus ceilings and trim.
Do I need primer?
Use primer on new drywall, stains, patched areas and big color changes. Primer coverage is similar to paint — about 300–400 sq ft per gallon.
How much extra paint should I buy?
Round up to the next whole gallon, which gives a natural touch-up margin. Buy extra if you're painting bare drywall or textured walls.
Is a quart enough for trim?
A quart (about 100 sq ft per coat) usually covers the baseboards, door frames and window trim of one average room.
Sources and further reading
- Lead-Based Paint Renovation, Repair and Painting Program rules — U.S. Environmental Protection Agency (homes built before 1978)
- Volatile organic compounds' impact on indoor air quality — U.S. Environmental Protection Agency
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.