Crown Paint Calculator: How Much Paint Do You Need?

The Crown Paint calculator is a free online tool that estimates how much paint you need for a decorating project. You enter your room’s dimensions, the number of doors and windows, and the number of coats you plan to apply, and it returns the required litres of paint. This article explains how the calculator works, how to use it accurately, and how to avoid common mistakes that lead to wasted paint or a second trip to the store.

Formula

The calculator uses a standard decorating formula based on the surface area of your walls and the coverage rate of the paint. The total wall area is calculated as the perimeter of the room multiplied by the wall height, minus the areas of doors and windows. The paint volume is then the wall area multiplied by the number of coats, divided by the coverage rate (typically 12 m² per litre for Crown paints).

In symbols:

Wall area = 2 × (length + width) × height − (doors × 1.9) − (windows × 1.5)

Paint needed (L) = (Wall area × coats) ÷ 12

Where:

• length and width are the room dimensions in metres (m)
• height is the distance from floor to ceiling in metres (m)
• doors and windows are the number of each in the room
• coats is the number of paint layers you intend to apply
• 1.9 m² is the average area of a standard internal door
• 1.5 m² is the average area of a typical window
• 12 m² per litre is the typical coverage rate for Crown emulsion paints

Worked Example

Using the default values from the calculator: a room 5 m long, 4 m wide, 2.4 m high, with one door, one window, and two coats.

1. Calculate the perimeter: 2 × (5 + 4) = 18 m
2. Multiply by the height: 18 × 2.4 = 43.2 m² (this is the total wall area before deductions)
3. Subtract the door area: 43.2 − 1.9 = 41.3 m²
4. Subtract the window area: 41.3 − 1.5 = 39.8 m² (this is the net wall area)
5. Multiply by the number of coats: 39.8 × 2 = 79.6 m² (total area to cover)
6. Divide by the coverage rate: 79.6 ÷ 12 = 6.63 litres

The calculator rounds this to 6.6 litres. For a real project, you would likely buy a 5-litre tin and a 2.5-litre tin, giving you 7.5 litres – a little extra for touch-ups.

How the Crown Paint Calculator Works

The calculator is a straightforward implementation of the coverage formula above. You input the room’s dimensions and the number of doors and windows, and it applies the standard coverage rate of 12 m² per litre. This rate is typical for Crown’s emulsion paints (both matt and silk finishes) and is printed on the tin as a guide.

The tool assumes you are painting all four walls, not the ceiling or the floor. If you are painting a ceiling, you need to measure the floor area (length × width) and use a separate calculation. The calculator also assumes a standard door and window size; if your doors are larger (e.g., double doors) or your windows are floor-to-ceiling, you should adjust the numbers manually.

How to Choose the Right Amount

When using the calculator, consider these factors that affect how much paint you actually need:

• Number of coats: New plaster or a dark colour change often requires three coats, while repainting the same colour may only need one.
• Surface porosity: Bare plaster and old, matt paint absorb more paint than a previously sealed surface.
• Paint type: Emulsion (water-based) paint covers roughly 12 m² per litre, but some premium ranges cover up to 16 m² per litre. Always check the tin.
• Application method: A roller uses less paint than a brush, and a sprayer uses the least but requires more overspray allowance.
• Trim and skirting: The calculator covers walls only. For woodwork, measure the actual surface area of skirting boards, door frames, and windowsills.

If you are between two sizes, it is safer to round up. Leftover paint can be stored for touch-ups, but running out mid-project means a colour match that may be slightly off.

Common Mistakes and Misconceptions

One frequent error is forgetting to subtract the area of doors and windows. While this only saves about 3–4 m² per room, it can add up to half a litre of unnecessary paint. Another mistake is using the floor area instead of the wall area. The floor area (length × width) is only useful for ceiling paint, not walls.

Many people also assume that one litre of paint covers exactly 12 m² regardless of the surface. In reality, coverage varies with the colour and the undercoat. For example, painting a bright red over white may require three or four coats, while a similar white over white may only need one. The calculator gives a good estimate, but always read the coverage information on the specific paint tin you buy.

