Soakaway Calculator UK
Estimate required soakaway storage, check your proposed dimensions, and review drain down performance for UK style SuDS planning.
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Enter your project values and click Calculate.
Expert Guide: How to Use a Soakaway Calculator in the UK
A reliable soakaway calculator helps you estimate whether rainwater from roofs, driveways, and hardstanding can be stored and infiltrated safely on your site. In the UK, this is not only a design question, it is often a planning, building control, and flood risk compliance question too. The best designs sit inside a Sustainable Drainage Systems (SuDS) strategy and align with recognised guidance such as Approved Document H, local authority drainage policies, and engineering approaches based on BRE Digest 365 percolation testing.
This page gives you a practical method to size a domestic scale soakaway quickly. It is not a substitute for detailed engineering on complex or high risk sites, but it does provide a clear first pass that is useful for homeowners, builders, and designers preparing early layouts, extension plans, and pre application documentation.
What the calculator is doing
The tool combines five core calculations:
- Runoff generation: catchment area multiplied by design rainfall depth and a runoff coefficient that reflects surface permeability.
- Climate uplift: increases rainfall depth to account for future climate intensity assumptions.
- Safety margin: applies a factor to allow for uncertainty and practical variation.
- Storage check: compares required storage volume against proposed soakaway geometry and void ratio.
- Drain down check: estimates how quickly stored water can infiltrate through base and sidewall surfaces using your soil infiltration rate.
In simple terms, your soakaway should be large enough to hold the storm runoff and permeable enough to empty in an acceptable time period. Many UK practitioners use 24 hours as a practical benchmark for drain down in domestic schemes, though your local authority or project constraints may require different values.
Regulatory context in the UK
Drainage design should always be linked to current policy and local requirements. For domestic and small development projects, three authoritative starting points include:
- Building Regulations Approved Document H (Drainage and waste disposal)
- Non statutory technical standards for sustainable drainage systems (England)
- Flood risk and coastal change guidance
These sources frame how runoff should be managed and why infiltration based methods like soakaways are usually preferred where ground conditions are suitable. In many councils, hierarchy based drainage statements are expected, and this often means demonstrating why infiltration is feasible before moving to other discharge options.
Input data quality matters more than software complexity
A polished calculator interface is useful, but the output is only as good as the assumptions fed into it. The three most common reasons soakaway designs fail on site are: unrealistic infiltration rates, underestimation of impermeable catchment area, and overlooked construction constraints such as groundwater, utility corridors, or proximity to foundations. If your assumptions are weak, even a mathematically correct result can be operationally wrong.
Typical UK rainfall statistics and what they mean for design
Annual rainfall gives useful context for regional wetness, although storm design should use event based depth or intensity data. The table below shows long term average annual rainfall figures often cited for UK locations (rounded values). These figures illustrate why one national default is rarely appropriate.
| Location | Approx. annual rainfall (mm) | Design implication |
|---|---|---|
| London | ~600 | Lower annual totals, but intense summer storms can still drive large short term runoff volumes. |
| Birmingham | ~750 | Moderate annual rainfall with increasing emphasis on resilience in urban catchments. |
| Manchester | ~900 | Higher annual wetness means drainage systems can have less recovery time between events. |
| Cardiff | ~1150 | Wet climate context supports robust storage and careful infiltration verification. |
| Glasgow | ~1250 | High rainfall climate increases sensitivity to under sizing and poor maintenance. |
Rainfall values are rounded and presented for planning context only. Use project specific design rainfall and return period criteria for final engineering.
Runoff coefficients for common surfaces
A runoff coefficient represents the proportion of rainfall that becomes direct runoff. Roofs and dense paving usually route most rainfall rapidly, while permeable landscapes retain and delay more water. Typical ranges used in practice are shown below.
| Surface type | Typical runoff coefficient range | Practical value often used in early sizing |
|---|---|---|
| Impermeable roof | 0.90 to 1.00 | 0.95 |
| Dense asphalt or concrete | 0.85 to 0.95 | 0.90 |
| Block paving (limited infiltration) | 0.70 to 0.90 | 0.85 |
| Compacted gravel | 0.50 to 0.80 | 0.70 |
| Landscaped soft area | 0.20 to 0.60 | 0.50 |
How to choose a realistic infiltration rate
The infiltration rate is usually the most sensitive variable in soakaway sizing. If this number is optimistic, the system might appear compliant on paper while draining slowly in reality. Good practice is to use a field based percolation test approach and adopt conservative design values rather than peak one off readings. In very low permeability soils, infiltration systems may not be suitable and alternative attenuation and controlled discharge routes could be required.
