How the amount is worked out
The calculator finds the total wall area, removes a typical area for each door and window, adds the ceiling if selected, and multiplies by the number of coats. Dividing by the paint’s coverage gives the litres or gallons needed.
Change the coverage under More options if your tin quotes a different number.
Getting a good result
Preparation matters more than the paint. Fill and sand cracks, wash grease from kitchens, and prime bare plaster or stains. Porous or freshly plastered surfaces soak up a lot of paint on the first coat, so you may use more than the figure shown.
Keep a little leftover paint, labelled with the room and date, for future touch-ups.
Measuring tips
Measure each wall’s width and the ceiling height with a tape measure. For irregular rooms, calculate each section separately and add the amounts together. Skirting boards, window frames and radiators are not included.
A worked example
A room measuring 5 m by 4 m with 2.5 m walls has 45 m² of wall. Taking off one door (1.8 m²) and two windows (3 m²) leaves 40.2 m². Two coats at 10 m² per litre need 8.04 litres, so you would buy 8.5 litres. Adding the ceiling brings the total to about 12 litres, or 12.5 to buy.
Questions people ask
How do I work out how much paint I need?
Find the wall area: 2 × (length + width) × height. Subtract doors and windows, multiply by the number of coats, and divide by the coverage on the tin. A 5 m × 4 m room with 2.5 m walls has 45 m² of walls before openings.
What is the usual coverage of paint?
Most emulsions cover about 10–13 m² per litre (roughly 350–400 ft² per US gallon) on a smooth, primed surface. Check the tin, because coverage varies by brand and finish.
How many coats do I need?
Two coats give an even, durable finish in most cases. One may be enough when repainting a similar colour on a sound wall, and going from dark to light often needs three or a primer.
Should I include the ceiling?
Choose Yes if you are painting it. Ceilings use the room’s floor area, and they are usually painted in a separate ceiling paint, so you may want to run the calculation twice.
Why does the calculator round up?
Paint is sold in fixed tin sizes, so you cannot buy an exact amount, and running out mid-wall can leave a visible colour difference. The exact figure is shown alongside the rounded amount to buy.