How to Calculate Paint Needed

Step 1: Measure Your Walls

Use a tape measure to get the dimensions of each wall you're painting.

For a rectangular room, you'll have four walls. Measure the length and height of each wall in metres. Don't worry about being exact to the millimetre, rounding to the nearest 10cm is fine for paint calculations.

Example Room Measurements

• Wall 1 (long wall): 5.2m × 2.4m = 12.48 m²
• Wall 2 (short wall): 3.6m × 2.4m = 8.64 m²
• Wall 3 (long wall): 5.2m × 2.4m = 12.48 m²
• Wall 4 (short wall): 3.6m × 2.4m = 8.64 m²
• Total: 42.24 m²

Step 2: Subtract Windows and Doors

You don't need to paint over windows and doors, so subtract their area from your total.

A standard UK door is about 2m × 0.8m = 1.6 m². A typical window might be 1.2m × 1.5m = 1.8 m².

For most rooms with a door and a couple of windows, subtracting 10-15% from your total area is a reasonable shortcut. In our example: 42.24 m² - 15% = 35.9 m².

If you have large patio doors or many windows, measure them individually and subtract the exact area.

Step 3: Apply Coverage Rate

Paint coverage depends on the type of paint you're using.

Standard Coverage Rates

These rates are averages from Crown, Dulux, and other major UK manufacturers. Check your specific tin for exact coverage, which can vary by formulation.

Step 4: Account for Multiple Coats

Most paint jobs need two coats for even coverage and durability.

Using our example: 35.9 m² ÷ 11 m²/litre = 3.26 litres per coat. For two coats: 3.26 × 2 = 6.52 litres.

When You Need More Than Two Coats

• Painting light over dark (e.g., white over navy) often needs 3 coats
• Bare plaster absorbs more paint, use a mist coat first then 2 full coats
• Cheap paint has less pigment and may need extra coats for solid coverage

Step 5: Add Wastage Allowance

Add 10% to your final calculation to account for:

• Paint left in the tray and roller
• Uneven absorption on textured walls
• Touch-ups and future repairs
• Spills and mistakes

Final calculation: 6.52 litres × 1.1 = 7.17 litres. Round up to 7.5 litres or 8 litres for safety.

Buying Paint in Standard Tin Sizes

UK paint typically comes in 1L, 2.5L, 5L, and 10L tins. For our example (7.5 litres needed), you'd buy either:

• 1 × 10L tin (cheapest per litre, but you'll have leftovers)
• 1 × 5L + 1 × 2.5L tins

Buying slightly more than you need is better than running short mid-job and having to match batches.

Quick Calculation Tool

Skip the manual maths and use our paint coverage calculator to get instant results. Just enter your room dimensions and it handles all the calculations above automatically.

Tips for Accurate Measuring

• Measure at least twice to confirm accuracy
• For non-rectangular rooms, break them into rectangular sections
• Ceiling paint uses the same coverage rates as wall paint
• Sloped ceilings or vaulted rooms need extra measuring
• Different brands have different coverage rates, check the tin label

When Coverage Rates Don't Apply

The standard rates assume smooth, previously painted walls. You'll get less coverage on:

• Bare plaster or new drywall (very absorbent)
• Heavily textured walls (artex, woodchip paper)
• Porous masonry or render

For these surfaces, add 20-30% more paint or use a primer/sealer first.