How much paint do I need per square metre?

It depends on the paint. A typical matt emulsion covers about 12 to 13 square metres per litre on one coat, gloss covers more (around 16), and rough exterior masonry far less (around 6 to 7). This calculator divides your paintable area by the coverage you enter, so always check the figure printed on the tin.

How does the calculator handle doors and windows?

Each opening is multiplied by its count and subtracted from the gross wall area before any paint is worked out. A standard 0.9 m by 2.0 m door removes about 1.8 square metres, and a 1.2 m by 1.2 m window removes about 1.44 square metres each.

How many coats of paint should I use?

Two coats is standard for a durable, even finish on most interior walls. Use a primer or undercoat first on bare plaster or when covering a dark colour, and you may need three coats when a light colour goes over a strong one. Each coat multiplies the paintable area, so set the coat count honestly.

Why does it round up to whole tins?

Paint is sold in fixed tin sizes such as 2.5 or 5 litres, so the calculator rounds the litres up to the next whole tin at the size you set. It shows both the exact litres needed and the rounded purchase quantity so you can see the gap.

Should I add extra for wastage?

Yes. Roller absorption, touch-ups, spills and keeping a little for future repairs mean real jobs use more than the bare theoretical figure. A 5 to 10 percent wastage margin is sensible; the calculator adds whatever percentage you choose.

Can I use feet instead of metres?

Yes. Switch the units to imperial and enter widths and heights in feet. The tool converts every dimension behind the scenes, keeps coverage consistent, and even shows the equivalent coverage in square feet per US gallon.

Paint Calculator — Gera Tools

This paint calculator tells you exactly how much paint to buy for a room or wall: how many litres, how many whole tins, and roughly what it will cost. Instead of guessing, you add each wall, subtract the doors and windows you are not painting, set how many coats you want, and enter the coverage printed on your tin. The result updates live as you type, so you can compare one coat against two, or a cheap paint against a higher-coverage one, before you go shopping. It works in metres or feet, supports several currencies, and runs entirely in your browser — nothing you enter is uploaded or stored anywhere but your own device.

How it works

The calculator measures area, not volume, then converts. First it adds up the gross area of every wall you list (width times height). Then it subtracts the openings — each door or window multiplied by how many of them there are — to get the net paintable area. That net area is multiplied by the number of coats, because two coats paint the same wall twice. Dividing by the coverage per litre gives the theoretical litres, and a wastage margin is added on top for roller absorption and touch-ups. Finally the litres are rounded up to whole tins at the size you choose, and multiplied by your price per litre for a cost estimate. Every intermediate number is shown in the “Show the working” panel so you can check it by hand.

Worked example

Suppose a small room has four walls totalling 30 square metres of gross area. It has one door of 0.9 by 2.0 metres (1.8 square metres) and two windows of 1.2 by 1.2 metres (1.44 square metres each, 2.88 in total). The openings remove 4.68 square metres, leaving a net area of about 25.3 square metres. Painting two coats means covering 50.6 square metres of surface. With a matt emulsion rated at 12 square metres per litre, that needs about 4.2 litres, and adding a 5 percent wastage margin pushes it to roughly 4.4 litres. Sold in 2.5 litre tins, you round up to two tins (5 litres). At a price of 6 per litre, the estimated spend on tins is about 30.

Formula note

The core relationship is:

litres = (net_area × coats ÷ coverage_per_litre) × (1 + wastage)

where net_area = sum(wall_width × wall_height) − sum(opening_width × opening_height × count). Tins to buy is ceil(litres ÷ tin_size), and the cost estimate is tins × tin_size × price_per_litre. When you work in feet, each width × height is converted to square metres using 1 ft = 0.3048 m before the area is summed, so coverage stays in consistent units throughout.

Use it for a single feature wall, a whole room, or a stairwell — add as many walls and openings as you need. The numbers are estimates to guide your purchase, so always confirm coverage and tin sizes against the product you actually buy.