Paint calculator
Enter the room dimensions or a wall area you measured, the number of doors and windows, and how many coats you want. The tool works out the surface that actually gets painted, converts it to litres at the usual spreading rate for water-based paint, and adds a small allowance for waste. The price is an editable default; the formula is not.
Openings come off the area: 1.9 m² per door and 1.5 m² per window, as typical sizes.
Two coats is normal. One coat rarely covers, especially over a colour change.
Default price from the Saudi Arabia market — checked August 2026 — change it to the quote you were given.
🔒 Worked out in your browser — nothing is sent to a server
Paint needed
10 L
9.66 litres exactly, and paint is sold in whole litres
Breakdown
| Wall area | 54m² |
| Doors and windows deducted | − 3.4m² |
| Net area per coat | 50.6m² |
| Total surface after coats | 101.2m² |
| Spreading rate | 11m²/L |
| Litres before waste | 9.2L |
| Waste allowance, 5% | + 0.46L |
| Estimated cost | 434.70 |
The cost is an estimate, in the same currency as the price you entered. The quantity assumes an average spreading rate; rough walls, or going from a dark colour to a light one, will use more.
How it's calculated
The whole calculation rests on the spreading rate: how many square metres one litre covers in one coat. The tool starts from the room perimeter multiplied by the height, which gives the area of the four walls, then subtracts doors and windows because they do not get painted.
The net area is then multiplied by the number of coats, since each coat uses a full quantity. That figure is divided by the spreading rate to give litres, and a small waste allowance is added to cover what stays on the roller and what goes into touching up.
The final number is rounded up, because paint is sold in whole tins and you cannot buy a fraction of a litre. The exact figure is shown next to the rounded one, so you can see how much spare you will end up with.
- Wall area = 2 × (length + width) × height
- Net area = wall area − (doors × 1.9 + windows × 1.5), plus the ceiling if you include it
- Litres = (net area × coats) ÷ spreading rate, then + 5% for waste
- The spreading rate used is 11 m² per litre per coat, the middle of the usual 9 to 13 range
- The price is an editable, dated default; the formula itself never changes
Worked example
A 4 × 5 metre bedroom with a 3 metre ceiling, one door and one window, painted in two coats:
- Wall area: 2 × (4 + 5) × 3 = 54 m²
- Openings: one door at 1.9 plus one window at 1.5 = 3.4 m², so the net area is 50.6 m²
- Two coats: 50.6 × 2 = 101.2 m² of covered surface
- Litres: 101.2 ÷ 11 = 9.2 L
- Waste at 5%: 9.2 + 0.46 = 9.66 L
You need 10 litres, which is a 9 litre tin plus a small one, or three 3.6 litre tins. The small surplus is worth having for touch-ups later.
FAQ
Does the quantity include the ceiling?
Not unless you tick "Include the ceiling". In many Gulf homes the ceiling is painted a different white and bought as a separate tin, so it is kept out by default. When you tick it, the floor area (length × width) is added to the net area.
Why does this differ from what my painter says?
Usually the spreading rate. The tool uses 11 m² per litre per coat on a primed, smooth wall, while the realistic range runs from 9 to 13. Rough render or bare plaster absorbs more, and going from a dark colour to a light one can need a third coat. If you have used the same product before, trust your own figure.
Is the price shown today's market price?
No. It is a default carrying the date it was last checked, and it is an editable field precisely because prices move. Replace it with the price your supplier quoted to get a realistic cost. The quantity in litres does not depend on the price at all.
Do I work out primer the same way?
Same method, lower spreading rate, because an unpainted surface absorbs more. As a rule of thumb treat primer as covering about 9 m² per litre per coat, so one primer coat on the same room needs roughly 6 litres. This tool sizes the topcoat; add primer as a separate calculation.
What if the room is not rectangular?
Switch to the wall area option and enter the total you measured. Measure each wall on its own (length × height), add them up, then take off the openings with the door and window fields as usual.
Related tools
Sources
✓ Reviewed September 2026- The spreading rate used is 11 m² per litre per coat, the middle of the 9 to 13 range paint manufacturers print on tins of interior water-based paint
- Default opening sizes: an interior door of about 0.9 × 2.1 m (1.9 m²) and a window of about 1.2 × 1.2 m (1.5 m²). These are common sizes, not a standard
- The 5% waste allowance covers what stays on the roller and tray and what goes into touching up. Trade practice runs from 5% to 10%; the lower figure is used so the quantity is not inflated
- The default price is dated in the pricing config and shown as an editable field, because it is the only number on this page that goes out of date
Assumptions used
- Spreading rate for water-based emulsion on a primed, smooth interior wall. Manufacturers quote 9-13 m2/L per coat; rough render, bare plaster and deep colour changes sit at the low end.
- A standard interior door leaf is about 0.9 x 2.1 m and a typical villa window about 1.2 x 1.2 m. Both are defaults the reader can override.
- Roller loading, tray residue and touch-ups. Trade practice is 5-10 percent; the lower figure is used so the estimate does not overstate.