Enter the net paintable area after deducting any openings.
What do these choices mean? The figure is the approximate area covered by 1 litre in one coat. A more absorbent surface gives lower coverage. Always replace it with the figure stated for your paint.
Buying 10–15% extra paint is a practical allowance. It covers absorption, paint left in the roller or tray, small losses and later touch-ups.
The diagram shows what is included in the area; it is not an exact room drawing.
Area breakdown
- Total wall area
- 36.4 m²
- Windows, doors and other openings
- −3.9 m²
- Paintable wall area
- 32.5 m²
- Paintable ceiling area
- 0 m²
- Paint coverage
- 10 m²/l
- Number of coats
- 2
Calculation:
This is an estimate. Before buying, check the coverage, number of coats and surface-preparation instructions for the exact product.
How much paint is needed for rooms of different sizes?
Based on 2.5 m-high walls, one 0.9 × 2 m door and one 1.5 × 1.4 m window, two coats, 10 m²/l coverage and a 10% allowance.
| Floor area | Room dimensions | Paintable wall area | Paint required | |
|---|---|---|---|---|
| 10 m² | 2.5 × 4 m | 28.6 m² | about 6.3 l | |
| 12 m² | 3 × 4 m | 31.1 m² | about 6.8 l | |
| 15 m² | 3 × 5 m | 36.1 m² | about 7.9 l | |
| 20 m² | 4 × 5 m | 41.1 m² | about 9 l | |
| 25 m² | 5 × 5 m | 46.1 m² | about 10.1 l | |
| 30 m² | 5 × 6 m | 51.1 m² | about 11.2 l |
A room with the same floor area can have a different perimeter, ceiling height and opening area. Treat the table as a guide.
How to calculate how much paint you need
The area is only the starting point: coverage, coats and the condition of the surface also affect the final amount.
Measure the wall area
For a rectangular room, use 2 × (length + width) × height. Enter walls separately when the room is irregular.
Subtract unpainted openings
Multiply the width, height and quantity of windows and doors. Allow separately for reveals if you intend to paint them.
Check the stated coverage
Use the m²/l per-coat figure printed on the tin or in the technical data sheet, rather than relying on a generic average.
Add a sensible allowance
An extra 10–15% helps cover surface absorption, application losses and keeping matching paint for touch-ups.
How to work out wall area from room dimensions
For a rectangular room, add its length and width, multiply by two, then multiply by wall height. This gives the gross area of all four walls.
A 4 × 3 m room with 2.6 m walls has 2 × (4 + 3) × 2.6 = 36.4 m² of wall. After a 1.8 m² door and 2.1 m² window are deducted, 32.5 m² remains.
How many square metres does one litre of paint cover?
There is no single rate for every emulsion. Interior wall paints commonly show a spreading rate in m²/l per coat; smooth prepared surfaces usually cover more than porous plaster or textured walls.
The calculator starts at 10 m²/l as a neutral planning value. Replace it with the coverage stated on the tin. The 8, 10 and 12 m²/l buttons are examples for more absorbent, normally prepared and smoother surfaces.
How many coats should walls receive?
Two finish coats are commonly planned, but the correct system depends on the product, substrate, colour change and manufacturer instructions. A strong colour change may need additional preparation or another coat.
Primer or mist coat is not counted as a finish coat. Estimate preparation products separately using their own coverage figures.
Why allow 10–15% extra paint?
A geometric calculation cannot predict the exact absorption of fresh plaster, filler or an existing coating. Paint also remains in rollers, brushes and trays.
Around 10% is often enough for a smooth prepared wall; a rough or absorbent surface may justify nearer 15%. Keep a labelled remainder for later touch-ups.
Paint calculator FAQs
Clear answers about wall area, paint coverage, coats, doors, windows and extra allowance.