What this planner works out
This planner calculates the wall area of a room, deducts allowances for doors and windows, and works out the paint volume, number of tins and cost for the number of coats you choose.
It covers walls only. The ceiling is a separate area and is not included, which is the most common reason a paint order comes up short.
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Before you measure
- Measure the room rather than trusting the plans. Rooms are rarely exactly square and older houses are rarely close.
- Deductions are fixed at 1.6 m² per door and 1.2 m² per window, which are reasonable averages. Large patio doors or bay windows need measuring separately and subtracting yourself.
- Coverage per litre is on the tin, typically 10 to 14 m² for emulsion. Bare plaster, textured surfaces and strong colour changes all reduce it.
- Two coats is standard. Allow three when covering a dark colour with a light one, or painting bare plaster.
New plaster is porous and will drink undiluted emulsion, leaving a patchy finish that peels. A mist coat — emulsion thinned with around 10% water — seals it. Count that as an extra coat when planning quantities, and note that it uses less paint per coat than a full one.
How this calculator works
Perimeter, height, deductions, then volume:
Wall area = 2 × (length + width) × height
Less (doors × 1.6) and (windows × 1.2)
Litres = wall area × coats ÷ coverage per litre
Tins = litres ÷ tin size, rounded upTins are always rounded up, because you cannot buy part of one. That rounding is why the tin size matters: the same job may need three 2.5 litre tins or one 5 litre tin plus a small one, at noticeably different prices.
The cost shown is paint only. Brushes, rollers, trays, filler, sandpaper, dust sheets and masking tape typically add £30 to £60 to a first decorating job.
Worked example: a 4 m by 3 m room
Using the default figures — 4 m by 3 m, 2.4 m high, one door, two windows, two coats, 10 m² per litre, 2.5 litre tins at £24:
- Wall area: 2 × (4 + 3) × 2.4 = 33.6 m², less 1.6 for the door and 2.4 for the windows = 29.6 m²
- Paint needed: 29.6 × 2 ÷ 10 = 5.92 litres
- Buy: 3 tins of 2.5 litres
- Paint cost: £72
Note that three 2.5 litre tins is 7.5 litres for a 5.92 litre job — over a litre and a half spare. A single 5 litre tin plus one 2.5 litre would be the same volume, but a 5 litre tin is usually cheaper per litre than two 2.5s. Checking the price per litre before buying often saves more than the calculation itself.
Common mistakes
- Forgetting the ceiling. It is not included here and typically needs a different paint.
- Using the tin's coverage figure on bare plaster. Porous surfaces absorb considerably more.
- Planning two coats over a dark colour. Strong colours usually need three, or a tinted primer.
- Buying exactly enough. Always leave a margin; a return trip mid-job is worse than a spare half tin.
- Not keeping the batch number. Colours vary slightly between batches, so buy all the paint at once.
Frequently asked questions
How much paint do I need for the ceiling?
The ceiling area is length × width — 12 m² for the room in the example. At 10 m² per litre and two coats, that is another 2.4 litres. Ceiling paint is usually a separate matt product formulated to reduce drips and hide imperfections, so it needs buying separately rather than sharing a tin with the walls.
Do I need to prime?
New plaster needs a mist coat. Bare wood, metal and filled patches need an appropriate primer or they will show through. Previously painted walls in sound condition generally do not, though a stain-blocking primer is worth using over water marks, nicotine or crayon, all of which bleed through emulsion repeatedly.
What is the difference between paint finishes?
Matt hides surface imperfections and is the usual choice for walls and ceilings. Eggshell and satin have a slight sheen and are more wipeable, which suits kitchens, bathrooms and hallways. Gloss and satinwood are for wood and metal. Durability generally rises with sheen, and so does how much the wall's flaws show.
How long between coats?
The tin will state a recoat time, typically two to four hours for emulsion. Cold and damp both extend it considerably — the Paint Drying Time Calculator applies an adjustment. Recoating too early lifts the first coat and produces a worse finish than waiting.
Is what I enter stored?
No. Room measurements and prices are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — guidance on decorating waste disposal and VOC regulations
- Health and Safety Executive — safe working at height and ventilation when using paints and solvents
Sources are checked at publication and can change — how I choose and check references.