Comparisons with Other Paint Calculators

Crown’s calculator is similar to those offered by other major paint brands, such as Dulux and Farrow & Ball. The main difference is the coverage rate used: Crown uses 12 m² per litre, while Dulux uses 16 m² per litre for some of its ranges. This means the same room might show a slightly different litre requirement depending on which brand’s calculator you use. Always match the calculator to the paint you intend to buy.

Some calculators also ask for the number of windows and doors, but not their sizes. If your room has unusually large windows (e.g., a bay window), you may need to manually adjust the result. In that case, measure the actual window area and subtract it from the total wall area before dividing by the coverage rate.

Cost Considerations

While the calculator gives you the volume of paint, it does not tell you the cost. Paint prices vary widely: a 5-litre tin of Crown emulsion costs roughly £20–£35, while premium designer brands can cost £60 or more for the same volume. To estimate the cost, multiply the litres required by the price per litre of your chosen paint. For the worked example above (6.6 litres), a mid-range paint at £5 per litre would cost about £33 for the paint alone.

Remember to budget for other materials: primer, undercoat, masking tape, rollers, and brushes. These can add 30–50% to the total project cost. If you are painting a large area, it may be worth buying a 10-litre tin, which is often cheaper per litre than smaller tins.

Safety and Preparation

Before painting, ensure the room is well ventilated. Water-based emulsions have low levels of volatile organic compounds (VOCs), but solvent-based paints (like gloss) require more ventilation. If you are sanding old paint, wear a dust mask, especially if the paint may contain lead (in homes built before 1978 in the US, or before 1992 in the UK).

For the actual painting, use a roller for large areas and a brush for edges and corners. Apply paint in a “W� pattern to ensure even coverage. Allow each coat to dry fully (usually 2–4 hours for emulsion) before applying the next. If you are painting over a glossy surface, sand it lightly and use a primer to help the paint adhere.

Frequently Asked Questions

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

A 12×12 foot room (about 3.7×3.7 m) with 8-foot (2.4 m) high walls has a wall area of about 35 m². Subtracting one door and one window leaves about 32 m². For two coats, you would need about 5.3 litres (32 × 2 ÷ 12). A single 5-litre tin would be tight, so buy a 5-litre and a 1-litre tin.

Does the calculator include the ceiling?

No, the calculator only covers walls. For a ceiling, use the floor area (length × width) and divide by the coverage rate. For the example room (5×4 m), the ceiling is 20 m². For two coats, you would need 20 × 2 ÷ 12 = 3.3 litres.

What if my walls are not perfectly rectangular?

If you have sloped ceilings or alcoves, break the wall into rectangles and triangles. Calculate each area separately and add them together. For a triangle, the area is 0.5 × base × height. Then subtract doors and windows as usual.

How accurate is the 12 m² per litre coverage rate?

It is a typical figure for emulsion paint on smooth, previously painted walls. Rough plaster or bare drywall can absorb up to 20% more paint, reducing coverage to about 10 m² per litre. If you are painting new plaster, increase the paint amount by 10–20%.

Can I use the calculator for exterior paint?

The calculator is designed for interior walls. Exterior surfaces are often rougher (brick, pebbledash) and require more paint. Coverage for exterior masonry paint is usually around 6–8 m² per litre, so you would need roughly double the amount. Use the same area calculation but adjust the coverage rate.

Should I buy extra paint for touch-ups?

Yes, it is wise to buy 10–15% more than the calculated amount. This covers any mistakes, extra coats, or future touch-ups. Store the leftover paint in a cool, dry place with the lid sealed tightly, and write the date and room name on the tin.

Related

• Drywall Pricing Calculator: Cost Per Sheet & Total
• Drywall Quote Calculator: How to Estimate Cost
• Drywall Estimator Calculator: How It Works

in length | Room length | m | 5 | 0
in width | Room width | m | 4 | 0
in height | Room 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) - (doors * 1.9) - (windows * 1.5) | 1
out litres | Paint needed | L | area * coats / 12 | 1