Dulux Paint Calculator: How Much Paint You Need

The Dulux paint calculator is a tool that estimates how much paint you need for a room by entering your wall dimensions, number of doors and windows, and coats of paint. You enter the room’s length, width, and height, plus the number of doors and windows, and it returns the total litres of paint required for your chosen number of coats. This article explains the standard formula behind such calculators, how to measure your room correctly, and how to avoid the most common estimating mistakes.

Formula

The standard Dulux paint calculator uses a simple geometric formula. First, calculate the total wall area, then subtract the areas of doors and windows, then divide by the paint coverage rate and multiply by the number of coats.

Total wall area (m²) = 2 × (room length + room width) × wall height

Paintable area (m²) = Total wall area − (number of doors × 1.9) − (number of windows × 1.2)

Paint needed (litres) = (Paintable area ÷ 10) × number of coats

Where:

• Room length and width are in metres (m)
• Wall height is in metres (m)
• 1.9 m² is the average area of a standard interior door
• 1.2 m² is the average area of a standard window
• 10 m² per litre is the typical coverage of Dulux interior paint for one coat

This formula assumes you are painting all four walls, not the ceiling. If you are painting the ceiling, you need a separate calculation: ceiling area = room length × room width.

Worked Example

Using the default values from the calculator above: a room 4 m long, 3 m wide, 2.4 m high, with 1 door and 1 window, and 2 coats of paint.

Step 1: Total wall area = 2 × (4 + 3) × 2.4 = 2 × 7 × 2.4 = 33.6 m²

Step 2: Subtract door and window areas = 33.6 − (1 × 1.9) − (1 × 1.2) = 33.6 − 1.9 − 1.2 = 30.5 m²

Step 3: Paint for one coat = 30.5 ÷ 10 = 3.05 litres

Step 4: For two coats = 3.05 × 2 = 6.10 litres

So you would need about 6.1 litres of paint. In practice, you would buy a 5-litre tin and a 1-litre tin, or a single 7.5-litre tin if available, to have a little spare for touch-ups.

How to Measure Your Room Accurately

The calculator is only as good as the numbers you enter. Measure in metres, not feet or centimetres, to keep the formula consistent. Use a metal tape measure and measure from floor to ceiling for the wall height, not from the skirting board to the ceiling.

• Measure the length and width of the room at floor level, in metres.
• Measure the wall height from the floor to the ceiling, in metres.
• Count the number of standard doors (about 0.9 m × 2.0 m) and windows (about 1.2 m × 1.0 m).
• For non-standard doors or windows, measure their actual area and subtract that manually.
• If you are painting a feature wall, only measure that wall’s length and height, and set the other dimensions to 0 (or use a single-wall calculation).

If your room has a sloping ceiling, an archway, or built-in wardrobes, the simple rectangle formula will overestimate. In those cases, measure each wall individually and add the areas together, then subtract the door/window areas.

Common Mistakes and How to Avoid Them

Most estimating errors come from unit mix-ups or forgetting to subtract openings.

• Mixing metres and feet: if you measure in feet, convert to metres first (1 foot = 0.3048 m). The formula expects metres.
• Forgetting the factor of 2: the perimeter of the room is 2 × (length + width). If you only add length + width, you get half the wall area.
• Not subtracting doors and windows: a standard door is about 1.9 m², a window about 1.2 m². Skipping this can add 10–15% extra paint to your estimate.
• Ignoring the number of coats: a dark colour over a light wall may need three coats, while a like-for-like repaint may need only one.
• Rounding up too early: calculate the litres to two decimal places, then round up to the nearest available tin size.

Another common mistake is assuming the coverage rate is the same for all paints. The 10 m² per litre figure is typical for Dulux interior emulsion, but textured paints, primers, and high-opacity paints can cover less or more. Always check the tin label for the specific coverage rate.

How to Read the Result

The calculator gives you litres of paint needed. Paint is sold in standard tin sizes: 1 L, 2.5 L, 5 L, and 10 L. You should round up to the nearest combination of tins that covers your number.

For example, if you need 6.1 litres, you could buy a 5 L tin and a 1 L tin (total 6 L), but that is slightly short. Better to buy a 5 L and a 2.5 L (total 7.5 L), leaving you with spare paint for touch-ups. If you need 3.05 litres, a 2.5 L and a 1 L tin gives you 3.5 L, which is enough.

The result assumes you are painting walls only. If you are also painting the ceiling, add the ceiling area separately: length × width, divided by 10, multiplied by coats. For a 4 m × 3 m room, the ceiling is 12 m², so one coat needs 1.2 L, two coats need 2.4 L.

