Understand your estimate
Paint is sold in tins, but a room is measured in square metres or square feet. This calculator works from individual wall dimensions, deducts measured doors and windows, and uses the coverage stated for your actual paint.
Enter the walls you intend to coat
Add one row for each wall you will paint. Enter the wall's actual width and height; walls may be different lengths or heights. Use the add-wall control to cover a hallway, alcove or extra partition. Remove a row if you are not painting that section. The calculator totals all wall rectangles without assuming every room has four matching walls.
Next add separate rows for door and window opening sizes. You can use a count when identical openings repeat, but do not count an opening twice. Set the number of coats and manufacturer coverage, then enter the tin size, waste allowance and optional quoted tin price. The output displays both liquid required and whole tins to buy.
Measure paintable wall area carefully
For each wall, measure horizontal width and vertical height at the surface to be painted. A 4 m wall 2.4 m high contributes 9.6 m². If a ceiling is also being painted, estimate it as a separate surface rather than silently including it among wall measurements. Be consistent about coving, panelled sections and areas covered by built-in furniture.
Doors and windows should use their actual dimensions. A door opening 0.8 m by 2 m subtracts 1.6 m² from gross wall area; two windows 1.2 m square remove another 2.88 m² together. Small trim details and reveal surfaces may still need paint. Record unusual returns separately rather than assuming a simple opening deduction captures every edge.
The paint formula and the important variables
Gross area is the sum of wall width multiplied by wall height. Opening area is the sum of opening width times opening height times count. The net paintable area is gross minus openings, provided the openings do not exceed the measured walls. Multiply net area by the number of coats before applying the product's coverage figure.
Required litres equal net square metres times coats divided by coverage in square metres per litre, then multiplied by any additional allowance. For USA users, coverage can be entered as square feet per US gallon; the app converts it internally. Tin capacity is converted separately, and the count is rounded up to whole tins, not to a fractional container.
Worked example with two coats
Take a rectangular room with two 4 m walls and two 3 m walls, each 2.4 m high. Total wall width is 14 m, so the gross wall area is 33.6 m². Deduct a 0.8 m by 2 m door and two 1.2 m by 1.2 m windows: 1.6 plus 2.88 gives 4.48 m² of openings.
The paintable wall area is 29.12 m². With two coats and manufacturer coverage of 10 m² per litre, the initial calculation is 5.824 litres. A 10% allowance takes that to 6.4064 litres. With 2.5-litre tins you need three complete tins, subject to checking the actual product and surface.
Follow the product label, not a universal rule
Coverage differs between brands, paint types, colours and finishes, and between a porous bare surface and a previously painted wall. A manufacturer may specify a typical range or different expectations for the first and subsequent coats. Use the figure relevant to your chosen product and substrate, not the example that happened to be prefilled.
Primer may be a separate material with its own coverage and number of coats. Similarly, ceilings, woodwork, radiators and exterior masonry may need different paints. Create separate estimates and shopping-list items for different materials rather than combining incompatible products into a single average tin requirement.
Tins, coats and practical buying decisions
The calculator rounds to the next whole tin of the size you entered. It does not optimise a mix of large and small tin sizes to minimise leftover paint. Compare supplier pack options after you know the required liquid volume; for example, two large tins and one small tin could sometimes cost less than several small tins.
Consider whether additional coats are needed because of a strong existing colour, repairs or coverage uncertainty. Ask whether the product requires thinning, stirring or special application conditions, and follow the manufacturer. If buying multiple tins, check batch consistency and keep enough paint for final touch-ups where sensible.
Allowances and common measuring mistakes
Allowances can cover wastage in trays and rollers, small missed areas and uncertainty in the surface measurement. Do not add an arbitrary extra coat through the waste percentage when the finish genuinely requires another full coat: update the coats field instead. That makes the estimate easier to understand and compare.
A frequent error is entering total floor area as though it were wall area. Another is forgetting the two shorter walls or treating window glass as paintable. Conversely, deducting every window while forgetting painted reveals can understate material slightly. Do not ignore the stated coverage unit when switching between litres and US gallons.
Make a final room-by-room purchase list
Check wall measurements, opening sizes, surface condition and whether you need primer. Confirm tin size from the label and compare the calculation to the manufacturer's own advice, especially on textured or highly absorbent surfaces. Keep the label or product code with your project notes so you can buy matching paint later.
Optional costs are based only on prices you enter. No live supplier pricing or currency conversion is used. Add the calculated tins to your project planner and record rollers, masking materials and preparation products separately. For specialist coatings, follow product-specific instructions rather than using a generic emulsion example.
Common questions
Does the calculator allow more than four walls?
Yes. Add as many wall rows as you need within the interface limit, including walls of different heights. The total area is calculated from each individual rectangle.
Do I subtract doors and windows?
You can enter each opening size and count to deduct it. Painted reveals and surrounding trim are not automatically added back, so assess those surfaces separately.
Why might a tin cover less than the stated label area?
Porosity, texture, application method and different coats change real coverage. Use the manufacturer's guidance for the specific material and consider preparation or primer where required.
Are US paint gallons the same as UK imperial gallons?
No. American inputs use US liquid gallons, while UK paint capacity is entered in litres. The app uses the correct conversion rather than treating gallon measures as equivalent.