Soakwell Volume Calculator
Calculate the volume and sizing of a soakwell needed for stormwater drainage from roof or catchment area, free in your browser.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the roof/catchment area (m²) and the rainfall event (mm).
- Read the litres to capture.
How it works
1 mm of rain on 1 m² is 1 litre, so litres = area × rainfall in mm.
Worked example
100 m² roof, 30 mm rain. = 3000 litres.
Common mistakes
- Using plan area vs actual catchment — use the roof's footprint area.
- Sizing to average rain instead of your local design storm.
Frequently asked questions
What size soakwell do I need?
It is based on roof catchment area and local rainfall intensity; the tool estimates the storage volume so runoff soaks away rather than pooling.
What is a soakwell?
An underground perforated chamber, common in Western Australia, that collects roof and stormwater runoff and lets it drain slowly into the soil.
What rainfall figure should I use?
Use your council's design rainfall intensity for the storm event you must cater for, not a typical day.
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Cite this tool
Zerdly. (2026). Soakwell Volume Calculator. Retrieved from https://www.zerdly.com/tools/soakwell-volume-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.