Paint Calculator
Calculate how many litres of paint you need to cover walls, ceilings, or a room. Accounts for doors, windows, coats, and paint coverage rate.
Did this tool work for you?
How to use this calculator
Net area = total wall area minus doors and windows. Coverage rate is typically 10–14 m² per litre for standard interior paint.
- 1
Enter the room dimensions and ceiling height.
- 2
Enter the number of doors and windows to subtract from paintable area.
- 3
Set the number of coats (2 is standard for good coverage) and your paint's coverage rate (check the tin — typically 10–14 m²/L).
Frequently asked questions
What formula does this paint calculator use?
Gross wall area = 2 × (length + width) × height. It subtracts 1.71 m² for each door (1.9 × 0.9 m) and 1.2 m² for each window (1.2 × 1.0 m) to get the net wall area. Litres for walls = net area × coats ÷ coverage. For a 5 × 4 × 2.5 m room that is 45.00 − 1.71 − 2.4 = 40.89 m², and with 2 coats at 12 m² per litre, 6.82 L.
Why are the door and window sizes fixed?
The tool only asks how many doors and windows there are, and uses the fixed sizes above for each one. Larger patio doors or tiny windows will be off by the difference, so adjust the count: for example, count a wide opening as two doors.
Does the ceiling use the same number of coats?
No. The ceiling is always included, always at one coat, whatever you enter for coats. It is calculated as length × width ÷ coverage, which is 1.67 L for a 5 × 4 m room at 12 m² per litre. If you are not painting the ceiling, read the Litres for walls line rather than the total.
Does the result include extra paint for waste or touch-ups?
No. The figures are exactly the litres needed for the areas and coats you entered, to two decimal places, and are not rounded to tin sizes. Add your own margin and round up to the sizes available in the shop.
What coverage figure should I enter?
Use the number printed on the tin, in square metres per litre for one coat. The default of 12 is just a starting point, and the tool cannot account for rough, dark or absorbent surfaces. A lower figure produces a higher litre count: the same 40.89 m² wall area at 10 m² per litre with 3 coats needs 12.27 L instead of 6.82 L.
Can it calculate primer, trim or skirting?
Not separately. Primer can be estimated by running the tool again with one coat and the primer’s coverage rate. Skirting boards, window frames and door leaves are not included in the area, and the doors and windows are fully deducted from wall paint.
What shapes of room does it handle?
Only a rectangular room with a flat ceiling at one height. Sloped ceilings, alcoves, chimney breasts and dividing walls are not modelled. Split an irregular room into rectangles, run each one, and add the results.
Does it recommend a paint finish or product?
No. Finish, sheen level, primer needs and surface preparation are product-specific, so follow the instructions on the tin and check with the supplier. This page only turns areas and coverage into litres.
Paint calculator — litres for walls and ceiling
Four walls minus the openings
A room’s wall area is its perimeter times its height, so the calculator doubles length plus width and multiplies by the ceiling height. Doors and windows are then taken off. This version assumes each door covers 1.71 m² and each window 1.2 m², which are built-in values rather than entries you can edit. The remaining area is the surface that actually receives paint, multiplied by the number of coats and divided by the coverage per litre. The ceiling is added separately as a single coat, so the total is always walls plus ceiling.
Worked examples
Four rooms were run through the calculator with the doors, windows, coats and coverage shown. The default 5 × 4 × 2.5 m room comes first, followed by a small room that needs three coats of a low-coverage paint, a larger room with more openings and a high-coverage paint, and a one-coat case with no openings.
| Room and settings | Net wall area | Walls | Ceiling | Total |
|---|---|---|---|---|
| 5 × 4 × 2.5 m, 1 door, 2 windows, 2 coats, 12 m²/L | 40.89 m² | 6.82 L | 1.67 L | 8.48 L |
| 3.5 × 3 × 2.4 m, 1 door, 1 window, 3 coats, 10 m²/L | 28.29 m² | 8.49 L | 1.05 L | 9.54 L |
| 6 × 4 × 2.7 m, 2 doors, 3 windows, 2 coats, 14 m²/L | 46.98 m² | 6.71 L | 1.71 L | 8.43 L |
| 5 × 4 × 2.5 m, no openings, 1 coat, 12 m²/L | 45.00 m² | 3.75 L | 1.67 L | 5.42 L |
What the examples show
The second room is only 10.5 m² on the floor, yet it needs more paint than the third, which is more than twice its size. The reason is the pair of choices behind it: three coats instead of two, and 10 m² per litre instead of 14. Coats and coverage multiply directly, so they often matter more than the room itself. The fourth row shows the baseline for the default room with no deductions and one coat, and the difference from the first row is the combined effect of a second coat and the door and windows.
Rounding and tin sizes
The figures are shown to two decimal places and are not rounded up to the next tin, because tin sizes vary between brands. After reading the total, add an allowance for touch-ups and uneven absorption, then choose the combination of tins that covers it. If you want to check how a change in the number of coats would alter your shopping list, it is quicker to rerun the page than to rework the arithmetic.
Where it stops
The calculator treats the room as a simple box with a flat ceiling. It does not model sloped ceilings, trim, radiators, textured finishes or the primer a bare surface may need, and it does not know your paint’s true coverage. Use the coverage printed on the tin and keep the leftover paint for later repairs. The sizes assumed for doors and windows are approximations, so adjust the counts if your openings are very large or very small.
Learn more from an authoritative source:
WikipediaFlooring Calculator
Calculate how many square metres of flooring you need for any room. Supports rectangular and L-shaped rooms, with waste percentage.
Tile Calculator
Calculate how many tiles you need for a floor or wall. Enter room dimensions and tile size to get tile count, boxes needed, and grout quantity.
Fence Calculator
Calculate the number of fence posts, rails, pickets, and panels needed for any fence length. Supports common picket and panel fence styles.
Brick Calculator
Calculate the number of bricks and bags of mortar needed for a wall. Supports single and double brick walls with standard or metric brick sizes.
Staircase Calculator
Calculate the number of steps, riser height, tread depth, and total stringer length for a staircase. Ensures compliance with building code guidelines.
Roofing Calculator
Calculate the total roof area for gable, hip, or flat roofs. Get roofing material quantities in squares, sheets, and tiles with waste allowance.
Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →