How to Calculate Paint Coverage — Step-by-Step UK Guide

What You'll Need

Before You Start

• Gather the paint tin data — you need the coverage rate (m² per litre) from the manufacturer. This is printed on every tin.
• Decide how many coats you need — typically two for walls and ceilings, plus a mist coat on bare plaster.
• Note the surface type — porous surfaces (bare plaster, lining paper, textured coatings) absorb more paint and reduce coverage.
• Use our Paint Coverage Calculator for walls and our Ceiling Paint Calculator for ceilings to get exact quantities.

Step-by-Step Instructions

Step 1: Measure Each Wall

Measure the width and height of each wall in metres. Write down each measurement. For irregularly shaped rooms, break the walls into rectangular sections and measure each one separately. Include chimney breasts, alcoves and any recesses — these all need paint.

Step 2: Calculate Total Wall Area

Multiply the width by the height of each wall to get its area in m². Add all wall areas together to get the total paintable wall area. For example, a room 4m × 3m with 2.4m ceilings: (4 × 2.4) + (3 × 2.4) + (4 × 2.4) + (3 × 2.4) = 33.6m².

Step 3: Measure Openings

Measure the width and height of each window and door. A standard UK internal door is approximately 762mm × 1981mm (about 1.5m²). A standard window is typically 1200mm × 1200mm (about 1.44m²). Measure your actual openings for accuracy.

Step 4: Deduct Openings

Subtract the total area of windows and doors from your total wall area. For example: 33.6m² total walls minus 1.5m² (door) minus 2.88m² (two windows) = 29.22m² of paintable wall.

Step 5: Calculate the Ceiling Area

If painting the ceiling, multiply the room length by the room width. For the 4m × 3m room example, the ceiling is 12m². Add this to your total if using the same paint, or note it separately for ceiling paint.

Step 6: Factor in Number of Coats

Multiply the total area by the number of coats. For our example: 29.22m² × 2 coats = 58.44m² of paint coverage needed for the walls. A mist coat on bare plaster counts as an additional coat.

Step 7: Divide by Coverage Rate

Check the coverage rate on your chosen paint tin (e.g. 12m² per litre). Divide the total coverage area by this rate: 58.44 ÷ 12 = 4.87 litres. Round up to the nearest available tin size — in this case, one 5L tin.

Step 8: Adjust for Surface Type

Reduce the coverage rate by 10-20% for porous surfaces (bare plaster, lining paper, textured coatings). For highly textured surfaces like Artex, reduce by 20-30%. These surfaces absorb more paint and have more surface area per m² of wall.

Step 9: Add Contingency

Add 5-10% extra to cover cut-in work around edges, touch-ups, roller splatter and the paint left in the tray and roller. Running out mid-wall gives a visible lap line. It is always better to have a small amount left over.

Step 10: Choose Your Tin Sizes

Round up to the nearest available tin sizes. Standard UK paint tins are 0.75L, 1L, 2.5L, 5L and 10L. Larger tins are better value. For our example (approximately 5L needed), one 5L tin is perfect. For 7L, buy one 5L plus one 2.5L from the same batch number.

Quick Reference: Standard Room Sizes

Common Mistakes to Avoid

• Not deducting openings: Forgetting to subtract windows and doors leads to buying too much paint. A room with three windows and a door can have 6-8m² less paintable area than the gross wall area.
• Ignoring surface porosity: Bare plaster, lining paper and textured surfaces absorb significantly more paint. The coverage rate on the tin assumes a sealed surface.
• Buying insufficient paint: Running out mid-wall leaves a visible lap line. Always round up and buy from the same batch number.
• Skipping the mist coat: Applying full-strength emulsion to bare plaster causes it to peel. Always apply a diluted mist coat first (50/50 paint and water).
• Mixing batches: Paint from different batches can vary in colour. Buy enough from one batch to complete the job.

Cost Estimate (2026 UK Prices)

Use our Paint Coverage Calculator and Ceiling Paint Calculator for exact quantities for your project.