Paint calculator
This is arithmetic on the area, coats and coverage you enter. Real coverage varies with the surface and the painter, so treat it as a buying estimate — it is usually better to have a little left over than to run out mid-wall.
Enter the wall size, how many walls, the coats and the coverage from the tin, and the tool works out the volume of paint and rounds it up to whole cans. Coverage and can size are editable because they change with the product and the surface.
Exemple détaillé
Four 12 ft × 8 ft walls are 384 square feet. Two coats is 768 square feet of painting. At 350 ft² per gallon that is 768 ÷ 350 ≈ 2.2 gallons. In 1-gallon cans you would buy 3 and keep the rest for touch-ups.
In metric: four 3.6 m × 2.4 m walls are 34.56 m², and two coats is 69.12 m² of painting. At 11 m² per litre that is 69.12 ÷ 11 ≈ 6.3 litres — in 4 L cans, buy 2.
Sur le chantier
Pricing a whole-floor repaint. Three 12 × 12 bedrooms, an 18 × 14 living room and a 20 × 4 hall, all 8 ft walls, all the same colour. Perimeters: 48, 48, 48, 64 and 48 ft — 256 ft of wall, × 8 = 2,048 ft². Two coats at 350 ft²/gal is 4,096 ÷ 350 = 11.7 gallons. That is 12 singles or three 5-gallon pails (15 gal); price both, because the pail is usually cheaper per gallon and the three gallons left over are the touch-up kit for the next two years.
Ceilings are their own takeoff. Same rooms: 144 × 3 + 252 + 80 = 764 ft² of ceiling. In flat ceiling white at two coats that is 1,528 ÷ 350 = 4.4 → 5 gallons. Run it separately — different paint, different can, and lumping it into the wall figure hides which one you are short of on the day.
The can's coverage is for a smooth wall. Rough-sawn siding, stucco, a heavily textured ceiling — the low end of the range on the label, or lower. If the can says 250–400 and the surface is rough, type 250. On 1,800 ft² of siding at two coats the difference between 350 and 250 is 10.3 versus 14.4 gallons: four cans, and a trip you would rather not make with the second coat half on. In metric the same siding is about 170 m²; at two coats, 8.6 versus 6.1 m² per litre is 39.5 versus 55.7 litres.
What went wrong?
Quick referencefor paint
- Wall area of a room
- perimeter × height — openings off if they are big
- A US gallon
- 3.785 L · 4 quarts · an imperial gallon is 4.546 L
- 350 ft² per gallon
- ≈ 8.6 m² per litre (ft²/gal × 0.0245 = m²/L)
- Gallons for 1,000 ft², two coats
- at 350: 5.7 · at 400: 5.0 · at 250: 8.0
- Litres for 100 m², two coats
- at 11 m²/L: 18.2 · at 14: 14.3 · at 8.6: 23.3
- A 5-gallon pail
- covers about 1,750 ft² one coat at 350 — a whole floor of walls
- Primer
- its own coverage on its own can; run it as a separate coat count
Waste & overagewhat to add, and why
- Painters buy about 10% over the count because the first coat on new drywall soaks in and the roller tray never empties — and a can short at 9 pm means a colour that does not quite match from the next batch.
- Round to whole cans; the buy line does that, but if the count lands on 4.1 gallons, ask whether a 5-gallon pail is cheaper than five singles.
- Textured or rough surfaces cover 20–30% less than the tin says — drop the coverage figure, do not raise the coats.
Mistakes to catchbefore you order
- Typing the floor area as the wall area — walls are perimeter × height, and the tool's walls field wants each wall's length and height.
- Forgetting the ceiling, or including it with wall paint when it needs its own product; run the ceiling separately.
- Reading the coverage per gallon off the tin and typing it against a litre can — the tool has a can-size field; set it to what you are buying.
La formule
paint = (area × coats) ÷ coverage · cans = paint ÷ can size, rounded up
Coverage is stated per unit of paint, so dividing the painted area by it gives the volume directly. Multiply the area by the coats first, since each coat is a full pass over the wall. Rounding up to whole cans is what you actually carry to the till.
Questions
- How much paint do I need for a room?
- Add up the wall area, multiply by the number of coats, and divide by the coverage on the tin. A gallon typically covers around 350 square feet per coat, so a 400 sq ft room in two coats needs about 800 ÷ 350 ≈ 2.3 gallons. In metric, at 11 m² per litre, 40 m² in two coats is 80 ÷ 11 ≈ 7.3 litres. This tool does that and rounds up to whole cans.
- How many coats should I plan for?
- Two is the honest default and what this tool assumes unless you change it. One coat rarely covers evenly, especially over a colour change or fresh drywall. A big colour shift or bare, thirsty surfaces can want a primer coat on top of that, so add it to the coat count.
- What coverage should I use?
- The tin states it, and it is the number that most affects the answer, so it is an editable field here. Smooth, sealed walls give the most coverage; rough, porous or textured surfaces drink paint and cut it down. When in doubt, use the lower end of the range on the can.
- Should I subtract doors and windows?
- For a rough estimate you can leave them in — the extra paint covers touch-ups and a second dip of the roller. If you are counting pennies on an expensive paint, subtract the big openings from the area you enter and you will buy a can or two less.
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