How Much Paint Do You Need for a Room? A Step-by-Step Method
Buying paint by eye usually ends one of two ways: a last-minute trip for another gallon that doesn’t quite match, or three half-used cans in the basement. A five-minute calculation avoids both. You need the wall area, the number of coats, and the coverage rate printed on the can.
Step 1: Measure the walls
Measure the length of each wall and the ceiling height in feet. For a rectangular room:
Gross wall area = 2 × (length + width) × height
For rooms that aren’t rectangles, measure each wall separately and add them up. Include closet interiors if you are painting them.
Step 2: Subtract doors and windows
Measure your actual openings if you can. If not, these standard sizes are reasonable placeholders:
| Opening | Typical size | Area to subtract |
|---|---|---|
| Interior door | 3 × 7 ft (0.9 × 2.1 m) | 21 ft² (about 2.0 m²) |
| Standard window | 3 × 5 ft (0.9 × 1.5 m) | 15 ft² (about 1.4 m²) |
| Sliding patio door | 6 × 7 ft (1.8 × 2.1 m) | 42 ft² (about 3.9 m²) |
Small openings matter less than you think; don’t bother subtracting outlets or light switches. Large openings like a double window or a wide archway are worth measuring.
Net wall area = gross area − openings
Step 3: Decide how many coats
Two coats is the standard recommendation for an even finish on walls, especially when changing color. One coat can be enough when repainting the same color over a sound surface. New drywall or a big color change usually needs primer first. Our guide to primer and number of coats covers the decision in detail.
Paintable area = net wall area × number of coats
Step 4: Divide by the coverage rate
Most interior wall paints list coverage of roughly 350–400 ft² per gallon per coat on smooth, previously painted surfaces. Check the label of the paint you plan to buy; some list lower figures, and rough or porous surfaces always use more paint.
Gallons = paintable area ÷ coverage per gallon
Use the lower end of the coverage range (350) unless your walls are smooth and already painted a similar color. It is the safer assumption.
In metric, 350–400 ft² per US gallon works out to about 8.6–9.8 m² per litre.
Step 5: Round to cans you can buy
Paint usually comes in quarts (¼ gallon), gallons, and 5-gallon buckets. Round up to the nearest combination:
- 1.6 gallons → 2 gallons
- 2.1 gallons → 2 gallons + 1 quart, or 3 gallons if you want touch-up paint left over
- 4.5 gallons or more → a 5-gallon bucket is often more economical than individual gallons
Worked example: a 12 × 14 ft bedroom
Room: 12 × 14 ft, 8 ft ceiling, one door, two windows, two coats.
- Gross wall area: 2 × (12 + 14) × 8 = 416 ft²
- Openings: door 21 ft² + two windows 30 ft² = 51 ft²
- Net wall area: 416 − 51 = 365 ft²
- Two coats: 365 × 2 = 730 ft²
- Gallons at 350 ft²/gal: 730 ÷ 350 = 2.09 gallons
- Gallons at 400 ft²/gal: 730 ÷ 400 = 1.83 gallons
Buy: 2 gallons plus a quart to be safe. If the walls are smooth and you’re staying close to the existing color, 2 gallons will likely cover it.
Metric version: 3.66 × 4.27 m room, 2.44 m high. Gross area 38.7 m², minus 1.95 m² door and 2.79 m² windows = 33.9 m². Two coats = 67.9 m². At 8.6 m²/L that is 7.9 L, a little more than 2 US gallons (7.57 L).
Quick reference: two coats of wall paint
Assumes one door and two windows deducted, 350–400 ft² per gallon.
| Room size | 8 ft ceiling: net wall area | Gallons (2 coats) | 9 ft ceiling: net wall area | Gallons (2 coats) |
|---|---|---|---|---|
| 10 × 10 ft | 269 ft² (25.0 m²) | 1.3–1.5 | 309 ft² (28.7 m²) | 1.5–1.8 |
| 10 × 12 ft | 301 ft² (28.0 m²) | 1.5–1.7 | 345 ft² (32.1 m²) | 1.7–2.0 |
| 12 × 12 ft | 333 ft² (30.9 m²) | 1.7–1.9 | 381 ft² (35.4 m²) | 1.9–2.2 |
| 12 × 14 ft | 365 ft² (33.9 m²) | 1.8–2.1 | 417 ft² (38.7 m²) | 2.1–2.4 |
| 14 × 16 ft | 429 ft² (39.9 m²) | 2.1–2.5 | 489 ft² (45.4 m²) | 2.4–2.8 |
| 16 × 20 ft | 525 ft² (48.8 m²) | 2.6–3.0 | 597 ft² (55.5 m²) | 3.0–3.4 |
This table covers walls only. Ceilings and trim are separate products and separate calculations; see estimating ceiling and trim paint.
Adjust for your surface
The label’s coverage rate assumes a smooth, sealed surface. Expect to use more paint when:
- Walls are textured (orange peel, knockdown, or heavy texture). Texture increases the real surface area.
- The surface is porous, such as new drywall, bare plaster, or unpainted wood. Prime first.
- You’re covering a dark or strong color with a light one. Expect an extra coat even with primer.
- You’re using a roller with a thick nap. It holds and applies more paint per pass.
Common mistakes
- Using floor area instead of wall area. A 12 × 14 room has 168 ft² of floor but 416 ft² of gross wall.
- Using the best-case coverage number. Labels give a range; plan on the low end.
- Forgetting the second coat. Most color changes need two.
- Buying from different batches without mixing. For a large job, pour all gallons of the same color into one bucket and stir (“boxing”) so the color is consistent from wall to wall.
- Throwing out the leftovers. Keep a labeled, sealed quart for touch-ups.
Calculate your room
The paint calculator does these steps for you: enter the room dimensions, doors, windows and coats, and it returns square footage and gallons in US and metric units.
FAQ
How much paint do I need for a 12 × 12 room?
With 8 ft ceilings, one door and two windows, the walls are about 333 ft². Two coats need roughly 1.7–1.9 gallons, so 2 gallons is usually enough.
Does one gallon of paint cover a room?
One gallon covers about 350–400 ft² in one coat. That is enough for one coat in a small room, but most rooms need two coats, so plan on at least 2 gallons for a typical bedroom.
Should I subtract doors and windows when calculating paint?
Yes, for standard doors and windows. Each door is about 21 ft² and each window about 15 ft², which adds up across a room. Skip small items like outlets.
Is it better to buy too much or too little paint?
A small surplus is better. Leftover paint is useful for touch-ups, and a second can bought later may vary slightly in color.