Paint Calculator m2: How Much Paint Do I Need?

This paint calculator works out how many litres of paint you need for a room or wall based on the total area in square metres (m2) and the coverage rate of the paint. You enter the surface area (or the length, width and height of the room), the number of coats, and the paint’s coverage in m2 per litre, and it returns the exact litres required. The result is a precise volume, so you can buy the right number of tins without guesswork.

Formula

The core formula is straightforward:

Paint needed (litres) = (Total surface area in m2 × Number of coats) ÷ Paint coverage (m2 per litre)

Variables:

• Total surface area (m2): the sum of all wall or ceiling areas you plan to paint, measured in square metres.
• Number of coats: how many layers of paint you will apply. Usually 2 for a colour change, 1 for a same-colour touch-up.
• Paint coverage (m2 per litre): the manufacturer’s stated spreading rate, typically 10–12 m2 per litre for emulsion paints.

The result is in litres. To convert to millilitres, multiply by 1000. To estimate tins, divide the litres by the tin size (e.g., 2.5 L, 5 L, 10 L) and round up.

Worked Example

Using the default values from the calculator above:

• Total surface area = 35 m2
• Number of coats = 2
• Paint coverage = 10 m2 per litre

Step 1: Multiply area by coats: 35 × 2 = 70 m2 (effective area to cover).

Step 2: Divide by coverage: 70 ÷ 10 = 7.0 litres.

So you need exactly 7.0 litres of paint. If you buy 2.5 L tins, you will need 3 tins (7.0 ÷ 2.5 = 2.8, rounded up to 3).

How to Measure Your Room’s Area

Most people don’t know the exact m2 of their walls offhand. Here’s how to get it:

1. Measure the length and width of the floor in metres (use a tape measure or laser measure).
2. Measure the ceiling height (usually about 2.4 m in UK homes, 8 ft in US homes).
3. Calculate the perimeter: 2 × (length + width). Multiply by height to get the total wall area.
4. Subtract the areas of windows and doors (each roughly 1.5–2 m2 for a window, 1.8–2 m2 for a door).
5. For the ceiling, multiply length × width.

Example: a 4 m × 3 m room with 2.4 m ceiling has walls of 2 × (4+3) × 2.4 = 33.6 m2. Subtract a door (2 m2) and a window (1.5 m2) → 30.1 m2 of wall. The ceiling adds 12 m2 if you’re painting it.

What Coverage Rate Should You Use?

Paint coverage is not a fixed number. It depends on:

• Surface type: smooth plaster absorbs less than bare plaster or old matt paint. Porous surfaces can need 20–30% more paint.
• Paint quality: premium paints often have higher solids and cover better, but cheap paints may need more coats.
• Colour change: going from dark to light or light to dark usually requires a primer or an extra coat.
• Application method: a roller uses less paint than a brush; spraying can waste more due to overspray.

Most manufacturers state a coverage of 10–12 m2 per litre for one coat. For a safe estimate, use 10 m2 per litre. If the surface is rough or very absorbent, drop to 8 m2 per litre.

Common Mistakes and How to Avoid Them

• Forgetting to subtract windows and doors: these can reduce the area by 10–15%, saving you a litre or more.
• Using the floor area instead of the wall area: a 20 m2 floor does not mean 20 m2 of wall; walls are usually 3–4 times larger.
• Not accounting for multiple coats: one coat of paint rarely gives full coverage. Always multiply by the number of coats.
• Rounding down tins: always round up to the nearest tin size. Running out mid-wall is a pain.
• Ignoring the primer: if you need a primer, that’s separate paint and separate coverage.

How to Read the Result and Buy Paint

The calculator gives you the exact litres, but paint is sold in tins (e.g., 1 L, 2.5 L, 5 L, 10 L). You need to buy enough tins to cover the litres, rounding up. For the example above (7.0 L), you could buy one 5 L tin and one 2.5 L tin (total 7.5 L) or three 2.5 L tins (7.5 L). It’s always wise to have 10% extra for touch-ups and mistakes.

If you’re painting a single wall, measure just that wall’s width and height. For a full room, add all walls. For a ceiling, use floor dimensions. The calculator works for any surface, including wood, metal, or exterior walls, as long as you know the coverage rate for that specific paint.

Limits of the Formula

This formula is a mathematical estimate. Real-world paint usage varies with:

• Surface porosity and texture (rough plaster, brick, or wallpaper absorb more).
• Paint viscosity and thickness of application.
• Temperature and humidity (paint dries faster in heat, which can affect coverage).
• Whether you use a primer or undercoat.

Always read the paint can for the manufacturer’s coverage rate. If you’re between two tin sizes, buy the larger one. It’s better to have a little extra than to run out.

Frequently Asked Questions

How many m2 does 1 litre of paint cover?

On average, 1 litre of emulsion paint covers about 10–12 m2 with a single coat on a smooth, primed surface. For rough or porous surfaces, it may cover only 6–8 m2. Always check the can for the exact figure.

Do I need to paint two coats?

Usually, yes. Two coats give a uniform, durable finish and are essential when changing colours or painting over bare plaster. One coat might suffice for a same-colour touch-up on a well-primed wall, but it’s rarely enough for a full repaint.

How do I calculate the area of a room for painting?

Measure the length and width of the floor, then the ceiling height. Multiply the perimeter (2 × (length + width)) by the height to get the wall area. Subtract windows and doors. For the ceiling, multiply length × width. Add the wall and ceiling areas together if painting both.

What if my walls are textured or rough?

Rough surfaces (e.g., Artex, brick, or textured wallpaper) have a larger actual surface area than the flat measurement. Increase the paint estimate by 20–30% or reduce the coverage rate to 8 m2 per litre. A thick-nap roller helps apply paint evenly.

Should I use a primer before painting?

Primer is recommended for bare plaster, new drywall, or when covering a dark colour with a light one. It seals the surface, reduces paint absorption, and can actually save you paint because the topcoat goes on more evenly. Primer has its own coverage rate, so calculate it separately.

Is it better to buy more paint than the calculator says?

Yes, add 10–15% extra for touch-ups, cutting in, and mistakes. Paint can also be stored for future repairs, so a little extra is practical. Just avoid buying far more than needed, as leftover paint has a limited shelf life (usually 2–5 years).

Related

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in area | Total surface area | m2 | 35 | 0
in coats | Number of coats | - | 2 | 1
in cov | Paint coverage | m2 per litre | 10 | 1
out litres | Paint needed | litres | (area * coats) / cov | 2
out tins | 2.5L tins needed | tins | ceil(((area * coats) / cov) / 2.5) | 0