What this calculator works out
Most paint calculators work forwards: you measure the room and they tell you how many tins to buy. This one works the other way round. You have a part-used tin in the garage and the question is whether it will finish the job, or how much wall it will actually do.
That is a harder question than it looks, because the coverage figure printed on a tin is a best case measured on a smooth, sealed, previously painted surface by someone who is not spilling any. Bare plaster drinks paint. Textured surfaces have far more area than their measured size suggests. A roller leaves a surprising amount of paint in the tray and the sleeve.
This calculator applies a realistic coverage rate for the surface you are painting, allows for waste depending on how you are applying it, and tells you the area a part tin will genuinely cover at the number of coats you want. Enter the area still to do and it will also tell you whether you are short, and by how much.
What you have and what you are painting
Realistic coverage rates
These are the figures this calculator uses, in square metres per litre for one coat. They sit below most tin figures deliberately, because tin figures describe ideal conditions.
| Surface | m² per litre | Why |
|---|---|---|
| Previously painted smooth plaster | 13 | Sealed and even — the easiest case, and roughly what the tin assumes |
| Bare plasterboard | 10 | Absorbent paper face, and the jointing compound absorbs differently again |
| New or bare plaster | 9 | Highly absorbent until sealed; the first coat largely soaks in |
| Textured, artexed or heavily patched | 8 | The real surface area is far greater than the measured area |
| Previously painted woodwork | 15 | Non-absorbent, and gloss and satin spread further than emulsion |
| Bare or primed woodwork | 11 | Grain absorbs, and end grain absorbs heavily |
| Exterior masonry, smooth render | 7 | Porous, and masonry paint is applied more thickly by design |
| Exterior masonry, pebbledash or rough render | 4 | Enormous true surface area; the worst case on this list |
A stated coverage of 14 m² per litre is measured under controlled conditions on a surface that is smooth, sealed, clean and in good condition, applied at the intended film thickness with nothing wasted. Every one of those conditions is generous. The figure is not a lie — it is a laboratory measurement, and it describes the paint rather than your wall. Treat it as a ceiling that you will not reach rather than as a prediction.
How this calculator works
Four steps, all of them straightforward, with the honesty in the assumptions rather than the arithmetic.
Paint remaining (litres) = tin size × how full it is
Usable paint = paint remaining × application factor
Coverage for one coat (m²) = usable paint × the rate for that surface
Coverage at your chosen coats (m²) = one-coat coverage ÷ number of coatsThe application factor is the part most calculators leave out. A roller and tray loses roughly a tenth of what you pour — paint left in the tray, absorbed into the sleeve, and dried on the rim. A brush is more frugal at around five per cent. A sprayer is the least efficient of the three for a small job, losing perhaps a quarter to overspray and to filling the hose and gun, though it comes into its own on large or awkward areas.
Dividing by the number of coats at the end is the right way round. A second coat over the first uses slightly less paint than the first did, because the surface is already sealed, but the difference is small enough on a part tin that assuming equal coats gives the safer answer.
The result is deliberately conservative. Running out three quarters of the way along a wall costs a trip to the shop and a visible join; having half a litre spare costs nothing.
Worked example: half a tin and a hallway
A 2.5 litre tin of matt emulsion, about half full, left over from doing the living room. The hallway walls are previously painted smooth plaster, measure about 18 m², and want two coats. It will be applied with a roller.
Entering those figures gives:
Usable after roller and tray losses: about 1.13 litres
Coverage at one coat: about 14.6 m²
Coverage at 2 coats: about 7.3 m²
Based on 13 m² per litre for that surface, less 10% application loss. The figure on your tin describes ideal conditions and will be higher.
That is 1.25 litres remaining, about 1.13 litres of it usable once the tray and the roller sleeve have taken their share, covering roughly 14.6 m² at one coat or 7.3 m² at two. Against 18 m² of hallway that is a long way short — you would get about 40% of the way through the job.
The instinct at this point is to do one coat instead and hope. On previously painted walls in the same colour that can work; over a different colour it will not, and the patchiness only becomes obvious once the paint has dried and the light changes. The useful output here is knowing before starting rather than at the point where the roller stops picking anything up — and knowing the shortfall in litres, so the right amount is bought rather than another 2.5 litre tin that leaves the same problem for next time.
Understanding the result
- Comfortably enough. More than about 15% spare. Proceed, and keep the remainder for touching up.
- Just about enough. Within 15%. Workable, but do not start late in the day, and think about whether a visible join would matter if you are wrong.
- Short. The calculator gives the shortfall in both square metres and litres. Buy before starting, not halfway through.
If you have to buy more of the same colour, the new tin will almost certainly be from a different batch, and mixed colours in particular can differ enough to see in daylight. It is not a manufacturing fault — pigment dispersion varies slightly between batches. The way round it is to mix the old and new paint together in a larger container before you start, so any difference is spread evenly rather than appearing as a line halfway up the wall. If you cannot mix, break at a corner or a natural edge rather than mid-wall.
Is the paint still usable?
A part tin is only useful if what is in it has survived storage, and paint does not last indefinitely once opened.
- A skin on top is normal. Water-based paint forms one where air has reached it. Lift the whole skin out in one piece and discard it — do not stir it in, or you will be picking rubbery fragments off the wall all afternoon.
- A foul or sour smell means it has gone. Water-based paint can spoil bacterially. Once it smells bad it cannot be recovered and should be disposed of. It will smell on the wall too.
- Frost damage is terminal. Emulsion stored in an unheated garage or shed over winter can freeze, and it comes back curdled or grainy. Straining will not rescue it.
- Separation is usually fine. Pigment settling out is normal and stirring thoroughly, for several minutes rather than several seconds, brings it back.
