The short answer depends on wall height and the paint tin
For the 10x12 ft room in this example, 9 ft walls, a 3×7 ft door and a 4×3 ft window leave 363 sq ft (33.72 m²) of wall. Two coats at an illustrative 12 m² per litre per coat need 5.62 L before touch-ups. One 5 L can is short; a 5 L and 1 L can together would supply 6 L if the same paint is sold in both sizes. Your answer depends on your own measurements and chosen product.
A room-size chart may assume a different height, openings or coverage. Check the tin or product sheet. This estimate covers wall topcoat only; ceiling paint, primer and labour are separate.
Measure the walls, then subtract what you will not paint
A 10×12 ft room has a 120 sq ft floor, while its four straight walls have a 44 ft perimeter. Multiply that perimeter by the height you will paint. For an irregular room, add each wall's width × painted height instead.
Measure each door, window or tiled section you will not coat. Do not deduct a door you plan to paint. Use feet for every length in this equation.
The example door is 3×7 = 21 sq ft and the window 4×3 = 12 sq ft; measure yours rather than assuming these sizes. Convert the remaining area to square metres for a tin labelled in m²/L. NIST gives 1 ft² = 0.09290304 m².
Worked example: 10x12 ft room with 9 ft walls
Assume two coats on straight walls, with only the measured door and window excluded. 12 m²/L per coat is an example input, not a product claim or a guarantee on bare plaster. Keep full precision until the final result.
| Step | Arithmetic | Result |
|---|---|---|
| Gross wall area | 2 × (10 + 12) × 9 ft | 396 ft² |
| Openings not painted | (3 × 7) + (4 × 3) ft² | 33 ft² |
| Net wall area | 396 − 33 | 363 ft² |
| Convert to m² | 363 × 0.09290304 | 33.72380352 m² |
| Two coats | 33.72380352 × 2 ÷ 12 m²/L | 5.62063392 L |
| If only 5 L cans are sold | ceiling(5.62063392 ÷ 5) | 2 cans (10 L bought) |
Here, 33.72380352 × 2 ÷ 12 = 5.62063392 L, or about 5.62 L. The doubled area represents two passes over the same walls, not a larger room.
Lower stated coverage means more litres. If the label already states coverage for a complete two-coat system, do not multiply by two again; check its wording before entering a per-coat rate.
A one-foot height change can change the can count
With the same openings, coats and example coverage, every extra foot of height adds 44 ft² of wall and about 0.68 L of paint. An 8 ft chart can therefore be short for a 9 ft room.
The 8 ft result is just 0.06 L below one 5 L can on paper; real coverage or touch-ups can erase that margin. The 9 ft result already exceeds 5 L. This is a sensitivity check, not a universal shopping list.
From calculated litres to cans on the shelf
5.62 L is the material estimate, not a pack size. If the same colour comes in 5 L and 1 L cans, those supply 6 L with about 0.38 L left on paper. With 5 L cans only, the purchase is two cans and 10 L. Check actual sizes and prices; least leftover does not always mean lowest cost.
An allowance for touch-ups is your choice, not a fixed rule. At an illustrative 10% extra, 5.62063392 × 1.10 = 6.18 L. A 5 L + 1 L combination is then short; 5 L + 2 L works if sold. Treat a new coat or primer as a separate calculation, not an allowance.
Ceiling, primer and difficult surfaces are separate estimates
The flat ceiling is 120 ft² = 11.1483648 m². At the same illustrative rate and two coats it needs about 1.86 L more. Combine it with the walls only if product, colour, coats and coverage really match. Calculate trim, doors and primer with their own products.
Bare plaster, rough texture or a colour change can alter coats and coverage. Dulux Pakistan and Sherwin-Williams advise measuring the surface and checking the selected paint's label. Its topcoat rate does not specify primer quantity.
Run your measurements in the existing paint calculator
- Measure walls and openings. Convert net ft² to m² by multiplying by 0.09290304, or check that step with the Area Converter.
- Open the Construction, Tiles & Paint Calculator, select Paint cans for a wall, and enter net wall area: 33.72380352 m² for this example, not 120 ft² of floor.
- Enter 2 coats, coverage per litre per coat, one available can size and price, and your chosen allowance. At 12 m²/L, 5 L cans and 0%, the result is about 5.62 L and two cans.
- Run another can size separately if available. The tool rounds one pack size upward; it does not choose mixed packs, deduct openings automatically or calculate primer and labour.
Keep your measurements and the tin's coverage with the estimate. A painter can use them to prepare a project-specific quote; this calculator does not price labour.