Interior Paint & Cost Calculator

Paint Calculator

Calculate how much paint you may need for any room. Select which walls to paint, include the ceiling, choose your paint type and get the optimal tin combination — with a cost estimate and mist coat guidance for new plaster.

Last reviewed 15 May 2026
Interactive wall selector
8 paint types
Optimal tin sizes
UK 2026 pricing
Rule of thumb: A standard 4×3m room (with 2.4m ceilings) requires approximately 5 litres of emulsion for two coats on the walls. One 2.5L tin covers about 15 m² in two coats. For new plaster, factor in an extra mist coat (diluted 70:30 with water) before applying topcoats.
Room Details
Enter dimensions and select walls to paint
Input Method
Room Dimensions
m
m
m

Standard UK ceiling: 2.4m. Victorian: 2.7–3.0m.

Please enter a valid ceiling height
Select Walls to Paint
Wall 1 (L) Wall 2 (W) Wall 3 (L) Wall 4 (W)

Tap walls or labels to toggle them on/off

Adds a separate ceiling calculation with its own paint type.

Doors & Windows

2 m² per standard door

1.5 m² per standard window

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Standard coverage applies — no adjustment.

Skirting boards, door frames, door faces and window frames.

Painting Guide

How to Calculate Paint for a Room

Running out of paint mid-wall is one of decorating's most avoidable frustrations. The calculation is straightforward — but there are several factors that can significantly change your requirement.

The Wall Area Formula

Total paintable area = wall perimeter × ceiling height, minus doors and windows. For a standard 4×3m room with 2.4m ceilings: perimeter = (4+3+4+3) = 14m, total wall area = 14 × 2.4 = 33.6 m². Subtract a door (2 m²) and window (1.5 m²) to get 30.1 m² paintable area. At 12 m²/litre for two coats, that's 5 litres — a 5L tin. Our calculator handles all of this automatically including selective wall painting.

Understanding Paint Coverage

Coverage rates printed on tins are achieved under ideal conditions — smooth, primed, previously painted surfaces. Real-world coverage is often 10–20% lower. Rough textured surfaces, very porous plaster, or painting a lighter colour over a dark one will all reduce coverage significantly. When in doubt, round up. A small amount of leftover paint is far less costly than running short and having to buy a new tin that may be a slightly different batch.

New Plaster — Always Mist Coat First

Fresh plaster is highly alkaline and extremely porous. Apply full-strength emulsion directly to it and the plaster will draw all the moisture out of the paint almost instantly, leaving a powdery, adhesion-free surface that will peel and flake. A mist coat — 70% emulsion, 30% water — soaks into the plaster, seals it, and creates a stable base for topcoats. Always allow 24–48 hours after the mist coat before applying any topcoats. Use cheap emulsion for the mist coat — there's no need to use your expensive topcoat paint diluted.

Buying the Right Tin Sizes

Larger tins are almost always significantly cheaper per litre — a 10L tin can be 30–40% cheaper per litre than buying the equivalent in 2.5L tins. The catch is that opened tins have a limited shelf life (typically 1–2 years if properly sealed). Our calculator recommends the optimal tin combination to cover your requirement with minimum waste. If you have upcoming rooms to paint, buying a larger tin now and storing it properly is usually better value.

Paint Coverage Reference
Typical coverage rates for common UK paint types (per coat, on a sound painted surface)
Paint Type m²/litre Notes
Matt Emulsion12Standard walls, most versatile
Silk / Satin13Washable, kitchens and bathrooms
Trade / Premium Emulsion15Higher pigment, better one-coat coverage
Ceiling Paint14Flat finish, less roller spatter
Gloss / Satinwood16Woodwork, doors and skirting
Masonry Paint8Exterior walls, rough surface
Textured / Thick Coat8Conceals imperfections, lower coverage
Primer / Undercoat10Bare surfaces, dark colour changes
How Much Does a Tin Cover?
Based on standard matt emulsion at 12 m²/litre
Tin Size 1 Coat 2 Coats Typical Use
1L12 m²6 m²Feature wall, small area, touch-ups
2.5L30 m²15 m²Small bedroom, single room
5L60 m²30 m²Standard living room or large bedroom
10L120 m²60 m²Whole house or trade use

Quick Reference

How Much Paint for Common Room Sizes?

