Updated 7 October 2026

How to use the paint quantity calculator

Use room mode for four walls of a rectangular room. Enter length, width and wall height, then the total area of doors and windows you will not paint. Tick the ceiling only if it will use the same paint and coverage assumption. For one wall, an irregular room or several measured surfaces, choose the net-area mode instead.

Set the planned number of coats and the coverage per coat from your product label or technical data sheet. The prefilled values are examples, not measurements of your room or a recommendation for a particular product. Add a reserve if you want an allowance for application losses or touch-ups, and enter the size of one tin. The result rounds the quantity up to whole tins of that size.

Measure walls rather than using the floor area

A room with a 12 m² floor does not have 12 m² of walls. Wall height and perimeter determine the wall area. For a rectangular room, multiply twice the sum of length and width by height. Measure the width and height of each unpainted opening, multiply them, then add those areas together.

Sloping ceilings, alcoves and walls of unequal height need separate measurements. Add the actual surfaces and enter their total in net-area mode. Exclude tiles, fitted units and other areas that will not receive this paint. Do not subtract the openings again when your entered area is already net.

Litres, coats and whole tins: the formula

Net area = 2 × (room length + room width) × wall height − opening area + optional ceiling area. Ceiling area = length × width. Paint litres = net area × coats ÷ coverage in m²/L. The reserve multiplies this quantity by 1 + reserve percentage ÷ 100.

Divide the final litres by the litres in one tin and round upward. This tool uses one tin size per calculation. It does not compare combinations of small and large tins or find the cheapest shop. An optional price is the price of one complete tin, so purchase cost equals tin count × tin price.

A worked example you can check

Take a room 3.8 m long, 3.2 m wide and 2.4 m high. Its walls total 33.6 m². Subtract 3.6 m² of measured openings to leave 30 m². At two coats and an illustrative 10 m²/L per coat, the base amount is 6 L. A 10% reserve brings it to 6.6 L.

With 5 L tins, the estimate requires two tins: 10 L purchased, which is 3.4 L above the estimate. That surplus comes from buying whole tins; it is not a prediction of paint left after the job. If the ceiling uses the same product, adding its 12.16 m² raises the estimate with that reserve to 9.2752 L.

Read the coverage figure before choosing a quantity

Coverage describes the area a stated volume can cover under specified conditions. Check whether the figure is per coat and which surface the manufacturer uses. Dulux’s measurement guidance calculates area, applies the product’s coat requirement and divides by its stated coverage. A different paint may have a different recommended number of coats.

New plaster, repaired areas and textured walls can change consumption. A strong colour change can also require a different coating system. Use product-specific guidance for preparation, primer and topcoat instead of treating a preset reserve as a correction for every surface. Calculate primer separately with its own coverage. If the ceiling uses a different paint, run a separate estimate for it.

Metres, feet and the two gallon sizes

Choose the units that match your measurements. Room lengths in feet use square feet for the openings; lengths in metres use square metres. Coverage and tin size have their own unit selectors. A US liquid gallon is 3.785411784 L; an Imperial gallon is 4.54609 L. They are not interchangeable.

The tool converts those quantities to litres internally. Changing a selector changes how the number is interpreted; it does not rewrite your measurements. Check all values after changing units. The result shows both m² and ft² to help you spot a mismatch.

Why an estimate can differ from the purchase or the job

Extra coats, a less favourable substrate, uneven application and paint retained in equipment can affect actual use. A manufacturer’s stated coverage is not a guarantee for every wall. Keep the calculation with its inputs so you can compare the estimate with the product instructions and your measurements.

The optional cost covers whole tins only. It excludes primer, brushes, rollers, masking materials, delivery, labour and additional taxes unless already included in the price you enter. A blank price produces no cost estimate; a zero price deliberately produces zero cost.

Common mistakes to check

• Entering floor area as wall area.
• Subtracting doors twice after measuring a net area.
• Using a two-coat coverage claim as though it were per coat.
• Mixing feet, metres, US gallons and Imperial gallons.
• Rounding the litres down before calculating whole tins.
• Putting the price per litre into the price-per-tin field.

Paint calculator FAQs

How much paint covers 30 m²?

At an entered 10 m²/L per coat, one coat needs 3 L and two coats need 6 L before reserve. Replace that coverage with your product’s figure.

Can I calculate a feature wall or just a ceiling?

Yes. Measure the net area and use net-area mode. Keep separate paints or colours in separate calculations.

Is the number of tins an exact shopping recommendation?

It is a rounded estimate for one chosen tin size. Actual consumption and available sizes can differ. Check the result before buying.

Does a 10% reserve guarantee enough paint?

No. It is an adjustable allowance, not a substitute for the correct surface, coats and coverage information.

Are my measurements saved?

The calculator does not store your entries or send them to a calculation server. Download the CSV before leaving if you need a record.

Sources and related tools

Dulux: measuring surfaces and applying product coverage · Bruguer: product selection and paint measurements · NIST: unit conversion factors

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