B&Q Paint Calculator: How Much Paint Do You Need?

The B&Q paint calculator is a tool that estimates how much paint you need for a room or surface based on its dimensions and the number of coats. You enter the wall height, wall width, number of doors and windows, and the calculator returns the litres of paint required for your chosen coverage. This article explains how the calculator works, how to use it effectively, and what to do when your room doesn’t fit the standard assumptions.

Formula

The B&Q paint calculator uses a simple area-based formula. The total paintable area is the wall area minus the areas taken up by doors and windows, then multiplied by the number of coats. The formula in symbols is:

Paint needed (L) = (H × W − D × 1.9 − Win × 1.1) × C / 10

• H = wall height in metres (typically 2.4 m)
• W = total width of all walls in metres
• D = number of standard doors (each subtracts ~1.9 m²)
• Win = number of standard windows (each subtracts ~1.1 m²)
• C = number of coats (1 or 2)
• 10 = average paint coverage in m² per litre (this is the standard figure used by most paint manufacturers)

The result is in litres. The calculator also shows the paintable area in square metres, which is useful if your paint brand specifies a different coverage rate.

Worked Example

Using the default values from the calculator above: wall height 2.4 m, total wall width 10 m, 2 coats, 2 doors, and 1 window.

1. Calculate the gross wall area: 2.4 × 10 = 24 m²
2. Subtract doors: 2 × 1.9 = 3.8 m² → 24 − 3.8 = 20.2 m²
3. Subtract windows: 1 × 1.1 = 1.1 m² → 20.2 − 1.1 = 19.1 m²
4. Multiply by coats: 19.1 × 2 = 38.2 m²
5. Divide by coverage: 38.2 / 10 = 3.82 L

The calculator rounds this to 3.8 litres. This matches the output shown in the block above.

Why the Coverage Rate Matters

The 10 m² per litre figure is a practical average, but it is not universal. Many emulsion paints claim coverage of 12–14 m² per litre for a single coat on smooth, primed plaster. Heavily textured surfaces, bare plaster, or porous brick can absorb far more, sometimes dropping effective coverage to 6–8 m² per litre. If your paint tin states a different rate, you can adjust the formula manually: divide your paintable area (after doors and windows) by the coverage stated on the tin, then multiply by the number of coats. The B&Q calculator’s default is conservative, which is safer than underestimating.

How to Measure Your Room Correctly

The accuracy of the calculator depends entirely on your measurements. Here are the steps to get them right:

• Measure the height from the floor to the ceiling, not to the top of the wall if you have coving or a picture rail – include the full height unless you are not painting above the rail.
• Measure the total width of all walls you plan to paint. For a rectangular room, add the lengths of all four walls. For an L-shaped room, measure each wall segment separately and add them.
• Do not subtract areas for skirting boards or architraves unless you are not painting them – they are usually painted with the wall.
• For doors and windows, use the standard deductions of 1.9 m² and 1.1 m² only if they are typical sizes. A large sliding door or a floor-to-ceiling window should be measured and subtracted manually.
• If you are painting the ceiling, treat it as a separate rectangle: length × width, and use the same coverage per litre.

Common Mistakes and Misconceptions

One frequent mistake is forgetting to multiply by the number of coats. Most paint jobs need two coats, especially when changing from a dark to a light colour. Another is using the floor area instead of the wall area – the calculator requires wall width and height, not room square footage. A third error is ignoring the subtractable areas for doors and windows; while they seem small, a typical door is about 2 m², and a window about 1 m², so a room with two doors and two windows loses about 6 m² from the paintable surface – roughly 0.6 litres of paint.

A common misconception is that one litre of paint always covers 10 m². In reality, coverage varies by brand, colour, and surface. Darker colours often require more coats, and some premium paints are thicker and cover more. Always check the tin for the specific coverage rate.

How to Adjust for Textured or Porous Surfaces

Rough plaster, brick, or textured wallpaper can increase paint consumption by 20–50%. If your walls are not smooth, reduce the coverage per litre in the formula. For example, if the paint tin says 12 m² per litre but your walls are rough, use 8–9 m² per litre instead. The calculator’s default of 10 m² per litre already leans conservative, so for very rough surfaces you may want to add 10–20% to the final result. Similarly, new plaster is highly absorbent and often needs a mist coat (a thinned first coat) before the main paint, which will use extra paint not accounted for in the calculator.

Frequently Asked Questions

Does the B&Q paint calculator include ceilings?

No, the standard calculator only estimates wall paint. To include a ceiling, measure its length and width, multiply to get the area in m², and divide by the coverage per litre (usually 10 m²/L). Add this to the wall paint estimate. Ceilings typically need two coats as well.

What if my room has an irregular shape or alcoves?

Measure each wall segment individually, including alcove walls and recesses, and add them all to the total width. The calculator treats the room as a simple rectangle, so irregular shapes require you to sum the widths of all paintable surfaces yourself before entering the total.

How much paint should I buy – should I round up?

Always round up to the nearest tin size. Paint is sold in 1 L, 2.5 L, 5 L, and 10 L tins. If the calculator says 3.8 L, you would need a 5 L tin. It is better to have a little extra for touch-ups than to run out mid-job. You can also buy two 2.5 L tins if you prefer smaller containers.

Does the calculator work for exterior walls?

Yes, the same formula applies, but exterior walls often have more texture (roughcast, brick) and may absorb more paint. Use a lower coverage rate, such as 6–8 m² per litre, and consider that exterior masonry paint often has a different coverage than interior emulsion. Measure the height to the eaves and include gable ends if you are painting them.

Why does the calculator ask for doors and windows separately instead of just entering the wall area?

Because subtracting a fixed area for each door and window is a quick, standard method used by most paint calculators. It saves you from measuring each opening. The default deductions (1.9 m² for a door, 1.1 m² for a window) are based on typical UK sizes. If your openings are larger, measure them and subtract manually from the total wall area before dividing by coverage.

Can I use the calculator for a single wall or just a feature wall?

Yes. Simply enter the height and the width of that one wall, set the number of doors and windows to 0, and choose the number of coats. For a feature wall, you might only need one coat if the paint is high-coverage, but two coats are safer for an even finish.

Related

• Drywall Calculator Metric: Sheets & Waste
• Drywall Calculator by Square Footage
• Drywall Calculator by Wall: Sheets & Cost

in h | Wall height | m | 2.4 | 0
in w | Total wall width | m | 10 | 0
in coats | Number of coats | 1 | 2 | 0
in doors | Number of doors | 2 | 0
in windows | Number of windows | 1 | 0
out paint | Paint needed | L | (h * w - doors * 1.9 - windows * 1.1) * coats / 10 | 1
out area | Paintable area | m² | h * w - doors * 1.9 - windows * 1.1 | 1