For a standard UK room with 2.4m ceilings, matt emulsion at 12 m²/litre, two coats on all four walls.

Room Size Floor Area Wall Area Litres (2 coats) Best Tin Option
2 × 2 m 4 m² 19.2 m² 3.2 L 1 × 2.5L + 1 × 1L
3 × 3 m 9 m² 28.8 m² 4.8 L 1 × 5L
4 × 4 m 16 m² 38.4 m² 6.4 L 1 × 5L + 1 × 2.5L (7.5L)
4 × 5 m 20 m² 43.2 m² 7.2 L 1 × 10L (saves vs two 5L)
5 × 5 m 25 m² 48.0 m² 8.0 L 1 × 10L
6 × 4 m 24 m² 48.0 m² 8.0 L 1 × 10L
Based on: matt emulsion 12 m²/L, 2.4m ceiling height, all four walls, no door/window deductions. Add ~10% for doors and windows.

Low-VOC Paint — What It Means and Why It Matters

VOC stands for Volatile Organic Compounds — the solvents in paint that evaporate as it dries and cause that distinctive paint smell. Traditional solvent-based gloss and oil-based paints can contain over 400 g/L of VOCs. In enclosed spaces this contributes to poor indoor air quality, headaches, and longer-term respiratory irritation.

UK and EU regulations (Directive 2004/42/EC) cap VOC levels for decorative paints — interior matt emulsion is limited to 30 g/L. Most modern water-based emulsions are well within this, many now marketed as "zero VOC" with under 1 g/L. Water-based gloss and satinwood have largely replaced traditional solvent-based products, offering comparable durability with significantly lower emissions.

When decorating enclosed rooms — especially bedrooms or children's rooms — look for paints labelled A+ VOC emission class (the EU's indoor air quality standard) or products carrying the EU Ecolabel. Keep windows open during application and for at least 24 hours after, regardless of stated VOC content.

Common Questions

Paint Calculator FAQs

For a standard 4×3m room with 2.4m ceilings (two coats, matt emulsion), expect to need around 4–5 litres for the walls. Add another 2–3 litres if painting the ceiling. Larger rooms, higher ceilings, rough surfaces or dark-to-light colour changes will all increase this. Our calculator gives you an estimated figure based on your specific room dimensions.
A mist coat is a diluted first coat (70% emulsion, 30% water) applied to fresh, bare plaster. New plaster is highly porous — full-strength paint applied directly will peel as the plaster draws moisture out too quickly. Apply the mist coat first, allow 24–48 hours to dry fully, then apply your topcoats. Use cheap emulsion for the mist coat — there is no need to dilute your expensive topcoat paint.
A standard 2.5L tin of matt emulsion covers approximately 30 m² in one coat, or 15 m² in two coats. A 5L tin covers around 60 m² in one coat. Premium trade paints may achieve 14–16 m²/litre. Always check the specific tin — coverage varies significantly by brand and formula, and real-world coverage on rough or porous surfaces will be lower than the label states.
Two coats as a minimum for any wall repaint. A single coat will look patchy when light hits the wall at an angle. Go to three coats if changing from a very dark to a very light colour, covering stains, or painting over new plaster (after the mist coat). One coat is only appropriate for a like-for-like refresh with a high-coverage trade paint.
Yes — tick "Include ceiling" in our calculator. Ceiling paint is calculated as Length × Width (the same area as the floor). Ceilings and walls typically use different paints — ceiling paint is usually a flat white designed to minimise roller spatter and avoid the shadows that a silk finish would show under overhead light.
Larger tins are almost always better value per litre — a 10L tin can be 30–40% cheaper per litre than the equivalent in 2.5L tins. Our calculator recommends the optimal combination of tin sizes to cover your requirement with minimum leftover paint. If you have other rooms to decorate soon, buying a larger tin and storing it properly (lid sealed, stored in a frost-free location) is usually the better choice.

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