About this Paint Calculator
Estimate paint litres from wall area, coats and coverage. Use this free Paint Calculator to create a quick, repeatable material estimate before requesting prices or preparing a detailed takeoff.
The calculator works in both metric and imperial units. Measurements are converted internally before the formula is applied, which prevents unit-system differences from changing the underlying result.
Measurements and information you need
The calculation uses Total wall length, Height, Doors and windows area, Number of coats, Coverage per litre / gallon. Measure the actual project rather than relying only on nominal room or product names. If the project is irregular, divide it into simple sections, calculate each section, then add the results.
- Total wall length: enter the measured or manufacturer-specified value in the unit shown beside the field.
- Height: enter the measured or manufacturer-specified value in the unit shown beside the field.
- Doors and windows area: enter the measured or manufacturer-specified value in the unit shown beside the field.
- Number of coats: enter the measured or manufacturer-specified value in the unit shown beside the field.
- Coverage per litre / gallon: enter the measured or manufacturer-specified value in the unit shown beside the field.
Formula and calculation method
Openings are deducted and the result is converted to litres or US gallons.
Worked metric example: Total wall length: 20 m; Height: 2.7 m; Doors and windows area: 4 m²; Number of coats: 2 ; Coverage per litre / gallon: 10 m²/L. With a 10% allowance where applicable, the net result is 10 L and the final result is 11 L. Purchase summary: 2 × 10 L cans.
How to use the calculator
- Choose Metric or Imperial and keep all measurements in that system.
- Enter the finished project dimensions and any product-specific coverage, density, spacing or quantity requested.
- Set waste to zero to see the pure mathematical result, or enter a realistic allowance for ordering.
- Select Calculate, then review the base quantity, added allowance and purchase summary separately.
- Compare the result with product packaging, supplier increments and the approved project documents before ordering.
How to read the result
In this metric example the result is expressed in L. Base quantity is the net calculated requirement, rounded up for whole-package results. Waste allowance is the extra quantity added where the calculation supports it. The selected unit system determines the result unit shown in the tool.
The main result is expressed in L. “Base quantity” is the net calculated requirement. “Waste allowance” is the extra amount added after the geometry or rate calculation.
Continuous results such as area, length, volume or mass retain useful decimal precision. Suppliers may still round them to their delivery or package increment.
Choosing a waste allowance
Waste is not a universal fixed percentage. Simple rectangular layouts may need only a small allowance, while diagonal patterns, many corners, fragile materials, colour matching or uncertain site dimensions can require more. Start with the net result, then use product instructions and installer experience to choose the final percentage.
Common mistakes to avoid
- Assuming one coat when the finish needs two or more.
- Using ideal coverage on porous or textured surfaces.
- Failing to separate products for walls, ceilings and trim.
Walls planning notes
Calculate each wall face separately and subtract openings only when the product supplier recommends it. Corners, returns, reveals, bond patterns and finish coats often need their own allowance.
Check before ordering or building
- Porous surfaces and major colour changes reduce effective coverage.
- Use the coating manufacturer’s coverage figure.
Measure critical dimensions at least twice and keep a written record of the assumptions used. Compare the calculated requirement with current manufacturer literature, local code and the supplier’s package or delivery increments.
Frequently asked questions
How accurate is the Paint Calculator?
The arithmetic and unit conversions follow the formula displayed on this page. Real-world accuracy depends on the measurements, product data, installation method, site conditions and chosen waste allowance.
Can I use metric and imperial units?
Yes. Switch the unit control before entering values. The calculator converts inputs to a common internal unit and then displays the result in the selected system.
Should I add a waste allowance?
Add waste when material will be cut, overlapped, spilled, compacted, damaged or sold only in whole packages. Use zero when you want the net mathematical quantity first.
Is this result suitable for ordering materials?
Use it as a planning and quotation check. Confirm package coverage, density, yield, minimum order and technical requirements with the manufacturer, supplier or qualified project professional.
Technical basis and references
Geometry is calculated in SI internally and converted only for display. Length, area and volume conversions use exact NIST conversion factors. Product yield, density and coverage values remain editable because they vary by manufacturer, formulation and market.