Paint Calculator
Measure the room in metres and get the area to paint, litres of paint, a suggested mix of local tin sizes (1, 2.5, 5 and 10 litre tins) and an estimated cost in GBP.
Formulas shown on this page · Last reviewed October 2026
Tins are chosen largest first, then the smallest tin that covers the remainder, so it may not be the cheapest mix. Cost assumes a flat price per litre.
How the paint calculator works
wall area (m²) = 2 × (length + width) × height − doors − windows
litres = (wall area + ceiling) × coats ÷ coverage (m²/L)
The calculator deducts 1.9 m² per door and 1.5 m² per window by default – both are editable. Coverage defaults to 13 m² per litre per coat. UK emulsion usually covers about 12–14 m² per litre per coat; new plaster needs a thinned “mist coat” first, which soaks up extra paint.
Worked example
| Step | Result |
|---|---|
| Room 4 × 3.5 m, 2.4 m walls | 36 m² of wall |
| Less 1 door and 1 window | 32.6 m² |
| Two coats at 13 m²/L | 5.02 L |
| Tins | 1 × 5 L + 1 × 1 L |
Choosing tin sizes
Paint is sold in 1, 2.5, 5 and 10 litre tins from DIY stores, builders’ merchants and paint shops. Larger tins are normally cheaper per litre, so the calculator fills the need with the biggest tins first and finishes with the smallest tin that covers what is left. This is a simple rule, not a price optimiser – always compare the price per litre of the sizes in store.
Ceiling paint and matt emulsion are commonly sold in 5 and 10 litre tins, which are cheaper per litre. Tick “Paint the ceiling too” to add the floor area of the room. Dark colours, bare plaster and textured walls can need an extra coat or a primer, and prices here are typical – check local prices and enter your own.
Frequently asked questions
How much paint do I need for a 4 × 3.5 m room?
With 2.4 m walls, one door and one window, the walls come to 32.6 m². Two coats at 13 m² per litre need about 5 L. Painting the ceiling as well raises that to about 7.2 L.
What tins should I buy for that room?
For 5 L the simple rule suggests 1 × 5 L + 1 × 1 L, which is 6 L in total and costs about £54.00 at £9.00 per litre.
How far does one 2.5 litre tin go?
At 13 m² per litre, a 2.5 L tin covers about 33 m² for a single coat, or about 16 m² with two coats.
Do I need to subtract doors and windows?
Yes, for an accurate figure. A typical door is about 1.9 m² and a window about 1.5 m². Small windows are often ignored to leave a little spare paint for touching up.