Start with the coverage on the can
Every paint lists a spread rate in its technical data sheet and on the label. Sherwin-Williams says its interior paints typically cover 350–400 ft² per gallon. Benjamin Moore's Regal Select data sheet lists 400–450 ft² per gallon. The calculator defaults to 350 ft², the conservative end, because real walls are rarely as smooth as the lab. Lower it for:
- Textured, rough or porous surfaces (stucco, brick, new unprimed drywall)
- Dramatic colour changes, especially light over dark
- Rolling with a thick-nap roller, which holds more paint
The formula
Wall area = 2 × (length + width) × height − openings. The calculator uses about 21 ft² per door and 15 ft² per window. Add length × width if you're painting the ceiling. Gallons = area × coats ÷ coverage. For a 12 × 12 ft room with 8 ft walls, one door and two windows: 384 − 21 − 30 = 333 ft² of wall. Two coats at 350 ft² per gallon is 1.9 gal, so buy 2 gallons.
Paint for common rooms
| Room | Wall area | Paint (2 coats) | With ceiling (2 coats) |
|---|---|---|---|
| 10 × 10 ft | 269 ft² | 1.54 gal | 2.11 gal |
| 10 × 12 ft | 301 ft² | 1.72 gal | 2.41 gal |
| 12 × 12 ft | 333 ft² | 1.9 gal | 2.73 gal |
| 12 × 14 ft | 365 ft² | 2.09 gal | 3.05 gal |
| 14 × 16 ft | 429 ft² | 2.45 gal | 3.73 gal |
| 16 × 20 ft | 525 ft² | 3 gal | 4.83 gal |
| Room | Wall area | Paint (2 coats) | With ceiling (2 coats) |
|---|---|---|---|
| 3 × 3 m | 24.1 m² | 4.8 L | 6.6 L |
| 3 × 3.6 m | 27 m² | 5.4 L | 7.6 L |
| 3.6 × 3.6 m | 29.8 m² | 6 L | 8.6 L |
| 3.6 × 4.2 m | 32.7 m² | 6.5 L | 9.6 L |
| 4.2 × 4.8 m | 38.5 m² | 7.7 L | 11.7 L |
| 4.8 × 6 m | 47.1 m² | 9.4 L | 15.2 L |
Ceilings are often painted with a dedicated ceiling paint. If yours differs from the wall paint, work it out separately as length × width × coats ÷ that paint's coverage.
Do you need primer?
Primer is worth it on new drywall and patches, on stained or glossy surfaces, and for big colour changes. USG's finishing guide includes a primer row in its estimating table at 400 ft² per gallon for new drywall. Many "paint and primer" products cover repainting over a sound, similar colour without a separate primer. The calculator shows one coat of primer at your coverage rate, which you can ignore if you don't need it.
Gallons, quarts and five-gallon buckets
Paint is sold in quarts (¼ gallon), gallons and 5-gallon pails. The calculator rounds up to the next quart. In practice:
- Under about 1 gallon: a gallon is often cheaper than three quarts, so check the prices.
- 3+ gallons of one colour: a 5-gallon pail usually costs less per gallon. Mix ("box") the gallons together if you buy several, so the colour is uniform.
- Keep a little left over for touch-ups, labelled with the room and the date.
