Room Paint Calculator: measuring notes and method
Separate wall paint from ceiling paint
Save the walls and ceiling as separate project items when they use different paint products or finishes.
Keep coats and coverage consistent
Container rounding changes what you buy, not coating consumption. If several surfaces use exactly the same product, combine their unrounded litre requirements before selecting containers.
CHECK A PRACTICAL EXAMPLE
A third finish coat changes consumption
Use the Nowra room dimensions: 4 × 4.5 m, 2.8 m high, with 3.6 m² of openings. Coverage is the recorded product maximum of 16 m²/L.
Two finish coats
- Paint required
- 5.5 L
- Containers to buy
- 1
Three finish coats
- Paint required
- 8.25 L
- Containers to buy
- 1
Litres increase even when the rounded 15 L container count stays the same. The case study’s three finish coats were user-specified; primer is a different product and is calculated separately.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Wall area = 2 × (length + width) × height − openings. Paint = area × coats ÷ coverage, with waste.
Product reference: Sherwin-Williams coverage guidance
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Length
- 6 m
- Width
- 4 m
- Wall height
- 2.4 m
- Openings / excluded area
- 0 m²
- Coats
- 2 coats
- Coverage per coat
- 10 m²/L
- Container size
- 4 L
- Waste allowance
- 5 %
Paint required: 10.08 L
Imperial worked example
- Length
- 19.685 ft
- Width
- 13.1234 ft
- Wall height
- 7.874 ft
- Openings / excluded area
- 0 ft²
- Coats
- 2 coats
- Coverage per coat
- 407.4584 ft²/US gal
- Container size
- 1.0567 US gal
- Waste allowance
- 5 %
Paint required: 2.6629 US gal
Follow the calculation
Lengths in this working are in metres.
- Net wall area = 2 × (6 + 4) × 2.4 − 0 = 48 m².
- Paint = 48 × 2 ÷ 10 × 1.05 = 10.08 L.
- Divide by 4 L per container and round up.
Unit conversions
Liquid conversions use US gallons (3.785411784 L), not UK imperial gallons (4.54609 L).
Reviewed 19 October 2026. How calculations are checked · Report a correction