Paint quantity:
wall area, coverage and coats

The formula, with a margin for waste

Paint quantity depends on only three things: how much surface you are covering, how far one litre of the product spreads, and how many coats you plan to apply. A large house exterior of 120 m² with paint rated at 10 m² per litre per coat needs 12 litres for one coat and 24 litres for two.

Most mistakes come from the first number. People guess floor size instead of wall area, forget the ceiling, or ignore doors and windows. Measure the walls themselves.

Coverage is printed on the tin, usually as a range such as 10 to 14 m²/L. Use the lower end for rough or porous surfaces and the upper end for smooth, sealed ones. Quoted figures assume ideal application, so a 10% allowance for spillage, tray loss and touch-ups is realistic.

Worked example: a 5 m × 4 m bedroom

The room is 2.7 m high, with one door of 2.0 m × 0.9 m and one window of 1.5 m × 1.2 m. The paint covers 12 m² per litre per coat and you want two coats.

If you also paint the 20 m² ceiling with the same product, that adds 20 ÷ 12 × 2 = 3.33 L before waste.

What to subtract and what to add

• Subtract doors and windows, or ignore them if they are small, which builds in a margin.
• Add the ceiling, skirting, door frames and window reveals if you are painting them.
• Add an extra coat's worth for a dramatic colour change; dark to light often needs three.
• Add a primer or undercoat quantity separately, because it has its own coverage rate.
• Subtract very large areas of tile, glass or built-in wardrobes.

Choosing tin sizes

Paint comes in standard sizes, commonly 1, 4, 10 and 20 litres, though they vary by brand and country. Buy to the next size up rather than to the exact figure, and keep any leftover for touch-ups. Where several tins are needed for one wall, mix them together or from the same batch number so slight colour variations do not show as a stripe.

Larger tins are cheaper per litre but are heavy and prone to waste if left half full. For 8.25 L, a 10 L tin is sensible, or 4 L + 4 L + 1 L if you would rather not carry one heavy tin.

Exterior walls, gables and awkward shapes

For a house exterior, treat each face as a rectangle and add triangles for gables. A gable is half base times height. A 6 m wide gable rising 2.5 m has an area of 0.5 × 6 × 2.5 = 7.5 m², which is easy to forget.

Add the faces: two long walls of 10 m × 3 m give 60 m², two short walls of 6 m × 3 m give 36 m², and two gables of 7.5 m² each give 15 m², a total of 111 m². Subtract windows and doors of about 14 m² and about 97 m² remain, which at 10 m²/L is 9.7 L per coat. Masonry paint on rough render may cover 5 to 7 m²/L on the first coat, so the real quantity could be nearly double. Test a small patch to find your own coverage before ordering.

Cost per square metre rather than cost per tin

A cheaper tin is only cheaper if it covers the same area and hides the colour in the same number of coats. Compare products on cost per square metre per coat, then multiply by the coats you expect.

The budget paint looks 30% cheaper per tin but, with a third coat needed, would cost more per finished square metre than the mid-range option. Hiding power, washability and finish quality also belong in the comparison.

How preparation and application change the amount

Two painters can use quite different quantities on the same room. Preparation matters first: filling cracks, sanding, cleaning off grease and applying a stain-blocking primer on patched areas let the finish coat cover evenly instead of soaking in. Bare, patched or dusty walls drink paint.

Application is the second factor. A roller with a thick nap uses more paint on a textured surface, while a spray gun wastes a considerable fraction as overspray. Thinning, where the manufacturer allows it, stretches the paint but reduces hiding power. Working from a decanted bucket rather than the tin stops the rim clogging and avoids contaminating the rest of the tin. Wet edges applied quickly give a more uniform finish and save the extra coat that patchy joins sometimes demand.

Mistakes that leave you short

• Using the tin's single-coat coverage and forgetting there will be two coats.
• Measuring in feet and dividing by metric coverage, or the reverse.
• Ignoring the porosity of the substrate.
• Trusting the manufacturer's best-case coverage on a rough wall.
• Buying exactly the calculated amount, with no margin for tray and roller loss.