How this calculator works
The paintable area is the four walls (the perimeter × the ceiling height) minus the doors and windows. Multiply by the number of coats and divide by how much area a gallon covers, usually about 350 ft². The ceiling is length × width, calculated the same way.
Worked example: a 12 × 12 ft room with 8 ft ceilings has 384 ft² of wall. One door and two windows take away 51 ft², which leaves 333 ft². Two coats at 350 ft² per gallon is 333 × 2 ÷ 350 = 1.90 gallons, so buy 2 gallons. The 144 ft² ceiling takes another 0.82 gallons for two coats.
Paint needed for common room sizes (two coats of walls)
Assumes 8 ft ceilings, one door, two windows and 350 ft² per gallon.
| Room size | Wall gallons | Ceiling gallons |
|---|---|---|
| 10 × 10 ft | 1.54 | 0.57 |
| 10 × 12 ft | 1.72 | 0.69 |
| 12 × 12 ft | 1.90 | 0.82 |
| 12 × 14 ft | 2.09 | 0.96 |
| 12 × 16 ft | 2.27 | 1.10 |
| 14 × 16 ft | 2.45 | 1.28 |
| 16 × 20 ft | 3.00 | 1.83 |
Tips before you buy
- Use primer on bare drywall, stains, patches and big colour changes. It is cheaper than extra coats of finish paint, and a tinted primer helps when going from dark to light.
- Rough or textured walls soak up 20 to 30% more paint. Choose the 250 ft² or 300 ft² option in the calculator.
- Buy all your paint for a room at once and mix the cans together ("boxing") so the colour is consistent.
- Buy a bit more than you need for touch-ups and keep the leftover, labelled with the room and the date.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How many gallons of paint do I need for a 12x12 room?
About 2 gallons for the walls with two coats, and another gallon for the ceiling. A 12 × 12 ft room with 8 ft ceilings has roughly 330 to 380 ft² of walls.
How much does a gallon of paint cover?
About 350 square feet per coat on smooth, previously painted walls. Textured walls and dark colours cover less.
Do I need two coats of paint?
Almost always. One coat rarely gives even coverage. Use a primer or a paint-and-primer in one when changing from dark to light.
Should I deduct doors and windows?
Yes, usually. A standard door is about 21 ft² and a typical window about 15 ft². Small windows are not worth deducting, because they are offset by trim and touch-ups.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.