Waterproofing primer Calculator: measuring notes and method
Use the primer’s own coverage
Primer and membrane can have very different yields. Do not carry the membrane’s coverage value into the primer calculation.
Measure the primed area
Only include surfaces that receive this primer. A substrate needing local treatment may have a smaller primed area than the membrane area.
CHECK A PRACTICAL EXAMPLE
An absorbent surface can change the quantity
Compare hypothetical product coverage rates for the same 24 m² substrate, one coat and 4 L containers.
6 m²/L coverage
- Primer required
- 4 L
- Containers to buy
- 1
3 m²/L coverage
- Primer required
- 8 L
- Containers to buy
- 2
Use the chosen membrane system’s primer instructions and substrate-specific consumption. A coverage comparison does not establish compatibility, dilution or suitability.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Coating = (area − excluded area) × coats ÷ coverage per coat × (1 + waste ÷ 100). Containers are rounded up.
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
- Total surface area
- 24 m²
- Openings / excluded area
- 0 m²
- Number of coats
- 2 coats
- Coverage per coat
- 6 m²/L
- Container size
- 4 L
- Waste allowance
- 5 %
Primer required: 8.4 L
Imperial worked example
- Total surface area
- 258.3339 ft²
- Openings / excluded area
- 0 ft²
- Number of coats
- 2 coats
- Coverage per coat
- 244.475 ft²/US gal
- Container size
- 1.0567 US gal
- Waste allowance
- 5 %
Primer required: 2.219 US gal
Follow the calculation
- Net area = 24 − 0 = 24 m².
- Coat area = 24 × 2 = 48 m².
- Quantity = 48 ÷ 6 × 1.05 = 8.4 L. Divide by 4 L and round up for containers.
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