How much paint do I need?
Wall area = 2 × (length + width) × wall height − doors − windows
Paint (gallons) = wall area × coats ÷ coverage per gallon
Most interior wall paints state a coverage of about 350 to 400 square feet per gallon for one coat on a smooth, primed surface (roughly 8.5 to 10 m² per litre). The calculator uses 350 by default, which leaves a little margin. Rough, porous or textured walls, fresh drywall and strong colour changes all absorb more paint, so use a lower coverage figure for them.
Worked example: a 14 × 12 ft bedroom
The walls are 8 ft high, with one door and two windows, and you want two coats.
- Perimeter: 2 × (14 + 12) = 52 ft. Gross wall area: 52 × 8 = 416 sq ft.
- Openings: one door (20 sq ft) and two windows (2 × 15 = 30 sq ft) = 50 sq ft. Net area: 416 − 50 = 366 sq ft.
- Two coats: 366 × 2 = 732 sq ft of coverage. At 350 sq ft per gallon: 732 ÷ 350 = 2.09 gallons.
So buy 2 gallons and 1 quart (2.25 gallons). Adding the 14 × 12 ft ceiling (168 sq ft, two coats) needs another 336 ÷ 350 = 0.96 gallons, so one more gallon of ceiling paint.
Standard sizes for doors and windows
The calculator subtracts 20 sq ft for each door (a typical interior door is about 3 × 7 ft) and 15 sq ft for each window (about 3 × 5 ft). For a large patio door or picture window, count it as two or three windows. Small openings barely change the result; many painters ignore them and keep the extra paint for touch-ups.
Gallons, quarts and litres
| Container | Volume | Covers (one coat, at 350 sq ft/gal) |
|---|---|---|
| 1 quart | 0.25 US gal (0.95 L) | about 87 sq ft (8 m²) |
| 1 gallon | 3.785 L | about 350 sq ft (33 m²) |
| 5-gallon bucket | 18.9 L | about 1,750 sq ft (163 m²) |
Once you need three or more quarts, a gallon usually costs less. Buying all of a colour at once, or mixing ("boxing") the cans together before you start, avoids small colour differences between batches.
How many coats?
- One coat: repainting the same or a very similar colour over sound paint.
- Two coats: the normal recommendation for an even finish, and for any colour change.
- Primer plus two coats: new drywall, patched walls, stains, or going from a dark colour to a light one. Primer coverage is similar to paint; work it out as a separate one-coat job.
Trim, doors and ceilings
Ceilings are usually painted with a flat ceiling paint, so keep them separate from the wall total; the calculator shows them as their own line. Trim and doors use a semi-gloss or gloss paint: one quart usually covers the baseboards, door frames and window casings of an average room.
Step by step: estimating paint for a room
- Measure the length of every wall at floor level and add them up for the perimeter.
- Measure the wall height from floor to ceiling.
- Count doors and windows; treat a double door or a large window as two.
- Read the coverage on the can, usually given per gallon or per litre.
- Decide on the number of coats and whether the ceiling and trim are part of the job.
Metric example
A room 4.2 × 3.6 m with walls 2.4 m high has 2 × (4.2 + 3.6) × 2.4 = 37.44 m² of wall. Subtract one door (1.9 m²) and two windows (2.8 m²) to get 32.7 m². Two coats at 8.6 m² per litre need 32.7 × 2 ÷ 8.6 = 7.6 litres, so two 5-litre tins or one 10-litre tin.
Common mistakes
- Using the floor area instead of the wall area. A 14 × 12 room has 168 sq ft of floor but 416 sq ft of wall.
- Forgetting the second coat, which doubles the paint.
- Trusting the best-case coverage on textured or unprimed walls. Use 250–300 sq ft per gallon for those.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8-ft walls the gross wall area is 2 × (12 + 12) × 8 = 384 sq ft. After one door and one window (35 sq ft) it is 349 sq ft, so two coats need about 698 ÷ 350 = 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover about 350–400 sq ft per gallon for one coat on a smooth, primed wall. Textured or porous surfaces can drop to 250–300 sq ft.
Should I subtract doors and windows?
Yes for an accurate estimate: roughly 20 sq ft per door and 15 sq ft per window. Small openings make little difference, and the leftover paint is useful for touch-ups.
How much paint for a ceiling?
Use the floor area: a 14 × 12 ft ceiling is 168 sq ft, so two coats need 336 ÷ 350 ≈ 1 gallon of ceiling paint.
How many litres of paint do I need for a room?
Multiply the wall area in square metres by the number of coats and divide by the coverage on the tin, often 8–10 m² per litre. A room with 32.7 m² of wall needs about 7.6 litres for two coats at 8.6 m² per litre.
Sources
- Paint manufacturers' product data sheets (e.g. Sherwin-Williams, Benjamin Moore, Behr): typical spread rate of 350–400 sq ft per gallon per coat for interior wall paint.
- NIST Handbook 44 Appendix C: US liquid gallon = 231 cubic inches = 3.785411784 litres.
Last reviewed 20 October 2026. Results are estimates; see our disclaimer.