How does it estimate paint coverage?

It calculates wall area as room perimeter times height, subtracts a standard area for each door and window, multiplies by the number of coats, then divides by your paint's coverage rate (litres per square metre or gallons per square foot).

What coverage rate should I use?

Most emulsion covers roughly 10–12 m² per litre (about 350–400 ft² per gallon) per coat. Check your tin, since primers and textured surfaces cover less. The default is 11 m²/litre.

How much area does each door and window deduct?

In metric the tool subtracts 1.9 m² per door and 1.4 m² per window; in imperial it subtracts 21 ft² per door and 15 ft² per window. These are typical opening sizes and keep the estimate from overbuying.

Why does the calculator use perimeter times height?

A room's four walls together have an area equal to the perimeter (2 × (length + width)) multiplied by the wall height. This counts all four walls in one step before openings are deducted.

How many coats of paint do I need?

Two coats is standard for good coverage on most walls. Use one coat for a light refresh over a similar colour, and three when going from dark to light or onto fresh plaster.

Should I round up when buying paint?

Yes. Buy whole tins covering at least the litres or gallons shown, and keep a little extra for touch-ups. The result is the minimum volume needed for the area and coats you entered.

Paint Calculator — Gera Tools

Q: Is my room data sent anywhere?

No. Every calculation runs entirely in your browser and nothing is uploaded to any server.

Paint calculator — how much paint to buy

Work out exactly how many litres or gallons of paint a room needs before you head to the shop. Enter the room length, width and wall height, subtract your doors and windows, choose the number of coats and your paint’s coverage rate, and the calculator returns the paint volume plus the paintable wall area. It works in metric or imperial and is handy for a single feature wall or a whole repaint.

How it works

The estimate uses the standard decorator’s method — total wall area equals the room perimeter times the wall height, minus a fixed deduction for each opening:

wall area  = 2 × (length + width) × height
paintable  = wall area − (doors × door area) − (windows × window area)
volume     = paintable × coats ÷ coverage rate

Default deductions are 1.9 m² per door and 1.4 m² per window (21 ft² and 15 ft² in imperial). The default coverage rate is 11 m² per litre per coat. Negative results are clamped to zero, so heavily glazed rooms never go below nothing.

Example

A 4 m × 4 m room with 2.4 m walls, 1 door, 1 window, 2 coats, coverage 11 m²/litre:

Wall area: 2 × (4 + 4) × 2.4 = 38.4 m²
Deductions: 1.9 + 1.4 = 3.3 m² → paintable = 35.1 m²
Painted area: 35.1 × 2 coats = 70.2 m²
Paint: 70.2 ÷ 11 = 6.38 litres (buy a 5 L + a 2.5 L tin)

Coverage rate	Paintable	Coats	Paint needed
11 m²/L	35.1 m²	1	3.19 L
11 m²/L	35.1 m²	2	6.38 L
12 m²/L	35.1 m²	2	5.85 L

Every calculation runs entirely in your browser and nothing is uploaded.