How to use the paint calculator
- Enter the room length, width and wall height, or switch to Wall area.
- Tick Paint the ceiling too if you are using the same paint. If the ceiling uses a different paint, calculate it separately with “Wall area”.
- Enter the number of doors and windows.
- Choose the number of coats (usually 2) and the coverage printed on the can.
The calculator gives the paint in gallons or litres and suggests the cans or tins to buy — it chooses a mix of sizes that covers what you need without buying lots of small, expensive cans.
Paint formula
Wall area = 2 × (Length + Width) × Height Paint area = Walls + Ceiling − Doors − Windows Paint = Paint area × Coats ÷ Coverage per gallon (or per litre)
Worked example
A 12 × 10 ft bedroom with 8 ft walls, one door and two windows, two coats, paint covering 350 ft² per gallon:
- Walls = 2 × (12 + 10) × 8 = 352 ft²
- Minus door (21) and windows (2 × 15) = 301 ft²
- Paint = 301 × 2 ÷ 350 = 1.72 gallons → buy 2 gallons
Paint coverage guide
| Surface | US (per gallon) | Metric (per litre) |
|---|---|---|
| Smooth, previously painted wall | 350–400 ft² | 10–12 m² |
| New drywall / plaster (after primer) | 300–350 ft² | 8–10 m² |
| Textured or rough plaster | 250–300 ft² | 6–8 m² |
| Brick or concrete block | 150–250 ft² | 4–6 m² |
| Exterior wood siding | 250–350 ft² | 6–9 m² |
Painting tips that save paint and money
- Buy one batch. Tinted paint can vary slightly between mixes; buy everything at once or box (mix) the cans together in a bucket.
- Trim and doors use a different finish. As a rough guide, one quart covers the trim and one door on both sides in an average room.
- Dark colours over light ones (or the reverse) may need a third coat or a tinted primer.
- Keep leftovers labelled with the room name for touch-ups.
- Roller nap matters: a 3/8 in (10 mm) nap for smooth walls, 1/2–3/4 in (12–20 mm) for textured surfaces, which also use more paint.
Painting the outside of a house
For exterior walls, measure the length of each wall and its height to the eaves. For gable ends, add the triangle: ½ × width × height of the gable. Add all the walls together and use the “Wall area” option. Subtract garage doors and large windows, but not small ones. Exterior surfaces are rougher and more absorbent than interior walls, so use the lower coverage figure from the table and plan for two coats. If the old paint is chalky or peeling, budget for washing, scraping and a coat of primer first — skipping preparation is the main reason exterior paint fails early.