Paint Calculator

How the paint calculator works

The calculator divides the area you are painting by how far one unit of paint goes, then adds a waste allowance:

Units needed = area (m²) ÷ coverage per unit (m²) × (1 + waste % ÷ 100)

A "unit" is whatever you choose to measure coverage in. If you enter coverage per litre, the answer is in litres. If you enter coverage per 4 L tin, the answer is in tins. The result isn't rounded, so round up to the next litre or tin when you buy.

Step 1: measure the area

For walls, add up the room's perimeter and multiply by the ceiling height. Then subtract the openings: a standard door is about 1.7–1.9 m², and a typical window about 1.5–2 m². Multiply by the number of coats, because the calculator works on total area painted.

Worked example: a 4 m × 3.5 m bedroom

1. Perimeter: 4 + 3.5 + 4 + 3.5 = 15 m
2. Wall area: 15 × 2.7 m ceiling = 40.5 m²
3. Less one door (1.8 m²) and one window (2.0 m²): 36.7 m²
4. Two coats: 36.7 × 2 = 73.4 m²
5. Coverage 12 m² per litre, waste 10%: 73.4 ÷ 12 × 1.10 = 6.7 litres

So you'd buy two 4 L tins (8 L). The spare litre is useful for touch-ups later, and it saves a trip back to the shop.

What coverage figure should I use?

Use the spread rate printed on the tin. Most interior acrylics state about 12–16 m² per litre per coat on smooth, sealed surfaces. Use the lower end, or even 8–10 m²/L, for:

• fresh plasterboard or render (seal or undercoat first)
• rough, textured or porous surfaces like bagged brick
• big colour changes, especially going from dark to light

Common mistakes

• Forgetting the second coat. Almost every job needs two.
• Including ceilings in the wall area. Ceilings usually take a different paint, so calculate them separately (the ceiling area is length × width).
• Using the best-case coverage from the tin on a surface that will drink paint.
• Skipping primer on bare surfaces, then needing an extra top coat anyway.

Frequently asked questions

How much paint for a standard room?

A typical 3 m × 4 m bedroom with a 2.4 m ceiling needs about 6 L for two coats on the walls. Allow around 2 L more for the ceiling.

How much waste should I allow?

10% is sensible for rollers and brushes. Allow 15–20% for spraying, which loses more to overspray.

Can I use this for exterior walls or fences?

Yes. Just use the exterior product's spread rate, which is often lower on rough cladding or timber.

Written by Sam Alex, civil engineer. About the author