What this calculator does
Paint needed is the wall area times the number of coats, divided by the coverage rate. A 5 by 4 metre room with 2.7 metre walls, less two doors and two windows, gives 42 m² of paintable wall and takes 7 litres for two coats at 12 m² per litre.
Coverage is where estimates go wrong, because the figure on the tin assumes a smooth, sealed, similarly coloured surface. Bare plaster, textured render or a dark colour being covered in light will all consume noticeably more, and a first coat on new plasterwork can use half again as much as the tin suggests.
The formula
The wall area is the room perimeter times the height, less a standard allowance for each door and window. Multiplying by coats and dividing by coverage gives the litres, which is then rounded up to whole tins. The ceiling area is reported separately, since it is usually painted with a different product.
| Term | Meaning |
|---|---|
| Coverage | Square metres per litre. Tins usually claim 10 to 14 for a smooth sealed surface. |
| Cutting in | Brushing the edges where a roller cannot reach, which uses proportionally more paint. |
| Sealer or primer | A first coat on bare or patchy surfaces, which is additional to the coats counted here. |
| Spreading rate | The same thing as coverage, and what paint technical data sheets call it. |
The inputs explained
| Field | What to enter |
|---|---|
| Room length (m) | Room length in metres. |
| Room width (m) | Room width in metres. |
| Wall height (m) | Wall height, floor to ceiling. |
| Doors | Number of doors, each deducted at a standard size. |
| Windows | Number of windows, likewise. |
| Coats | Coats. Two is normal; three if covering a strong colour. |
| Coverage per litre (m²) | Coverage per litre from the tin. Reduce it for porous or textured surfaces. |
| Price per litre ($) | Price per litre, for the cost estimate. |
When to use it
Buying paint for a room
Paint comes in fixed tin sizes, so knowing the requirement decides how many to buy.
Budgeting a repaint
Multiplying the litres by the price gives a material cost before labour.
Comparing products
A more expensive paint with better coverage can work out cheaper per room.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much paint does each coverage rate need?
The same room with paints of different coverage.
| Coverage per litre | Paint needed | Buy | Paintable wall area |
|---|---|---|---|
| 8 m²/L | 10.50 litres | 3 × 4 L tins | 42.00 m² |
| 12 m²/L | 7.00 litres | 2 × 4 L tins | 42.00 m² |
| 16 m²/L | 5.25 litres | 2 × 4 L tins | 42.00 m² |
Questions
How much paint do I need per square metre?
Roughly a twelfth of a litre per coat on a smooth sealed surface, which is the usual 12 m² per litre. Bare plaster, render or timber will absorb considerably more on the first coat, sometimes half again, so treat the tin figure as a best case.
Do I need to deduct doors and windows?
For an accurate estimate, yes, and this does so automatically. Two doors and two windows remove about 6 m² from a typical room, which is around 15% of the wall area and most of a litre over two coats. For a rough figure you could ignore them and have spare.
How many coats do I need?
Two over a similar colour on a sealed surface. Three when covering a dark colour with a light one, or a strong colour at all. New plasterwork needs a sealer or thinned first coat before the finish coats, which is additional again.
Should I buy extra?
A little, yes. Touch-ups are inevitable and matching a colour later from a new batch is unreliable, even with the same code. Rounding up to the next tin usually provides enough, which is why the calculator reports whole tins.
For the floor in the same room, see the flooring calculator. For wall coverings instead, see the wallpaper calculator.