Enter the room size or the total area. For example, a 12×10 ft room, 10 ft high, with 1 door and 2 windows needs ≈ 6.48 litres for 2 coats at 120 coverage.
Measure by
ft
ft
ft
Each door counts as 21 and each window 15 sq ft, left out
Paint the ceiling too?
sq ft
Add wall length × height, minus doors and windows
Coats
Printed on the can; usually 100–140
৳
Paint needed
6.48 litres
Area to paint
389 sq ft
In gallons (4 L)
1.62
4-litre cans to buy
2
Buy 6.48 litres of paint in total.
How this was calculated
Four walls = 2 × (12 + 10) × 10 = 440 sq ft
Less doors and windows = 1 × 21 + 2 × 15 = 51 sq ft
For a room, enter length, width, height and the number of doors and windows; pick 'Yes' to paint the ceiling too.
Pick the number of coats.
Enter the can's coverage; add a price per litre for the cost.
Paint formulas
What
Formula
Example (12×10 room, 10 ft high)
Four walls
2 × (length + width) × height
2 × 22 × 10 = 440 sq ft
Less doors & windows
doors × 21 + windows × 15
1 × 21 + 2 × 15 = 51 sq ft
Ceiling (optional)
length × width
12 × 10 = 120 sq ft
Litres
area × coats ÷ coverage
389 × 2 ÷ 120 = 6.48 litres
Gallons
litres ÷ 4
6.48 ÷ 4 = 1.62 → two 4-litre cans
Examples
One bedroom: 12×10 ft, 10 ft high, 1 door, 2 windows, 2 coats, coverage 120 — 389 sq ft, ≈ 6.48 litres. With the ceiling, 509 sq ft, ≈ 8.48 litres.
Two-bedroom flat: 1,200 sq ft of wall in all, 2 coats, coverage 120 — 20 litres (five 4-litre cans). At Tk 450 a litre that’s Tk 9,000 (example price).
Inside and outside paints differ: interiors usually get plastic/emulsion paint, outer walls a weather-resistant (weathercoat-type) paint. Their coverage differs, so work them out separately.
New walls: once fresh plaster is fully dry, the usual steps are primer, putty if needed, then 2 coats. Repainting the same shade often needs just 1 coat.
Mind the monsoon: paint doesn’t hold on damp walls, and coverage drops. Paint while the walls are dry.
Same batch: machine-tinted shades vary slightly between batches. Buy the whole job’s paint at once, with a little extra.
Labour is separate: painters are usually paid per sq ft; don’t mix that up with the paint cost.
More: how the math works
The coverage on the can is how many square feet one litre covers in one coat. So total paint = area × coats ÷ coverage.
With a room size, the four walls come first: two long walls (length × height) and two short walls (width × height), i.e. 2 × (length + width) × height. Then 21 sq ft per door and 15 per window come off. Painting the ceiling adds length × width.
Can coverage usually assumes a smooth, primed wall. Rough walls, fresh plaster or a light colour over a dark one take a bit more in practice, so keep 5–10% extra.
The result shows litres, gallons (4 L) and how many 4-litre cans to buy. Buying the small remainder in 1-litre cans saves money.
Frequently asked questions
What is the paint formula?
Litres = area × coats ÷ coverage. 1,000 sq ft, 2 coats at 120 coverage is 1,000 × 2 ÷ 120 = 16.67 litres.
How much paint for a 12×10 ft room?
With 10 ft walls the four walls are 440 sq ft. Less 1 door (21) and 2 windows (15 each) leaves 389 sq ft. Two coats at 120 coverage is 6.48 litres; with the ceiling, 509 sq ft takes 8.48 litres.
Where do I find the coverage?
On the can or in the maker's catalogue, usually 100–140 sq ft per litre per coat. Rough or fresh plaster lowers it.
Should I leave out doors and windows?
Yes. In room mode the calculator leaves out 21 sq ft (3 × 7 ft) per door and 15 sq ft (5 × 3 ft) per window. For bigger openings, work out the area yourself and use total-area mode.
Do I need to count primer separately?
Yes. New walls take one coat of primer before paint. Primer usually covers more than paint; work out one coat separately with the coverage on its can.
What about putty?
Fresh plaster needs wall putty for a smooth finish. Divide the area by the coverage on the bag (sq ft per kg) to get the kg.
Which can sizes should I buy?
The calculator shows how many 4-litre cans you need. Topping up the remainder with 1-litre cans is often cheaper. Can sizes vary by brand, so check the label.
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