Understanding the topic
Paint estimates begin with surface area, not floor area. You need the width and height of each wall, the number of coats and the coverage rate printed on the paint tin. Coverage varies with the product, texture and application method.
Quick reference
Use these reference points alongside the worked examples below.
| Term | Meaning / value | Why it matters |
|---|---|---|
| Wall area | Width × height | Calculate each wall separately |
| Total area | Sum of wall areas | Before subtracting large openings |
| Coats | Usually more than one | Multiply the paintable area |
| Coverage | m² per litre | Use the manufacturer’s stated rate |
| Allowance | Extra for waste and touch-ups | Adjust for surface and job |
Step-by-step method
- Measure the width and height of each wall in metres.
- Multiply each width by height and add the areas.
- Subtract significant door and window areas if appropriate.
- Multiply by the number of coats.
- Divide by the coverage rate and allow for waste, then select tin sizes.
Worked examples
Example 1: Four-wall room
A 4 m × 3 m room with 2.4 m walls has perimeter 14 m and wall area 14 × 2.4 = 33.6 m² before openings.
Example 2: Two coats
If paintable area is 30 m², two coats cover 60 m² of surface.
Example 3: Paint quantity
At 12 m²/L, 60 m² ÷ 12 = 5 L before any extra allowance.
Common mistakes and useful cautions
- Do not use the floor area as the wall area.
- Rough or porous surfaces may use more paint than the label estimate.
- A dramatic colour change or primer requirement can change the number of coats.
Practice questions
Try each question before revealing the answer.
Question 1: A 5 m wall is 2.5 m high. What is its area?
Answer: 12.5 m².
Question 2: Two coats on 20 m² cover how much total area?
Answer: 40 m² of coating area.
Question 3: At 10 m²/L, how many litres for 40 m² before waste?
Answer: 4 L.
What to remember
- Measure the width and height of each wall in metres.
- Divide by the coverage rate and allow for waste, then select tin sizes.
- Do not use the floor area as the wall area.