The room we are painting
Take an ordinary double bedroom: 4 m long, 3.5 m wide, 2.7 m from floor to ceiling. One standard door, two windows. We are painting the walls and the ceiling, two coats, in a light colour over an existing light colour — so no primer, and no dramatic colour change to fight.
Measure the finished internal dimensions, not the outside of the building and not the plan on the estate agent's brochure. Measure at about waist height where walls are usually straightest, and if two opposite walls disagree by more than a few centimetres, use the larger figure. Paint bought against the larger figure gets used; paint bought against the smaller one sends you back to the shop.
Step one: wall area
The perimeter of the room multiplied by its height. There is no need to measure each wall separately in a rectangular room:
2 × (4 + 3.5) × 2.7 = 40.5 m²
That is the gross wall area, before anything is taken out of it. Note how quickly height matters: the same floor plan with a 3 m ceiling gives 45 m², more than a tenth extra, which at the end of this calculation is the difference between two tins and three.
Step two: subtract the doors and windows
A standard internal door is about 0.8 × 2.0 m, so 1.6 m². Two windows at roughly 1.2 × 1.4 m are 1.68 m² each, 3.36 m² together. Total openings: 4.96 m².
40.5 − 4.96 = 35.54 m² of wall to paint
Whether to subtract openings at all is the first place people disagree, and both camps have a point. Subtracting is more accurate. Not subtracting builds in a cushion that quietly covers the cutting-in, the touch-ups and the roller tray you tip over. Our position is to subtract properly and add the cushion explicitly as a waste percentage, because a cushion you chose is better than a cushion you inherited by accident.
One thing not to do: subtract a wardrobe, a bed or a radiator. You will paint behind all of them, or at least you should, and the day you move the wardrobe you will want the wall behind it to match.
Step three: add the ceiling if you are painting it
4 × 3.5 = 14 m²
35.54 + 14 = 49.54 m² total paintable area
Ceilings usually take a different product from walls — a dedicated matt ceiling paint, often brilliant white — so in practice you will calculate them as a separate job with their own tin. Keep the areas separate in that case rather than adding them, or you will buy one product to cover two different coverage rates.
Step four: coats and coverage
Coverage is printed on the tin, usually as a range: 10 to 12 m² per litre per coat is typical for interior emulsion on a smooth, sealed, previously painted wall. Take the pessimistic end of the range unless you have good reason not to. The optimistic end assumes a laboratory substrate and a roller technique you probably do not have on a Saturday afternoon.
49.54 × 2 coats ÷ 10 m²/L = 9.9 litres
Two coats is the honest default. One coat is realistic only when you are refreshing the same colour on a sound surface. Going lighter over a dark wall commonly needs three, and a strong red or yellow can need four, because those pigments have poor natural opacity regardless of price.
Step five: the waste allowance
Add 10% for paint. It is not really waste in the sense of spillage; it covers the fact that your wall is not the wall on the datasheet, that cutting-in uses more per square metre than rolling, and that the last stretch of a tin never quite comes out.
9.9 × 1.10 = 10.9 litres
Step six: round up to whole tins
Shops sell 2.5 L and 5 L tins, not 10.9 L. This is the step that decides what you actually pay:
| Option | Volume | Left over |
|---|---|---|
| Three 5 L tins | 15 L | 4.1 L |
| Two 5 L + one 2.5 L | 12.5 L | 1.6 L |
| Two 5 L tins | 10 L | 0.9 L short |
Two 5 L tins plus one 2.5 L is the right answer here, and it is exactly the answer people fail to reach in the shop, where the mental arithmetic says "about ten litres" and the hand reaches for a third 5 L tin. The difference is roughly the price of a decent roller set, on one room.
One caveat that overrides the arithmetic: if the colour is mixed to order rather than off the shelf, buy it all in one visit. Tinting machines are accurate, not perfect, and a top-up tin mixed three weeks later can be visibly different on a large wall.
Where the estimate goes wrong
- Trusting the coverage figure on the tin
- It is measured on a smooth, sealed, non-absorbent surface. New plaster can drink half of a first coat. Old distemper, heavy filler repairs and textured finishes all push actual coverage down, sometimes to 6-8 m² per litre.
- Forgetting that colour changes the coat count
- Hiding power is a property of the pigment. Deep reds, bright yellows and anything covering a dark base can need a third or fourth coat that no area calculation predicts. Where you expect this, price the extra coat before you choose the colour.
- Measuring the floor and calling it the ceiling
- They are the same only in a room with vertical walls and no sloping ceiling. In a loft conversion, measure the ceiling planes separately and add them.
- Buying one big tin instead of two smaller ones
- 5 L is usually cheaper per litre than 2.5 L, which is why the arithmetic above is worth doing rather than eyeballing: sometimes the larger tin genuinely wins, and sometimes it leaves you with four litres you will never open.
- Ignoring the second product
- Walls, ceiling, woodwork and radiator are four different products with four different coverage rates. A single number for "the room" is the fastest way to be wrong about all four.
The whole calculation in one place
walls 2 × (L + W) × H − openings + ceiling → × coats ÷ coverage → × (1 + waste) → ÷ tin size, rounded up
For our bedroom: 40.5 − 4.96 + 14 = 49.54 m²; × 2 ÷ 10 = 9.9 L; + 10% = 10.9 L; two 5 L tins and one 2.5 L.
Every figure here is an estimate for planning a purchase. Check the coverage stated on your own product, and where the job is large or the paint expensive, confirm with your supplier before ordering.