Quick answer: Paint needed = (wall area − doors − windows) × coats ÷ coverage per gallon. A 12 × 10 ft room with 8 ft walls, one door, two windows and two coats needs 302 sq ft × 2 ÷ 350 = 1.73 gallons, so buy 2 gallons. The calculator adds 10% spare for touch-ups.
Buying too little paint means a second trip and a risk of slightly different batches. Buying too much wastes money. This calculator works out the wall area of a room, takes off the doors and windows, multiplies by the number of coats and divides by what one gallon or liter covers.
Enter the room size in feet or meters, and it shows how much paint you need, plus a suggested amount to buy that includes a 10% spare for touch-ups and uneven walls.
How the amount is worked out
Paintable area = wall area − doors − windows (+ ceiling)
Paint needed = paintable area × coats ÷ coverage per gallon or liter
A standard door is counted as 20 sq ft (1.85 m²) and a window as 15 sq ft (1.4 m²). If yours are much bigger, add a few extra doors or windows. The ceiling area is length × width.
Tips
- Two coats is normal for a color change. One coat may do when repainting the same color.
- Rough, porous or freshly plastered walls soak up more, and a primer coat helps. Dark colors over light ones may need more coats.
- Coverage on the can is for smooth, sealed surfaces. Treat it as the best case.
- Paint is sold in fixed sizes, so round up to the next size and keep the leftover for touch-ups.
A worked example
Example. A room 12 ft × 10 ft with 8 ft walls has 2 × (12 + 10) × 8 = 352 sq ft of wall. Take off one door (20 sq ft) and two windows (30 sq ft) and 302 sq ft needs paint. Two coats is 604 sq ft. At 350 sq ft per gallon that is 1.73 gallons, so buying 2 gallons covers it with a little spare. Painting the ceiling as well raises it to 2.41 gallons, so buy 2.75.
Paint for common room sizes
| Room | Area to paint | Paint needed (2 coats) | Suggested to buy |
|---|---|---|---|
| 10 × 10 ft, 8 ft walls | 270 sq ft | 1.54 gallons | 1.75 gallons |
| 12 × 10 ft, 8 ft walls | 302 sq ft | 1.73 gallons | 2 gallons |
| 15 × 12 ft, 8 ft walls | 382 sq ft | 2.18 gallons | 2.5 gallons |
| 20 × 15 ft, 9 ft walls | 580 sq ft | 3.31 gallons | 3.75 gallons |
Each room has one door (20 sq ft) and two windows (15 sq ft each), walls only, at 350 sq ft per gallon, with 10% spare.
Common mistakes to avoid
- Forgetting the number of coats. A color change usually needs two, and dark over light can need more.
- Trusting the coverage on the can for rough or porous walls. They soak up more paint, so allow extra or use a primer.
- Buying exactly what you need. Paint batches vary slightly in color, so keep a little spare for touch-ups and buy it all at once.
Frequently asked questions
How much paint do I need for a 12 × 10 room?
With 8 ft walls, one door and two windows, two coats need about 1.7 gallons, so buy 2 gallons. Painting the ceiling as well brings it to about 2.4 gallons.
How much area does a gallon of paint cover?
About 350 square feet (roughly 32 m²) on smooth walls. In metric terms that is about 8 to 10 m² per liter. Check the label of your paint, because some cover more and some less.
Do I need to subtract windows and doors?
It gives a more accurate result and saves money on large rooms. For small openings, ignoring them adds a safety margin. This calculator subtracts them for you.
Should I buy extra paint?
Yes, around 10%. It covers touch-ups, uneven surfaces and mistakes, and lets you match the color later. The suggested amount here includes that spare.
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Sources and further reading
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