Every paint estimate comes down to one formula:
paint volume = (paintable area x number of coats) / coverage rate
purchase volume = paint volume x (1 + waste allowance)
Everything else is deciding what goes into those four numbers. Get the coverage rate wrong and you can be out by half a tin on a single room. Get the area wrong and you are out on every room.
Step 1: Measure the paintable area
Measure each surface separately, because walls, ceilings, and trim rarely take the same paint or the same number of coats.
Walls. Multiply the room perimeter by the wall height. For a room 4.0 m by 3.5 m with a 2.4 m ceiling:
perimeter = 2 x (4.0 + 3.5) = 15.0 m
gross wall area = 15.0 x 2.4 = 36.0 m2
Ceilings. Length times width. A rectangular ceiling is the easy case; for L-shaped rooms, split the shape into rectangles and add them.
Trim. Skirting, architrave, and casings are measured as a length, then converted to area using the girth — the developed width of the profile once you flatten it out. A typical skirting board is about 0.15 m (6 in) of paintable width:
trim area = trim length x girth
Doors and window frames are usually counted per unit rather than measured, but if you want a number, treat a standard door leaf as roughly 3.5 m2 for both faces.
Step 2: Deduct openings, but only the big ones
Subtract doors and windows from the gross wall area. Typical presets:
| Opening | Metric | Imperial |
|---|---|---|
| Door | 0.84 m x 1.98 m | 3 ft x 7 ft |
| Window | 1.0 m x 1.2 m | 4 ft x 3 ft |
Continuing the example, with one door and two windows:
openings = (0.84 x 1.98) + 2 x (1.0 x 1.2) = 1.66 + 2.40 = 4.06 m2
net wall area = 36.0 - 4.06 = 31.94 m2
Do not deduct sockets, switches, or small vents. The cutting-in around them uses more paint per square metre than a roller does on open wall, so the two effects cancel out. Deducting them makes the estimate worse, not better.
Step 3: Pick a coverage rate that matches the surface
Coverage — the spreading rate — is the number that most estimates get wrong. It is printed on the tin, and it assumes a smooth, sealed, previously painted surface. Anything rougher or more absorbent will drink more paint.
| Surface | Typical coverage |
|---|---|
| Smooth, sealed, previously painted plaster | 12–14 m²/L |
| Bare or porous plaster, first coat | 8–10 m²/L |
| Primer or sealer | 10–12 m²/L |
| Lightly textured wall or woodchip | 8–10 m²/L |
| Rough render, brick, or masonry | 4–7 m²/L |
| Gloss or satin on prepared trim | 14–16 m²/L |
A warning about units: UK and EU tins quote noticeably higher figures than US cans for a comparable paint, because they assume a different film thickness. A US can stating 350–400 ft²/gal is not describing a worse paint than a UK tin stating 12 m²/L. Do not convert coverage figures between the two systems and do not compare them — take the number printed on the tin you are actually buying, in the units it is printed in.
Colour matters too. Strong reds and yellows, and any light colour going over a dark one, may need an extra coat regardless of what the coverage figure says.
Step 4: Count the coats honestly
Two coats is the working default and the right answer for most repaints. Use three when you are covering a dark colour with a light one, painting over a patchy or heavily filled surface, or using a weakly pigmented colour.
One coat is realistic only when you are refreshing an identical colour on a sound surface. Estimating one coat and painting two is the single fastest way to run out of paint mid-wall — and a second tin from a different batch rarely matches.
Step 5: Add primer where it is needed, not everywhere
Primer is a separate calculation with its own coverage rate and its own coat count, usually one coat. Prime when the surface is new plaster or plasterboard, bare timber, filled or patched, stained, or when you are making a drastic colour change.
Priming a sound, previously painted wall in a similar colour is usually wasted material and wasted time.
Step 6: Add a waste allowance
An 8% allowance is a sensible starting point. It absorbs roller loading, tray residue, what stays in the tin, drips, and touch-ups.
Push it higher — 10% to 15% — for textured surfaces, cutting-in-heavy rooms with lots of trim, spray application, or when you want a genuine touch-up reserve left at the end of the job.
Step 7: Convert litres into tins
Paint is sold in fixed sizes, so the last step is a packing problem, not a division. Common sizes are 1 L, 2.5 L, 5 L, and 10 L in metric markets, and quart, gallon, and 5-gallon in the US.
Working the example through, with walls and ceiling in the same paint at 12 m²/L, two coats, and 8% waste:
coat area = (31.94 net wall + 14.0 ceiling) x 2 coats = 91.88 m2
raw volume = 91.88 / 12 = 7.66 L
purchase volume = 7.66 x 1.08 = 8.27 L
You cannot buy 8.27 L. A 5 L plus a 2.5 L plus a 1 L gives you 8.5 L and leaves 0.23 L spare. A single 10 L tin also covers it, but leaves 1.73 L — better if you want a touch-up reserve, worse if you are counting cost. Always round up to the next buyable combination; never round the litres down to make the tins fit.
The Siteometry Paint Calculator does this whole chain in the browser — areas, openings, coats, primer, waste, and the cheapest tin combination — for walls, ceilings, and trim in one project. If you only need the areas, the Square Footage Calculator will handle the awkward room shapes.
Estimator's checklist
Before you buy:
- Walls, ceiling, and trim are measured as separate surfaces.
- Doors and windows are deducted; sockets and switches are not.
- The coverage rate matches the actual surface, not the best case on the tin.
- Coats are counted per surface, not assumed across the whole job.
- Primer is calculated separately, with its own coverage and coat count.
- Waste allowance is applied after the coats, not before.
- Litres are rounded up to real tin sizes.
- Everything for one continuous surface is bought in one batch.
If the job runs across more than one visit, buy the full quantity up front. Batch-to-batch colour variation is small, but it is visible on a wall lit from the side — and no amount of estimating accuracy will fix it once it is on.