How many square metres does a litre of paint actually cover?

Plan on roughly 11 square metres per litre per coat on a sound, previously painted wall, the figure Resene uses for a standard low-sheen wall paint, but the exact rate is on your tin, because it is set product by product. Rough, porous, bare or dark surfaces take more paint per square metre, and a big colour change can need extra coats, so plan on less real coverage than the label figure, never more.

The number on the tin is a target for the painter, not a promise about your wall.

Where does the number on the tin come from?

Resene lists it as "Theoretical coverage" on its data sheets and "Coverage" on its labels. The worked example on Resene's paint calculator page uses SpaceCote Low Sheen at 11 square metres per litre, on walls that are previously painted and in good condition. That last phrase is the important part.

The other figure you will hear is 16 square metres per litre. That is the old one, and it does not describe a modern wall paint. According to Resene's explanation of spreading rates, it dates from pre-metric days, when the standard recommendation was 800 square feet per gallon, mostly for enamel paints. Resene says a waterborne paint spread at that rate gives only about 25 microns a coat, and it instead quotes spreading rates that give a film about 35 microns thick per coat.

That gives you the real meaning of the figure. It is the area a litre should cover if it goes on at the thickness the paint was designed for. Stretch it further and the film is thinner than intended. Resene says it plainly: put on too little and you waste time and money on an extra coat.

Why does real life never match the label?

Because the label assumes a well-behaved surface, and most surfaces are not.

• Porous surfaces soak paint in. Bare plaster, new stopping, bare timber and weathered paint all absorb part of the first coat before any of it builds a film. Resene notes porous surfaces will require more paint.
• Rough and shaped surfaces have more area than their measurements. A textured ceiling, rough-sawn timber or corrugated iron has far more real surface than its length times width. For roofs, Resene suggests taking the footprint area and adding about 40 percent to allow for pitch, corrugations and overhangs.
• Colour changes eat coats. Resene's own example: an off-white over an off-white may be acceptable in one coat, while a bright yellow over a dark purple feature wall may take four.
• Application loss. Paint left in the tray, on the roller sleeve and on the drop sheet never reaches the wall.

How much does the surface actually change it?

Timber shows the effect most clearly, because stain manufacturers publish rates for different surfaces of the same product. Resene's data sheet for Woodsman Decking Oil Stain gives these typical rates per litre per coat:

Deck surface	Typical coverage per litre, per coat
New dressed smooth pine	20 m²
New smooth hardwood (e.g. kwila)	20 m²
New hardwood, tread side up	12 m²
Weathered decking, smooth	10 to 12 m²
Weathered decking, tread side up	6 to 7 m²

Same product, same litre, and the coverage drops to around a third between a new smooth board and a weathered grooved one. Walls are less extreme than decks, but the same logic applies: the rougher and thirstier the surface, the further below the label you end up.

Cabot's says much the same thing on its Aquadeck data sheet, which gives approximately 12 square metres per litre depending on the porosity and condition of the timber.

How do I work out litres for a wall?

Resene's formula is simple: surface area divided by spreading rate, multiplied by the number of coats.

1. Measure the area. Length of the wall times its height. For a room, add the lengths of all the walls together and multiply by the ceiling height.
2. Take out what you are not painting. Subtract doors and windows.
3. Look up the spreading rate for the exact product on its data sheet or label.
4. Decide on coats. Two is normal for a repaint. A colour change, fresh plaster or a bare surface may need a primer or sealer first, and sometimes a third coat.
5. Divide, multiply, round up. Or put the numbers into our paint calculator and let it do the sums.

A worked example. A bedroom 4 metres by 3.5 metres with a 2.4 metre stud has 15 metres of wall, so 36 square metres. Take off about 3 square metres for a door and a window and you have 33 square metres. At 11 square metres per litre, one coat is 3 litres and two coats are 6 litres. The ceiling is a separate calculation with its own product.

That 6 litres is the best case. If the walls are patched in places, the old colour is dark or the room has a lot of fresh stopping, allow more. Resene's own example rounds 7.6 litres up to 8, which leaves enough to finish the job and go back for touch-ups later.

What changes when you go dark to light, or light to dark?

Opacity, not area. The spreading rate stays the same. What changes is how many coats it takes before the old colour stops showing through.

Going from a strong or dark colour to a pale one is the classic case. The first coat looks patchy and grey, and people assume they bought bad paint. Usually the right answer is a tinted undercoat or primer rather than coat after coat of topcoat, and that decision is covered in when you need a primer. Going pale to deep can take extra coats too, so ask the supplier whether the colour you have picked usually needs a tinted undercoat.

Should you ever spread it further to save a tin?

No. Overspreading is the most expensive way to save paint. The film ends up thinner than the product was designed for, the wall looks patchy in raking light, and you buy the paint anyway when the extra coat goes on. What looks like two coats of thinly rolled paint can be closer to one proper coat, which is the point made in what two coats actually means.

The honest way to save money is to measure properly, buy the right quantity once, and keep the leftovers sealed for touch-ups.

When is DIY maths good enough?

For a single room repaint in a similar colour, the formula above is all you need, and running short by half a litre just means a trip back to the shop. It gets harder with a whole house, a big colour change, or a mix of bare, patched and painted surfaces, because each area behaves differently.

If you would rather have someone measure it properly and quote the job, get in touch for a free quote.