What is a paint calculator?
A paint calculator helps you estimate how much paint to buy before starting a decorating job. You enter the wall area you want to cover, the paint’s coverage rate, and the number of coats, and the calculator returns the volume of paint needed in litres. It saves you from guessing, so you avoid both running short halfway through and buying far more than the job requires.
How the calculator works
The estimate rests on three inputs:
- Wall area — the surface you intend to paint, in square meters or square feet.
- Coverage rate — how far one litre of paint goes, expressed in square meters per litre (m²/L).
- Number of coats — how many layers you plan to apply.
Multiply the area by the number of coats, then divide by the coverage rate, and you have the litres required.
Paint formula
If you enter the wall area in square feet, the calculator converts it to square meters before applying the formula. The conversion is:
Understanding the coverage rate
Coverage rate, sometimes called spreading rate, tells you how much surface a single litre of paint will cover in one coat. Most wall paints cover somewhere between 6 and 14 square meters per litre, and the figure is usually printed on the tin. The default of 10 m²/L is a reasonable middle value for a typical emulsion on a smooth, primed wall.
The real coverage you get depends on several things:
- Surface texture: rough, porous, or unpainted surfaces soak up more paint and reduce coverage.
- Paint type: thick or high-opacity paints often cover less area per litre than thin ones.
- Application method: rollers and brushes can apply paint more thickly than sprayers.
- Colour change: covering a dark wall with a light colour can lower effective coverage.
When in doubt, use the figure printed on your specific paint can rather than the default.
Why coats matter
A single coat rarely gives an even, opaque finish, especially over a different base colour or a fresh, porous surface. Two coats are standard for most interior walls, and you may need a third when making a dramatic colour change or covering stains. Because each coat uses roughly the same amount of paint, doubling the coats roughly doubles the paint you need.
This is an estimate
Treat the result as a planning figure rather than an exact requirement. Coverage rates vary with surface and technique, and some paint is always lost on rollers, trays, and trim. It is sensible to round up and buy a little extra so you have enough to finish the job and keep some for touch-ups later. Buying paint from the same batch also helps the colour stay consistent across the wall.
Worked examples
Example 1: two coats on a 40 m² wall
With a coverage rate of 10 m²/L and two coats:
You would need about 8 litres of paint to give the wall two even coats.
Example 2: one coat on a 50 m² wall
With a coverage rate of 12 m²/L and a single coat:
About 4.17 litres covers the wall once, so a 5-litre can would be enough with a little to spare.
Frequently asked questions
How much paint do I need per square meter?
It depends on the coverage rate of your paint. At a typical 10 m²/L, one litre covers about 10 square meters in a single coat, so you need roughly 0.1 litres per square meter per coat. Check the tin for your paint’s exact figure.
How many coats of paint should I apply?
Two coats are standard for most interior walls and give an even, durable finish. A single coat may be enough for a refresh in the same colour, while a third coat helps when covering dark colours, stains, or very porous surfaces.
Can I enter the wall area in square feet?
Yes. The calculator converts square feet to square meters automatically using 1 m² ≈ 10.7639 ft² before working out how much paint you need.
Why does the estimate not match what I actually used?
Coverage rates are averages. Rough or porous surfaces absorb more paint, application method changes how thickly paint goes on, and some paint is always lost on rollers and trays. Buy a little extra and keep the leftovers for touch-ups.
Should I round the result up?
Yes. Round up to the nearest can size and add a small margin so you do not run out mid-job. Leftover paint is useful for repairs, and buying from one batch keeps the colour consistent.
Need a different construction estimate? Try the BTU calculator to size an air conditioner, or the area calculator to work out the surface you are covering. This paint calculator lives at https://www.mega-calculator.com/construction/paint/.