🎨 Paint Calculator
Work out how much paint a room actually needs: wall area minus the doors and windows, the ceiling if you are painting it, multiplied by the number of coats and divided by the coverage printed on the tin. It returns litres, US gallons and how many tins to buy.
How much paint a room needs, and why the tin's coverage figure lies
Paint quantity is a simple calculation that goes wrong for one reason: the coverage printed on the tin is measured on a smooth, sealed, previously painted surface under laboratory conditions. Real walls are rougher, thirstier and more awkward than that, and a textured or freshly plastered surface can take half again as much paint for the same area.
Litres = (wall area − openings + ceiling) × coats / coverage
| Surface | Calculation | Area |
|---|---|---|
| Wall perimeter | 2 × (4.5 + 3.6) | 16.2 m |
| Walls, gross | 16.2 × 2.4 | 38.88 m² |
| One door | − 1.9 m² | −1.9 m² |
| Two windows | − 2 × 1.5 m² | −3.0 m² |
| Walls to paint | 33.98 m² | |
| Ceiling | 4.5 × 3.6 | 16.2 m² |
| Total, two coats | 50.18 × 2 | 100.36 m² |
| At 11 m² per litre | 9.1 litres |
Nine litres means two 5-litre tins, and the second will be mostly full. That leftover is not waste — it is what you touch up with in two years' time, when matching a colour from a fresh tin would be impossible because batches differ slightly.
What coverage you should actually assume
| Surface | Litres per coat per 10 m² |
|---|---|
| Previously painted, smooth | 0.8 – 1.0 |
| Lining paper or light texture | 1.0 – 1.2 |
| New plasterboard | 1.2 – 1.4 |
| Bare plaster, first (mist) coat | 1.4 – 1.8 |
| Textured or woodchip | 1.5 – 2.0 |
Bare plaster is the big one. It is porous and pulls the water out of the paint, so the first coat should be a mist coat — emulsion thinned with roughly 10% water — which soaks in and seals the surface. Skipping it does not save paint; it causes the top coats to sit unevenly and sometimes peel away in sheets later.
Coats, and when two is not enough
Two coats is the normal answer and the one to budget for. One coat only works when refreshing the same colour on a sound surface. Three become necessary in three situations: covering a dark colour with a light one, covering a strong colour such as red or deep blue, which bleed through for several coats, and painting bare or patched plaster where the mist coat does not count as a top coat.
A tinted primer is cheaper and faster than a third top coat. Grey primer under a strong colour, or a primer tinted towards the final shade, routinely saves one full coat of expensive paint — which on the room above is 4.5 litres.
Doors and windows: deduct or not?
This calculator deducts 1.9 m² per door and 1.5 m² per window, which are standard allowances. For a room with one door and a couple of windows the deduction is around 5 m², roughly 13% of the wall area. Some decorators ignore openings entirely as a built-in safety margin, and for a single small room that is a defensible shortcut.
It stops being defensible on a larger job. A house with twelve openings has 25 m² of deduction, around 4 litres of paint across two coats, which is a real cost. Count them on anything bigger than one room. If you are also painting the door and window frames, that is gloss or eggshell at a different coverage and belongs in a separate calculation.
Finish changes how much you need
Matt emulsion covers best per litre and hides surface imperfections, but marks easily and cannot be scrubbed. Silk and satin cover slightly less per litre, resist moisture and wipe clean, and show every bump in a wall under side light. Gloss and eggshell are for wood and metal and cover considerably less area, so a door takes more paint than its size suggests once both faces and the edges are counted.
For areas rather than volumes, the square footage calculator handles irregular rooms, and the area calculator deals with gables, arches and other shapes the simple perimeter method cannot.
A worked example
A hallway and stair: walls 28 m² of smooth previously painted surface, plus a 4 m² ceiling, two coats of matt at 11 m² per litre. That is 32 × 2 / 11 = 5.8 litres, so one 5-litre tin plus a 2.5-litre. But the existing colour is a deep navy and the new one is off-white, so budget a tinted primer at 3 litres and three top coats rather than two: 8.7 litres of emulsion, which is two 5-litre tins. The primer is the line item that prevents a fourth coat.
Once you have this figure, the square footage calculator is where it usually goes next. the area calculator is the related one people reach for afterwards.
Frequently Asked Questions (FAQs)
How much paint do I need for a room?
Work out the wall perimeter times the ceiling height, subtract about 1.9 square metres per door and 1.5 per window, add the ceiling if you are painting it, multiply by the number of coats and divide by the coverage on the tin. A typical 4.5 by 3.6 metre room needs around 9 litres for two coats including the ceiling.
How many square metres does a litre of paint cover?
Tins usually claim 10 to 14 square metres per litre for matt emulsion, but that is measured on a smooth sealed surface. Assume 10 to 11 on previously painted walls, less on textured surfaces, and as little as 6 on bare plaster.
How many coats of paint do I need?
Two for most jobs. One is only enough when refreshing the same colour on a sound surface. Use three when going light over dark, covering a strong colour, or painting bare plaster — though a tinted primer is usually cheaper than a third top coat.
Do I need to subtract doors and windows?
For one small room you can skip it as a safety margin. On anything larger, count them: twelve openings across a house is about 25 square metres and four litres of paint over two coats.
What is a mist coat and do I need one?
Emulsion thinned with roughly 10% water, used as the first coat on bare plaster. Fresh plaster is porous and draws water out of unthinned paint, which leaves the finish patchy and can make it peel later. On bare plaster it is not optional.
Should I buy extra paint?
Yes, round up rather than down. A part tin is cheap compared with a second trip, and keeping the leftover means you can touch up later with the exact batch — colours vary slightly between production runs even within the same shade.
Does paint finish affect how much I need?
Yes. Matt covers the most per litre, silk and satin slightly less, and gloss or eggshell considerably less. Woodwork also has more surface than it looks, since a door means both faces plus the edges.