Worked examples
Small bathroom, 8 × 5 ft, 8 ft walls, 1 door, no windowSmall bathroom, 2.44 × 1.52 m, 2.44 m walls, 1 door, no window
Room length 8 ft · Room width 5 ft · Wall height 8 ft · Doors 1 · Windows 0 · Coats 2 · Coverage per gallon 350 ft²/gal · Paint ceiling too No · Door size 21 ft² · Window size 15 ft²Room length 2.44 m · Room width 1.52 m · Wall height 2.44 m · Doors 1 · Windows 0 · Coats 2 · Coverage per litre 8.59 m²/L · Paint ceiling too No · Door size 1.95 m² · Window size 1.39 m²
- Paint needed: 1.07 galPaint needed: 4 L
- Wall area (net): 187 ft²Wall area (net): 17.4 m²
- Buy (rounded up to the quart): 1.25 galBuy (rounded up to the litre): 5 L
Living room, 18 × 14 ft, 9 ft walls, 2 doors, 3 windows, ceiling tooLiving room, 5.49 × 4.27 m, 2.74 m walls, 2 doors, 3 windows, ceiling too
Room length 18 ft · Room width 14 ft · Wall height 9 ft · Doors 2 · Windows 3 · Coats 2 · Coverage per gallon 350 ft²/gal · Paint ceiling too Yes · Door size 21 ft² · Window size 15 ft²Room length 5.49 m · Room width 4.27 m · Wall height 2.74 m · Doors 2 · Windows 3 · Coats 2 · Coverage per litre 8.59 m²/L · Paint ceiling too Yes · Door size 1.95 m² · Window size 1.39 m²
- Paint needed: 4.23 galPaint needed: 16 L
- Wall area (net): 489 ft²Wall area (net): 45.4 m²
- Ceiling area: 252 ft²Ceiling area: 23.4 m²
- Primer (1 coat): 2.12 galPrimer (1 coat): 8 L
- Buy (rounded up to the quart): 4.25 galBuy (rounded up to the litre): 17 L
Accent wall, 12 × 8 ft (entered as length 6 ft, width 0)Accent wall, 3.66 × 2.44 m (entered as length 1.83 m, width 0)
Room length 6 ft · Room width 0 ft · Wall height 8 ft · Doors 0 · Windows 0 · Coats 2 · Coverage per gallon 350 ft²/gal · Paint ceiling too No · Door size 21 ft² · Window size 15 ft²Room length 1.83 m · Room width 0 m · Wall height 2.44 m · Doors 0 · Windows 0 · Coats 2 · Coverage per litre 8.59 m²/L · Paint ceiling too No · Door size 1.95 m² · Window size 1.39 m²
- Paint needed: 0.55 galPaint needed: 2.1 L
- Wall area (net): 96 ft²Wall area (net): 8.9 m²
For one wall, enter half its width as the length and 0 as the width. The formula doubles the length.
Trim, doors and other surfaces
Doors and trim are usually a different sheen (semi-gloss) and are bought separately. A rough budget: a quart covers the trim in a typical bedroom, and a standard door takes very little paint per side (approximate). For fences and decks, use the area of the surface (both sides of a fence) and the stain's coverage rate, which can be much lower than paint on rough wood. The results suggest a mix of cans (5 gal, 1 gal and quart in imperial; 10, 5, 2.5 and 1 L tins in metric) that covers the paint needed with the least leftover, with a small allowance against buying many small cans. With the ceiling switched on, the wall and ceiling paint are listed separately, since ceiling paint is usually a different product.
Common mistakes
- Using the best-case coverage from the label on a textured or porous wall.
- Planning one coat for a colour change. Two coats is the realistic plan.
- Forgetting closets, which are often painted with the room.
- Buying the exact amount in separate cans without mixing them together. Slight tint differences show in raking light.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 12 × 12 × 8 ft room, 1 door, 2 windows, 2 coats at 350 ft²/gal | 1.9 gal | Sherwin-Williams paint calculator FAQ (coverage) | Uses the published coverage |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 3.66 × 3.66 × 2.44 m room, 1 door, 2 windows, 2 coats at 32.5 m²/gal | 7.2 L | Sherwin-Williams paint calculator FAQ (coverage) | Uses the published coverage |
Frequently asked questions
How much paint for a 12 × 12 room?
About 1.9 gallons for two coats on the walls (333 ft² net), so buy 2 gallons. Add about 0.8 gallon for two coats on the ceiling.
How much does a gallon of paint cover?
Typically 350–400 ft² per coat (Sherwin-Williams). Some products list 400–450. Check the can.
How much paint for one wall?
Width × height × coats ÷ coverage. A 12 × 8 ft wall with two coats needs 192 ÷ 350 ≈ 0.55 gallons, so a gallon (or 2–3 quarts).
How much paint for a bedroom?
Most bedrooms between 10 × 10 and 12 × 14 ft need about 1.5–2.5 gallons for two coats on the walls.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- Sherwin-Williams: paint calculator FAQ (coverage per gallon)
- Benjamin Moore Regal Select technical data sheet (spread rate)
- USG Sheetrock Brand Installation and Finishing Guide (J371): fastener, compound and tape estimating table
- Behr Premium Plus Interior technical data sheet (coverage 250–400 sq ft per gallon; primer for big colour changes)
- Dulux Trade Vinyl Matt product data sheet (spreading rate up to 17 m² per litre)
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.