Paint Calculator — How Many Litres Do I Need?

How to Calculate How Much Paint You Need

Working out the right amount of paint before you start decorating saves both money and frustration. There is nothing worse than running out of paint halfway through a wall, or discovering that the shade you bought is no longer in stock when you go back for another tin. Our paint calculator takes the guesswork out of the process by using industry-standard coverage rates from leading UK paint manufacturers.

The basic formula is straightforward: measure the total wall area, subtract the area of doors and windows, then divide by the coverage rate per litre and multiply by the number of coats you plan to apply. Our calculator does all of this for you in an instant.

Understanding Paint Coverage Rates

Coverage rate tells you how many square metres one litre of paint will cover in a single coat. This varies depending on the type of paint, the surface you are painting, and the application method. Here are the standard rates used by most UK manufacturers:

How Many Coats of Paint Do I Need?

For most interior redecorations, two coats of emulsion deliver the best results. Here is a quick guide:

• One coat — Suitable when repainting with the same or a very similar colour, using a high-opacity paint.
• Two coats — The standard for most jobs. Provides even colour and good durability.
• Three coats — Needed when covering a dark colour (e.g., red or navy) with a much lighter shade, or when painting new plaster after a mist coat.

Do I Need Primer?

Primer is a preparatory coat that helps paint adhere to the surface and provides a uniform base. You should use primer in the following situations:

• New plaster: Apply a mist coat first — this is standard emulsion diluted with water (typically 70% paint, 30% water). This seals the plaster without trapping moisture. Allow it to dry fully before applying your top coats.
• Bare wood: Use a wood primer or all-purpose primer to prevent the paint soaking in unevenly.
• Stained surfaces: A stain-blocking primer prevents marks from bleeding through.
• Dramatic colour changes: A grey-tinted primer helps when going from a very dark to very light colour.

Standard Dimensions Used in This Calculator

To keep things simple, our calculator uses standard UK dimensions for doors and windows:

• Standard door: 1.98m x 0.76m = approximately 1.5 m² (accounting for the frame, we use 1.9 m²)
• Standard window: 1.2m x 1.2m = approximately 1.44 m² (we use 1.5 m² including the frame)

Popular UK Paint Brands and Their Coverage

The UK paint market offers excellent choices at every price point. Here are some of the most popular brands and how they compare:

• Dulux — The UK's best-selling paint brand. Their standard matt emulsion covers approximately 13 m²/litre. Dulux Trade tends to have higher coverage at 14–16 m²/litre.
• Crown — Good mid-range option. Coverage of around 12–14 m²/litre depending on the product line.
• Johnstone's — Popular with professionals. Trade matt emulsion offers 14–16 m²/litre coverage.
• Farrow & Ball — Premium brand with a coverage rate of about 12 m²/litre. The price per litre is significantly higher but the colour range is exceptional.
• Wickes / B&Q own brand — Budget-friendly options typically covering 10–12 m²/litre. You may need an extra coat compared to premium paints.

Tips for Getting the Best Coverage

1. Prepare your surfaces: Fill holes, sand rough patches, and clean off grease or dust. Good preparation is 80% of a good paint job.
2. Use the right roller: Short-pile rollers (around 10mm) give the best finish on smooth walls. Medium-pile (15mm) works well on lightly textured surfaces.
3. Maintain a wet edge: Work in manageable sections and keep the edge wet to avoid visible lap marks.
4. Do not thin the paint unless you are applying a mist coat on new plaster. Thinned paint covers less area per litre.
5. Store paint properly: Leftover paint lasts for years if sealed tightly and stored in a cool, dry place. Label tins with the room and date.