Area, coats, coverage
Paint quantity comes from three numbers: the area to be painted, the number of coats, and the coverage of the paint in square metres per litre per coat. Multiply area by coats, divide by coverage, and you have litres. Then add a bit for touch-ups later, because the same tint from a different tin never quite matches a wall that has been on for two years.
litres = area (m²) × coats ÷ coverage (m²/L) × (1 + touch-up allowance)
Coverage is printed on every can and it is the number to trust over any rule of thumb. Quality interior wall paints on a smooth, sealed surface sit around 14 to 16 m²/L. Cheaper paints, textured surfaces, and strong colour changes all pull that down. Sealers and undercoats on new plaster are lower again, 10 to 12, because the surface drinks them.
Measuring the room
Walls are the perimeter of the room times the wall height. Deduct the doors at about 1.7 m² each and the windows at their actual size. Do not deduct the small stuff, power points, a fireplace; the touch-up allowance covers it. The ceiling is length times width. Trim is the awkward one: a door has two faces and a frame, roughly 4.2 m² of surface once you count the edges, and skirting and architraves add a band around the room. The calculator estimates that band at 250 mm high, which suits standard 90 mm skirting and 65 to 90 mm architraves.
| Room | Net walls | Ceiling | Wall paint, 2 coats | Ceiling paint |
|---|---|---|---|---|
| Bedroom 3 × 3, 2.4 m high | 24.6 m² | 9 m² | 3.9 L | 1.4 L |
| Bedroom 4 × 3.5, 2.4 m high | 31.8 m² | 14 m² | 5.0 L | 2.2 L |
| Living 6 × 5, 2.7 m high | 52.4 m² | 30 m² | 8.2 L | 4.7 L |
| Hallway 8 × 1.2, 2.7 m, 4 doors | 42.9 m² | 9.6 m² | 6.7 L | 1.5 L |
Figures use 14 m²/L, one door and 2.5 m² of window unless stated, and a 10 percent touch-up allowance.
Coats
Two top coats is the standard over a sound surface in a similar colour. Going from a dark colour to a light one, or onto a strong colour, often needs three, or a tinted undercoat first, which is cheaper than a third coat of the finish. New plasterboard needs a sealer before any of that, one coat over everything, and the calculator adds it when you choose the new plasterboard option. Ceilings normally take two coats of a flat ceiling white, which hides roller marks better than any other finish.
Cans
Paint in Australia commonly comes in 1, 4, 10 and 15 L cans, and some ranges in 2 and 6 L. Per litre, the big cans are much cheaper, so the calculator shows the litres as 10 L cans plus 4 L cans to make up the remainder. If the remainder is small, a 1 L or 2 L may be better value than a 4 L; if it is over about 8 L, the next 10 L is.
Worked example
A bedroom 4 × 3.5 m, 2.4 m high, one door, 2.5 m² of window, new plasterboard, ceiling and trim included. Walls are 15 × 2.4 = 36 m² less 1.7 and 2.5, so 31.8 m². Two coats at 14 m²/L with 10 percent over is 5.0 L, so a 4 L and a 1 L, or a 6 L. Ceiling 14 m², 2.2 L, a 4 L can. Sealer for 45.8 m² at 11 m²/L is 4.6 L, so a 4 L and a 1 L. Trim is about 8 m², 1.5 L of enamel for two coats, a 2 L or a 4 L.
Common questions
How many m² does a litre of paint cover?
About 14 to 16 m² per coat for interior wall paint on a sealed surface. Sealers 10 to 12. Check the can.
How much paint for a 4 × 3 room?
About 4.3 L for two coats on the walls and 1.7 L for the ceiling.
Do I need sealer on new plasterboard?
Yes, one coat, or the joints flash through the top coats.
How many coats?
Two over a similar colour. Three, or a tinted undercoat, for a big colour change.
Quantities only. Use the coverage printed on the can you are buying, and the manufacturer's system for the surface.