As a rough context:
- Very low rates can indicate heavy clay and potential failure risk for infiltration systems.
- Moderate rates can support practical domestic soakaways if storage volume is adequate.
- High rates can reduce drain down time significantly but still require checks for groundwater and receiving strata stability.
Step by step method for domestic use
- Measure all connected impermeable area feeding the soakaway, including roof plan area and any hardstanding that actually drains to it.
- Select a runoff coefficient that reflects the real surface condition, not an idealized condition.
- Choose a design storm depth consistent with project risk appetite and local expectations.
- Apply climate uplift and a safety factor to avoid under design.
- Enter proposed soakaway dimensions and void ratio (crate systems are often around 95% voids, rubble systems much lower).
- Use tested infiltration data where possible and run the drain down check.
- Confirm that both criteria pass: storage capacity and drain down time.
Worked interpretation example
Suppose a detached house has 120 m² roof area draining to one soakaway. With a runoff coefficient of 0.95, design storm of 75 mm, and climate uplift of 40%, the effective depth becomes 105 mm. The runoff volume is approximately 11.97 m³ before safety factor. If a 1.10 safety factor is used, required storage is about 13.17 m³.
If the proposed soakaway is 3 m x 2 m x 1.2 m with 95% void ratio, available storage is about 6.84 m³, which is below the required amount. Even if soil infiltration is moderate, storage deficiency alone means the design does not pass. This is exactly the type of issue a calculator should highlight early, before excavation design and procurement.
Crate soakaway versus rubble pit
Modern geocellular crate systems provide much higher void ratios and predictable geometry than traditional rubble pits. That often means smaller footprint for the same storage target. Rubble systems can still work in some contexts, but they are harder to model accurately and can lose effective volume over time due to settlement and migration of fines if detailing is poor.
- Crate systems: high void ratio, compact footprint, repeatable capacity, easier to model.
- Rubble pits: lower effective voids, bulkier for equal storage, greater variability in performance.
Installation and location checks you should never skip
Even a well sized soakaway can create structural or nuisance problems if placed incorrectly. Always check separation distances to buildings, boundary constraints, tree root zones, and underground services. Avoid placing infiltration structures where long term groundwater conditions or contamination pathways create unacceptable risk. For larger schemes or difficult ground, professional geotechnical and civil engineering input is essential.
Maintenance planning is part of the design
SuDS performance is strongly linked to maintenance. Silted downpipes, blocked leaf guards, and sediment laden upstream gullies can progressively reduce soakaway function. Include simple maintenance access and routine checks in your design documents:
- Inspect inlets and catchpits at least twice yearly and after major storms.
- Remove leaf litter and sediment before it is transported into the soakaway.
- Record standing water duration after heavy rainfall to identify early performance decline.
- Schedule remedial cleaning when recurring surcharge signs appear.
Common mistakes in soakaway calculations
- Ignoring climate uplift and using historical rain depth without resilience margin.
- Assuming all paving is permeable when compaction or subbase design actually drives high runoff.
- Treating manufacturer brochure infiltration claims as a substitute for site testing.
- Checking storage only and forgetting drain down compliance.
- Not revisiting the model when layout changes increase connected area.
When to move from calculator to professional design package
A web calculator is ideal for concept decisions and early feasibility. You should escalate to a detailed drainage design process when any of the following apply: complex multi plot development, challenging geotechnical conditions, protected water environment constraints, high groundwater, known flooding issues, or local authority requests for formal hydraulic evidence. At that stage, detailed rainfall profiles, critical duration testing, staged storage routing, and formal reporting become necessary.
Final takeaway
For UK projects, a soakaway is not just a hole in the ground. It is an engineered infiltration asset that must balance runoff, storage, emptying time, and compliance. A robust calculator helps you make better early decisions: whether infiltration is plausible, what approximate volume is needed, and how quickly your design may recover after a storm. Use conservative assumptions, validate infiltration with field data, and align your proposal with national guidance and local policy expectations. Done properly, soakaways can provide reliable, low energy drainage performance and support wider flood resilience goals.