Calculate how much paint you need for your room
Estimating paint quantities correctly saves both money and trips to the DIY store. Buy too little and you end up with a mismatched finish when the second batch of paint has a slightly different tint from the first. Buy too much and you are left with tins that slowly dry out in the garage. Getting it right first time makes your decorating project smoother and more cost-effective. In the UK, wall paint is typically sold in 2.5-litre and 5-litre tins, with most standard emulsion paints covering approximately 12-14 square metres per litre on a smooth, sealed surface. Coverage varies significantly depending on the paint type (matt, silk, or specialist), the surface condition (new plaster absorbs much more paint than a previously painted wall), and the colour -- darker colours and certain reds or yellows often require additional coats for even coverage. Most rooms require two coats of paint for a professional finish. If you are painting over a dark colour with a lighter shade, you may need a dedicated undercoat or primer plus two topcoats, effectively three coats in total. New plaster should be sealed with a mist coat (diluted emulsion) before applying full-strength coats. This calculator accounts for the number of coats and deducts standard allowances for doors and windows to give you an accurate estimate.
To calculate how much paint you need: 1. Measure and enter the room length in metres. This is the longer dimension of the room. 2. Measure and enter the room width in metres. This is the shorter dimension. 3. Enter the room height. Standard UK ceiling heights are 2.4 metres in modern builds and up to 2.7-3.0 metres in older properties. Measure from floor to where the wall meets the ceiling. 4. Select the number of coats. Two coats is standard for most repainting jobs. Choose three if you are covering a very different colour or painting new plaster (after the mist coat has dried). 5. Expand advanced options to adjust the number of doors and windows. Each door or window deducts approximately 1.5 square metres from the total wall area, as you typically do not paint these surfaces with wall paint. 6. View the results. The calculator shows the total litres of paint needed and the number of standard 2.5-litre tins to purchase (rounded up, since you cannot buy a fraction of a tin).
The paint calculation involves computing the total paintable wall area and dividing by the coverage rate: First, calculate the total wall area. A room has four walls: two walls of (length x height) and two walls of (width x height). Total wall area = 2 x (length + width) x height. Next, deduct non-paintable areas. Each door or window deducts approximately 1.5 m2 from the total area. Adjusted area = total wall area - (number of doors and windows x 1.5). Then multiply by the number of coats to get the total area to be covered: total paint area = adjusted area x coats. Finally, divide by the coverage rate to get litres needed: litres = total paint area / coverage rate. The calculator uses a coverage rate of 12 m2 per litre, which is a conservative estimate suitable for most standard emulsions on previously painted surfaces. The number of 2.5-litre tins is the litres needed divided by 2.5, rounded up to the nearest whole number. For example, a room measuring 4 m x 3 m with 2.4 m ceilings and one door: total wall area = 2 x (4 + 3) x 2.4 = 33.6 m2. After deducting 1.5 m2 for the door: 32.1 m2. With two coats: 64.2 m2. At 12 m2 per litre: 5.35 litres, which rounds up to 3 tins of 2.5 litres (7.5 litres total, providing a comfortable margin). Coverage rates vary by product. Premium paints often cover up to 14 m2 per litre, while budget paints may only manage 10 m2. Check the manufacturer's coverage information on the tin label and adjust your expectations accordingly.
This calculator estimates wall paint only and does not include ceiling paint, woodwork paint (gloss or satinwood), or primer. For ceilings, calculate the floor area (length x width) and multiply by the number of coats. Paint coverage figures assume a smooth, sealed surface. New or bare plaster requires a mist coat first -- dilute standard emulsion with 10-20% water. Textured surfaces such as Artex or rough render absorb significantly more paint and may require 50% or more additional material.