Paint is easy to over-buy and annoying to under-buy: a second trip for one more tin, and a slightly different batch colour on the last wall. Working it out properly takes a few minutes. This guide walks through the area, the coats and the tin sizes, using spread rates from Plascon’s product data.
1. Work out the area
For a room, wall area = perimeter × height − doors and windows. The perimeter is 2 × (length + width). Measure the ceiling separately, as length × width, because it often takes a different paint. For the outside of a house, wall area = perimeter × eaves height, plus each gable (half the gable width × its height), less doors and windows.
2. Know the spread rate
A spread rate tells you how many square metres one litre covers in one coat on a smooth, sealed surface. Plascon gives a theoretical spread rate of 12 m² per litre for both Double Velvet, an interior paint, and Wall & All, an exterior paint, and 9 m² per litre for its Natural Deck Coating. Real coverage is lower on rough, porous or textured surfaces, and on new plaster, so treat the published rate as the best case.
Formula: litres = area × coats ÷ spread rate
3. Decide the coats
Two coats is normal for a repaint in a similar colour. A big change in colour, especially dark to light, usually needs a third. New plaster needs a plaster primer first: Plascon sells a Plaster Primer for this, and it soaks into the fresh surface so the top coats go on evenly.
4. Buy the right tins
Plascon’s Double Velvet and Wall & All come in 1, 5 and 20 litre tins, and Natural Deck Coating in 5 litres. Buy the biggest tins that suit the job, and a 1 L tin or two for the remainder rather than a whole extra 5 L. For large areas in one colour, buying all the paint at once from one batch avoids slight colour differences between tins.
Worked example: a bedroom
A room 3.6 × 3.6 m with 2.4 m walls, one door (about 1.9 m²) and two windows (about 1.5 m² each):
- Walls = 2 × (3.6 + 3.6) × 2.4 − 1.9 − 3.0 = 29.66 m².
- Two coats at 12 m² per litre = 29.66 × 2 ÷ 12 = 4.94 litres, so one 5 L tin.
- Ceiling = 3.6 × 3.6 = 12.96 m²; two coats = 2.16 litres, so three 1 L tins (or a 5 L tin if you want spare).
The paint calculator does this for any room and picks the tins.
Worked example: outside walls
A house with a 40 m perimeter, 5 m to the eaves, two gables 8 m wide and 2.5 m high, and 12 m² of doors and windows:
- Walls = 40 × 5 + 2 × ½ × 8 × 2.5 − 12 = 208 m².
- Two coats at 12 m² per litre = 34.7 litres: one 20 L tin and three 5 L tins.
Rough or textured plaster and bagged brick take more paint; the exterior paint calculator lets you choose the surface and increases the area for texture.
Decks
For a deck, include both faces of any railings and the steps. At Plascon’s 9 m² per litre for deck coating, a 4.8 × 3.6 m deck with railings and steps typically takes two 5 L tins for two coats. The deck coating calculator works it out.
Tips
- Fill and sand cracks before painting; paint shows every flaw.
- Keep a little paint from each colour, labelled with the room, for touch-ups.
- Check the spread rate on the tin you buy; it varies between products.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.