Paint estimating is area × coats ÷ coverage. The skill is in choosing a realistic coverage figure for the surface in front of you, and in remembering the parts of a room that aren't flat walls: ceilings, woodwork, doors and the primer under it all.
Measuring a room
Add the lengths of all the walls to get the perimeter, then multiply by the wall height. Take off doors and large windows. Ceilings are length × width. For stairwells and sloped ceilings, split the surface into rectangles and triangles. Measure the woodwork separately: skirtings and architraves in running metres, and doors by count.
Coverage rates in the real world
| Surface | Typical coverage |
|---|---|
| Smooth, previously painted wall | 12–16 m²/L |
| New plasterboard (after primer or mist coat) | 10–12 m²/L |
| Textured ceiling, rough render | 6–9 m²/L |
| Brick, block, bagged render | 4–7 m²/L |
| Bare or weathered timber | 8–10 m²/L |
The tin's figure is a best case, measured on a smooth, sealed surface with a fresh roller. Assume a bit less, especially on your first job with a new product.
How many coats
- Same colour refresh: often one coat, but two gives a more even finish.
- Colour change: two coats, and three for strong colours or dark to light. A tinted primer can save a coat.
- New plaster or board: a coat of sealer undercoat, then two finish coats.
- Patched walls: spot-prime the repairs first, or they'll show through as dull patches ("flashing").
Deductions
Deduct doors and large windows. Don't bother with small openings: cutting in around them uses as much paint as the area you'd save.
Woodwork and trim
Doors, frames and skirting take a different product (enamel in semi-gloss or gloss) and are often missed. As a rough guide, one litre covers the trim of a typical bedroom with two coats. Allow about 0.5 litre per door, both sides, for two coats.
Spraying vs rolling
Airless spraying uses 20–30% more paint than rolling because of overspray, but it's much faster on large, empty areas such as new builds and ceilings. Masking takes longer, so spraying pays off most on big, open jobs.
Exterior work
Outside, the surface makes a huge difference. Rough render, bagged brick and brick can use twice the paint of smooth siding or boards. Gable ends are triangles: ½ × width × height. Soffits and fascias are extra, and so are the access costs: ladders, towers or scaffold. Weather windows matter too. Most paints need dry conditions and a minimum temperature to cure properly.
Buying the right sizes
Paint is sold in 1 L, 4 L, 10 L and 15 L tins. Bigger tins are cheaper per litre, and mixing several tins together (boxing) keeps the colour consistent across a large wall. Our calculator picks a sensible combination so you don't overbuy.
A worked example
A 4.2 × 3.6 m bedroom with 2.4 m walls, one door and two windows: walls 32.5 m², ceiling 15.1 m². Two coats at 14 m² per litre: 4.6 L for the walls and 2.2 L for the ceiling. Buy a 4 L and a 1 L tin for the walls, a 4 L of ceiling white, and a 1 L of semi-gloss enamel for the woodwork.
Prep is the real cost
On most repaint jobs, preparation (filling, sanding, caulking, masking, protecting floors and furniture) takes longer than painting. Price it as its own labour line, and the client will understand why two painters quote very different prices.
Use the paint calculator for rooms and the exterior paint calculator for outside walls with gables and textured surfaces.
Common questions
How much paint for one wall?
Multiply the wall's width by its height, double it for two coats, and divide by the coverage. A 3.6 × 2.4 m wall with two coats at 14 m² per litre needs about 1.2 litres, so buy two 1 L tins, or a 4 L tin if you want some left for touch-ups.
Do I need primer?
On new plaster or board, bare wood, stained areas and big colour changes, yes. On a sound, previously painted wall in a similar colour, usually not.
Checklist before you send the price
- Walls, ceilings and woodwork measured separately
- Coverage set for the real surface, not the tin's best case
- Primer or mist coat where needed
- Number of coats agreed, especially for colour changes
- Preparation, protection and clean-up priced as labour
- Access equipment for high walls and stairwells
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.