Paint calculator litres and tins

How much paint you need, worked out

Litres of paint = the area you are painting, times the number of coats, divided by how many square metres one litre covers. The area is the walls less the doors and windows. The calculator runs exactly that sum, the one a decorator scribbles on the back of a quote.

The part people get wrong is the coverage figure. The number printed on a tin is a best case. The calculator starts at 10 square metres per litre for emulsion, a more cautious figure explained below, and you can change it to whatever your tin or your experience says.

Here is a worked example: a through-lounge 5.5m by 4.9m, ceilings 2.4m high, one door and one window, two coats of matt emulsion.

• Perimeter: 5.5 + 4.9 + 5.5 + 4.9 = 20.8m.
• Wall area: 20.8 × 2.4 = 49.92m².
• Less one door (1.5m²) and one window (1.0m²): 49.92 − 2.5 = 47.42m².
• Two coats: 47.42 × 2 = 94.84m².
• At 10m² per litre: 94.84 ÷ 10 = 9.48 litres.
• Tins: one 10L tin, with 0.52 litres left over for touch-ups.
• The ceiling too: answer Yes, same paint, and the calculator adds 5.5 × 4.9 = 26.95m², two coats = 53.9m², another 5.39 litres. The total is 14.87 litres: a 10L and a 5L tin, with only 0.13 litres spare, so buy a 1L as well if the walls are rough.

What a litre of paint really covers

Manufacturers quote coverage as "up to" a figure, and it assumes a smooth, sound, sealed surface in a similar colour. Dulux Matt says up to 14m² per litre and adds that actual coverage depends on the surface. Even the makers' own figures wobble: the Crown Trade Matt Vinyl listing says up to 17m² per litre in its description and up to 14m² per litre on its data sheet.

Real walls are patched, porous and sometimes a very different colour underneath. That is why the calculator defaults to 10m² per litre for emulsion. It is the figure Tikkurila gives for most of its paints, and the one both the Wickes and B&Q calculators use. Use the tin's figure on a smooth, already-painted wall, and 10 or less on anything rough or thirsty.

The manufacturer figures for other finishes, all from the makers' own product pages, are below. Treat each one as the most you will get.

New plaster needs an extra coat

Yes, bare new plaster needs a mist coat before the top coats, and the calculator counts it for you. Fresh plaster soaks up the first coat like a sponge, so a single tin of emulsion goes nowhere near as far on it as the tin suggests.

Dulux's own advice on painting new plaster is to wait, because plaster can take up to 4 weeks to dry out completely, and then put on a mist coat of watered-down emulsion: about ten parts paint to one part water. Or use a plaster sealer instead. The Dulux Matt product page says the same in its own words: on a new, bare, absorbent surface, seal with the paint thinned 10% with water.

In the calculator, answer Yes, add a mist coat to Bare new plaster? and leave coats at 2. It adds the mist coat as one extra coat, so the lounge above goes from 9.48 to 14.23 litres: a 10L and a 5L tin. The mist coat is slightly thinned, but bare plaster drinks it, so counting it as a full coat is a fair allowance and any spare goes on touch-ups.

Ask about the plaster before you quote. A room skimmed last week is a different job from one painted ten years ago, in both paint and drying time.

How to measure a room for paint

For a plain rectangular room, choose A whole room and enter the length, the width and the height from floor to ceiling. The calculator doubles length plus width to get the perimeter and multiplies by the height. Skirting and coving take a little off the true area, so leaving them in gives you a small margin.

Doors and windows come off the total. Each door counts as 1.5m², which is a 762mm by 1981mm door: Beesley & Fildes gives 1981mm as the usual UK door height and 762mm as one of the two most popular widths. For windows you pick Standard (about 1m²) or Large or patio doors (about 4m²), the two presets Tikkurila uses. The size you pick applies to every window in the count, so for a room with two small windows and a set of patio doors, choose Standard and enter 6: two, plus four for the patio doors.

Small openings are worth thinking about. Cutting in around a little window uses paint too, so if you are unsure, take off less rather than more.

• Chimney breasts and alcoves: walk the tape round the room, into each alcove and round the breast, then choose One wall and enter that total as the wall width. The sides add up. That option leaves out the ceiling, so run the ceiling separately.
• Sloping ceilings and stair walls: run each one as One wall. For a triangle, enter half the height, which gives half the rectangle.
• Feature wall in a different colour: run it as its own calculation so you buy the right tin of each.
• What the calculator does not include: woodwork (skirting, door frames, the doors themselves), primer, undercoat, or a wastage allowance beyond rounding up to whole tins.

Which tins to buy, and what is left

The calculator rounds your litres up to whole 1L, 2.5L, 5L and 10L tins and shows what will be left over. It takes the fewest tins it can, as long as that buys no more than about a litre beyond the closest fit. So 9.48 litres is one 10L tin, not a 5L, a 2.5L and two 1L tins. Bigger tins usually cost less per litre, so that is normally the better buy too.

Check what your paint actually comes in. Ranges differ: Dulux Matt is sold in 2.5L, 3L, 5L, 6L, 7L, 7.5L and 10L, while Quick Dry Satinwood stops at 2.5L. The calculator only offers the four common sizes, so if your paint comes in 3L or 7.5L, take the litres figure and pick the tins yourself.

A strong colour change takes more coats than the usual two, and the Dulux product pages say so. Going from dark red to off-white, budget a third coat rather than running out on the last wall.

Keep the leftover. A labelled tin with the room and the date saves a match later when a picture comes down or a chair scuffs the wall. On a job, hand it to the customer: they paid for it, and it saves you a callback for touch-ups.

Using it to price a decorating job

For a decorator, paint is the line on the quote that is easiest to get exactly right and most often guessed. Run each room through the calculator, write the litres and tins against it, and the customer can see you have measured rather than rounded.

Price the coats you will actually put on. If the plaster is new or the colour change is big, say so on the quote and price the extra coat, because that conversation is easier before the job than halfway through it.

Labour is the bigger number. The day rate calculator works out what a day needs to earn once your costs and unpaid days are counted, and the quote template turns the lot into a clean A4 quote with what is included and what is not.

Common questions