If you are painting woodwork (skirting boards, door frames, window frames), that is a separate job with different paint (satin or gloss) and a different coverage rate, usually about 12 m² per litre for a single coat.

Limitations of the Formula

The simple rectangle formula works well for standard rooms with flat walls and regular openings, but it has limits.

• It does not account for wall texture: rough plaster, brick, or heavily textured wallpaper absorb more paint, reducing coverage to 6–8 m² per litre.
• It does not account for the colour change: painting white over dark red may need an extra coat or a primer.
• It does not account for the paint type: matt emulsion, silk, and eggshell have slightly different coverage rates.
• It does not account for the application method: a roller uses less paint than a brush, and a sprayer uses more.
• It does not account for the condition of the wall: new plaster needs a mist coat first, which is a thinned emulsion that covers about 8 m² per litre.

For most interior rooms, the formula is accurate to within 10–15%. If you are painting a complex space with alcoves, bay windows, or multiple openings, measure each wall individually and add the areas, then subtract the openings. That manual method is always more accurate than a single rectangle formula.

Dulux Paint Calculator vs. Other Methods

Dulux’s official website offers a paint calculator that uses the same basic formula but may include additional fields for ceiling painting, woodwork, and specific product coverage rates. The official tool also lets you choose the paint type (e.g., Dulux Matt, Dulux Silk, Dulux Once) to get a more tailored estimate.

Other paint brands (e.g., Crown, Farrow & Ball, Johnstone’s) have similar calculators, but their coverage rates differ slightly. Dulux typically quotes 10 m² per litre for interior emulsion, while some premium brands quote 12–13 m² per litre for their thicker paints. Always use the coverage rate printed on the tin you plan to buy, not the default in a calculator.

If you do not have internet access, a simple rule of thumb is: measure the perimeter of the room (in metres), multiply by the wall height, subtract 2 m² per door and 1.5 m² per window, then divide by 10 and multiply by coats. This gives you litres.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

A 12×12 foot room is about 3.66 m × 3.66 m. With a standard 2.4 m ceiling, the wall area is 2 × (3.66 + 3.66) × 2.4 = 35.1 m². Subtract one door (1.9 m²) and one window (1.2 m²) to get 32 m². For two coats, you need 32 ÷ 10 × 2 = 6.4 litres, so a 7.5 L tin or a 5 L + 2.5 L combination.

Does the Dulux paint calculator include the ceiling?

No, the standard calculator only estimates wall paint. You need a separate calculation for the ceiling: multiply the room length by the width, divide by 10, and multiply by the number of coats. For a 4 m × 3 m room, the ceiling is 12 m², needing 1.2 L per coat.

How many coats of paint should I use?

Most Dulux interior paints are designed for two coats for full, even coverage. If you are painting a similar colour over a light wall, one coat may suffice, but two is safer. If you are painting a light colour over a dark colour, or using a low-opacity paint, you may need three coats or a primer first.

What is the coverage of a 5-litre tin of Dulux paint?

At the standard coverage rate of 10 m² per litre, a 5-litre tin covers 50 m² for one coat, or 25 m² for two coats. A typical 4 m × 3 m room with one door and one window has about 30.5 m² of paintable wall area, so a 5 L tin covers just over one coat, and you would need a second tin for a second coat.

Can I use the calculator for exterior paint?

No, exterior paint has a different coverage rate (usually 8–10 m² per litre for masonry) and different surface conditions. Exterior walls are rougher and more porous, so the formula would underestimate. Use a dedicated exterior paint calculator or the coverage rate printed on the exterior paint tin.

What if my room has non-standard doors or windows?

Measure the actual height and width of each door or window, multiply to get the area in square metres, and subtract that total from the wall area manually. For example, a large patio door is about 2.4 m × 1.8 m = 4.32 m², which is more than twice a standard door. Enter the number of standard doors as 0 and subtract the extra area yourself.

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in length | Room length | m | 4 | 0
in width | Room width | m | 3 | 0
in height | Wall height | m | 2.4 | 0
in doors | Number of doors | - | 1 | 0
in windows | Number of windows | - | 1 | 0
in coats | Number of coats | - | 2 | 1
out area | Total wall area | m² | 2 * (length + width) * height | 1
out paintable | Paintable area | m² | 2 * (length + width) * height - (doors * 1.9 + windows * 1.2) | 1
out litres | Paint needed | L | (2 * (length + width) * height - (doors * 1.9 + windows * 1.2)) / 10 * coats | 2