Soakaway Size Calculator
Enter the area that drains to the soakaway, the design rainfall, your soil type, the fill and how deep you can dig. The calculator gives the soakaway volume, its plan size and how many crates or tonnes of gravel to buy.
- Roof or drive area
- Gravel or crates
- Pit size
A design rainfall of about 25 mm covers a heavy storm in most of the UK. Use more in very wet areas.
A soakaway holds storm water until the ground can absorb it. It has to store the rain that falls in a heavy storm on everything draining to it. Crates are 95% empty space, so a crate soakaway is about a third of the size of one filled with gravel.
Soakaway size chart
25 mm rainfall, loam soil. Search for an area.
| Area (m²) | Water (L) | Gravel pit (m³) | Crate pit (m³) | Crates |
|---|---|---|---|---|
| 20 | 425 | 1.42 | 0.45 | 2 |
| 40 | 850 | 2.83 | 0.89 | 4 |
| 60 | 1275 | 4.25 | 1.34 | 6 |
| 80 | 1700 | 5.67 | 1.79 | 8 |
| 100 | 2125 | 7.08 | 2.24 | 10 |
| 150 | 3188 | 10.63 | 3.36 | 15 |
Gravel volumes are for 30% void. Crates are 0.23 m³ each.
Roof area and volume
Loam soil, 1 m deep. Slide the roof area.
The section shows the pit lined with membrane, fed from the downpipe on the left.
How to size a soakaway
Storm, soil and fill. Step through it.
Drained area
Roof and paving.
Before digging, do a percolation test: fill a test hole with water and time how fast it drains.
Gravel, crates or a chamber
Soakaway types. Compare them.
Rubble or stone filled.
Plastic cells.
Perforated concrete rings.
Wrap crates in geotextile membrane so soil does not wash in and block them.
Before digging
Site checks. Tick each item.
Tick each item as you go.
Never put a soakaway where water can run back toward the house, or on a slope above a neighbour's property.
What you get
Soakaway design. Each tile explains one result.
In litres.
Of the pit.
Square, at your depth.
Crates or gravel.
Drainage advice.
Building control may ask for a BRE Digest 365 design for larger areas or new builds.
Frequently Asked Questions
No question matches that search.
How big should a soakaway be?
Big enough to hold the water from a heavy storm, divided by the fill's void ratio. For a 60 m² roof with gravel fill, about 4 m³.
How far should a soakaway be from a house?
At least 5 metres, so water does not soften the foundations.
Are crates better than gravel for a soakaway?
They hold about three times as much water for the same size, so the hole is smaller.
Can I put a soakaway in clay?
Clay drains very slowly. Do a percolation test, and if it fails, use a storage tank or a drain.
How deep should a soakaway be?
Usually about 1 to 1.5 metres, with the base above the water table.
What is a percolation test?
A test of how fast water drains from a hole, used to check the soil can take a soakaway.
Do I need permission for a soakaway?
For new builds and extensions, building control will usually want to see the design.
Storm water handled
Work out the water to store, allow for your soil and divide by the fill's void. Then dig a pit that size well away from the house.
More size calculators
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