How does the paint calculator work?
The calculator measures the wall area, removes the doors and windows, multiplies by the number of coats, and divides by how much a gallon covers. The formula is gallons = (wall area − openings) × coats ÷ coverage, where wall area is 2 × (length + width) × height. A 12 × 12 ft room with 8 ft walls, two coats, one door, and one window needs about 2 gallons.
How much does a gallon of paint cover?
One gallon of interior paint covers about 350 to 400 square feet in a single coat on a smooth wall. Rough, textured, or porous surfaces soak up more, so they cover closer to 250–300 sq ft per gallon. The calculator uses 350 sq ft by default; lower it under Advanced for textured walls.
How many coats of paint do I need?
Plan on two coats for most jobs — it gives even color and full coverage. You can get away with one coat when repainting the same color, and you may need a third coat (or a tinted primer) when a deep or bright color goes over a light wall, or a light color goes over a dark one.
How to figure out paint by hand
- 1
Measure the room. Measure the length and width of the floor and the height of the walls, in feet.
- 2
Find the wall area. Add the length and width, double it, and multiply by the height: 2 × (length + width) × height. A 12 × 12 room with 8 ft walls is 2 × 24 × 8 = 384 sq ft.
- 3
Subtract doors and windows. Take off about 20 sq ft for each door and 15 sq ft for each window. One door and one window leaves 384 − 35 = 349 sq ft.
- 4
Multiply by the number of coats. Most jobs need two coats, so double the area: 349 × 2 = 698 sq ft to cover.
- 5
Divide by the coverage and round up. A gallon covers about 350 sq ft, so 698 ÷ 350 = 2 gallons. Always round up.
Paint reference charts
Quick-reference tables for sizing a paint job.
How much paint by room size (two coats)
Gallons needed for the walls of a room with 8 ft ceilings, two coats, one door and one window, at 350 sq ft per gallon. Round up — buy a little extra for touch-ups.
| Room size | Wall area | Paint (2 coats) |
|---|---|---|
| 8 × 10 ft | 288 sq ft | 2 gallons |
| 10 × 10 ft | 320 sq ft | 2 gallons |
| 10 × 12 ft | 352 sq ft | 2 gallons |
| 12 × 12 ft | 384 sq ft | 2 gallons |
| 12 × 14 ft | 416 sq ft | 3 gallons |
| 12 × 16 ft | 448 sq ft | 3 gallons |
| 14 × 16 ft | 480 sq ft | 3 gallons |
| 16 × 20 ft | 576 sq ft | 4 gallons |
| 20 × 20 ft | 640 sq ft | 4 gallons |
Source: Socko calculation: (wall area − openings) × coats ÷ 350 sq ft per gallon.
How far a gallon of paint goes
A gallon covers more on a smooth, sealed wall and less on rough or thirsty surfaces. Lower the coverage in the calculator’s Advanced setting to match.
| Surface | Coverage per gallon |
|---|---|
| Smooth, primed or previously painted drywall | 350–400 sq ft |
| Bare or new drywall (more absorbent) | 250–350 sq ft |
| Rough or textured walls | 250–300 sq ft |
| Smooth wood, trim, and doors | 350–400 sq ft |
How many coats you need
Use this to set the “coats” field. Two coats is the safe default for most rooms.
| Paint job | Coats | Primer? |
|---|---|---|
| Repainting the same or similar color | 1–2 | No |
| Standard color change on a painted wall | 2 | Usually no |
| Light color over a dark wall | 2–3 | Yes (tinted) |
| Dark or bright color over a light wall | 2–3 | Yes (tinted) |
| Bare drywall, wood, or patched spots | 2 | Yes |
Source: Socko reference, based on manufacturer coverage and finish guidance.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons. A 12 × 12 room with 8 ft walls is ~384 sq ft of wall (~349 after a door and window). Two coats is ~698 sq ft, and a gallon covers ~350 sq ft, so you need 2 gallons.
How much does a gallon of paint cover?
About 350–400 sq ft per coat on a smooth wall, and closer to 250–300 sq ft on textured or porous surfaces.
How many coats of paint do I need?
Two coats for most jobs. One coat if you are repainting the same color; three coats (or a tinted primer) for a big color change.
Do I need primer?
Prime bare drywall, wood, patches, stains, and big color changes. Over a sound painted wall, a paint-and-primer usually makes a separate primer unnecessary.
Should I subtract doors and windows?
Yes — it removes ~20 sq ft per door and ~15 sq ft per window. With only a small opening or two the difference is minor, and leaving them in just gives you a bit of spare paint.
How much paint do I need for one wall?
Multiply width × height, then × coats ÷ 350. A 12 × 8 ft wall is 96 sq ft — one coat is about a quart (~90 sq ft), and two coats (192 sq ft) still fit in one gallon.
How much extra paint should I buy?
Round up and keep about 10% extra (a quart is plenty) for touch-ups and a future exact-color match.
This tool is for estimation and education, not financial advice. See our methodology for how these figures are calculated and sourced.