Soakaway Crate Calculator
Estimate the soakaway crate volume and number of crates needed for patios, driveways, extensions and garden drainage projects. Adjustable for soil type and rainfall event depth.
Enter drainage area details
Patio drainage — typically straightforward domestic installation.
50mm = heavy storm (typical design event). 75mm = extreme event.
Advanced: crate specification (optional)
Check your crate specification sheet
Typical: 95% for polypropylene drainage crates
Extra capacity allowance. Default 10%.
Soakaway crate estimate
Enter your drainage area dimensions to see crate estimate
Common rainfall design events
| Rainfall event | Depth | Typical use |
|---|---|---|
| Light shower | 10–20mm | Minor top-up drainage |
| Heavy rain | 30–50mm | Standard domestic design event |
| Extreme storm | 75–100mm | For high-risk or regulated sites |
50mm is the most common domestic design event for garden patios and paths.
How soakaway crate calculations work
The basic calculation is: drainage area × rainfall depth = water volume. This volume is adjusted for a safety factor and a soil drainage multiplier, then divided by the effective storage capacity of each crate (crate volume × void ratio) to give crate count.
Soil type matters significantly. Clay soils drain much more slowly than sandy soils — this calculator applies a multiplier to account for the slower infiltration rate, so more storage is needed before the water disperses.
Always install soakaway crates at least 5m from any building foundation, wrapped in geotextile membrane, and at least 300mm below finished ground level. Do not install where the water table is high.
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Frequently asked questions
Planning estimates only
These results are estimates for planning purposes only. Actual material quantities can vary based on site conditions, compaction, wastage, product size, installation method and supplier guidance. For structural, safety-critical or regulated work, always consult a qualified professional.