- Rough general guide. Unopened emulsion keeps for a few years in frost-free conditions; opened and resealed properly, perhaps a year or two; solvent-based gloss keeps rather longer but skins more heavily.
Storing a part tin well is mostly about excluding air: seal the lid down properly, store it upside down briefly to let the paint seal the rim, keep it somewhere that will not freeze, and write the room and the date on the lid.
Things that affect the result
- Colour change. Going from dark to light, or covering a strong colour, may need three coats or a tinted undercoat regardless of what the coverage arithmetic says.
- The mist coat on new plaster. Fresh plaster needs a first coat thinned with water, and it is absorbed rather than covering. That coat uses less paint but achieves less; the calculator's bare plaster rate accounts for absorbency but not for the extra coat.
- Temperature and humidity. Paint applied in a cold room goes on thicker and covers less.
- Roller nap. A long-pile sleeve for a textured surface holds and deposits considerably more paint than a short one.
- How much is really left. Judging a tin by eye is unreliable. Weighing it is better: emulsion is roughly 1.3 to 1.5 kg per litre, so subtracting the weight of an empty tin and dividing gives a much better estimate than looking.
- Cutting in. Edges and corners use disproportionately more paint per square metre than open wall.
Common mistakes
- Trusting the tin's coverage figure. It describes ideal conditions on a sealed, smooth surface.
- Forgetting the surface area of texture. Pebbledash can have several times the area its measurements suggest, which is why its rate is so much lower.
- Measuring the wall and forgetting the number of coats. Two coats halves the area a tin will do.
- Starting a wall you cannot finish. A join within a single wall face shows in raking light almost regardless of technique.
- Stirring the skin in. Lift it out whole.
- Pouring leftovers down the sink or a drain. Liquid paint is not permitted in household waste and should never go into the drainage system.
- Buying a fresh tin without checking what is in the garage. The reason this calculator exists.
Left with paint you will not use
Around half of the paint bought in the UK for a job is not used on it, and most of it goes into storage rather than to any purpose.
- Do not pour it away. Liquid paint must not go down a sink, drain or gully, and liquids are not accepted in kerbside household waste.
- Water-based paint can be hardened. Leave the lid off in a well-ventilated place, or stir in sawdust, sand, soil or a proprietary paint hardener until solid. Once hardened, the tin can normally go in household waste — check your council's rules, as they differ.
- Solvent-based paint is different. Gloss, oil-based undercoat and their thinners are hazardous waste and should go to a household waste recycling centre, not into general rubbish.
- Usable paint can be donated. Community RePaint schemes across the UK collect leftover paint in reasonable condition and redistribute it to community groups, charities and households on low incomes. Your council's website will say whether there is a scheme locally.
- Empty tins are recyclable. Metal tins scraped clean and dry are generally accepted; plastic tubs depend on the council.
Frequently asked questions
How can I tell how much paint is left without opening the tin?
Weigh it. Emulsion weighs roughly 1.3 to 1.5 kg per litre, and an empty 2.5 litre metal tin is around 250 g. Weigh the tin on kitchen scales, subtract the empty weight, and divide by about 1.4 to get litres. Judging by eye or by sloshing is unreliable, and people consistently overestimate — the sides of the tin hold a surprising amount of the apparent volume.
Why does the tin claim better coverage than this calculator gives?
Because the tin figure is a laboratory measurement of the paint, taken on a smooth, sealed, previously painted surface at the intended film thickness with no waste. It is accurate for what it measures. Your wall is rarely that surface, and applying paint by roller in a real room always loses some. The figures here are deliberately pitched at what people actually achieve rather than what the paint is capable of.
Does a second coat use as much as the first?
Usually slightly less, because the first coat has sealed the surface and reduced absorption. On bare plaster the difference can be substantial; on previously painted smooth walls it is small. This calculator assumes equal coats, which errs on the side of having paint left over.
Can I thin paint to make it go further?
Only within limits, and only where the manufacturer says so. Emulsion can be thinned modestly with water — that is exactly what a mist coat on new plaster is — but thinning to stretch a tin reduces the film thickness, the opacity and the durability, and you generally end up applying an extra coat to compensate. It rarely gains anything on the second coat of a finish.
Will a new tin of the same colour match?
Not necessarily. Batch-to-batch variation is normal, particularly with strong or mixed-to-order colours, and can be visible in daylight even when it looks identical in the tin. If you have to add a new tin mid-job, mix old and new together in a larger container first so any difference is spread across the whole area rather than appearing as a line.
How long does opened paint last?
Sealed properly and kept somewhere frost-free, opened emulsion is often usable for a year or two and solvent-based paint rather longer. What kills it is air, which forms a skin, and cold, which can curdle water-based paint permanently. A tin that has spent a winter in an unheated shed is a much poorer bet than its age suggests.
Is it safe to put old paint in the bin?
Not while it is liquid. Water-based paint can be hardened with sawdust, soil or a paint hardener and then usually goes in household waste, though councils vary. Solvent-based paints and thinners are hazardous waste and belong at a household waste recycling centre. Nothing should go down a drain.
Is what I enter stored?
No. Everything is calculated in your browser and nothing is transmitted or retained.
Related tools
References
These organisations publish the guidance behind this page.
- British Coatings Federation — the UK trade body for paint manufacturers, on coverage, storage and leftover paint
- GOV.UK — rules on household and hazardous waste, and disposing of paint legally
- Health and Safety Executive — safe handling, ventilation and storage of paints and solvents
- Approved Documents, GOV.UK — Approved Document F, on ventilation while decorating indoors
External guidance changes. Check the current position at the source before relying on it for a decision that matters.