Soakaway Design Calculator

Step 1: Project Details & Site Location

Fields marked * are required

Project Name *

Project Reference

Site Address

Site Postcode

📍 Site location: enter a postcode above to drop a pin

Client Name

Engineer/Technician

Calculation Date

Project Notes

Step 2: Soil Infiltration Rate

Infiltration testing should be conducted following BRE Digest 365 methodology.

BRE Digest 365 Soakage Test Procedure

Excavate a trial pit to the proposed soakaway invert depth. Record the internal length, width, and depth.
Fill the pit with water to a minimum of 75% of the effective depth (measured from the pit base upward).
Allow the water to drain away completely. This first fill saturates the surrounding soil and is NOT measured.
Refill the pit with water to at least 75% of the effective depth again.
Record the time for the water level to drop from the 75% depth mark to the 25% depth mark. This is the soakage time t_p.
Repeat steps 4–5 a minimum of three times and use the worst (slowest) result.

Depth marks explained: If your pit is 1.0m deep, the 75% mark is 0.75m up from the base and the 25% mark is 0.25m up from the base. You are measuring how fast water drains through the middle 50% of the pit.

1. Trial Pit Dimensions

Enter the internal dimensions of the trial pit as excavated.

Pit Length (m) *

Pit Width (m) *

Pit Depth: Effective Depth (m) *

Total usable depth of the trial pit from base to invert level

2. Water Level Measurements

Enter the water depth (measured up from the pit base) at each mark. These are used to calculate the volume of water that drained during the test.

Water Depth at 75% Mark (m) *

Water depth from base when timing starts (75% of pit depth)

Water Depth at 25% Mark (m) *

Water depth from base when timing stops (25% of pit depth)

3. Soakage Time

Time for water level to drop from the 75% mark to the 25% mark. Use the slowest result from at least 3 repeat tests.

Time from 75% to 25% (HH:MM:SS) *

Format: hours:minutes:seconds, e.g. 00:15:30 = 15 minutes 30 seconds

Soil Type	Rate (m/s)	Enter as
Clean gravel	1×10⁻² to 1×10⁻³	0.01 – 0.001
Coarse sand	1×10⁻³ to 1×10⁻⁴	0.001 – 0.0001
Medium / fine sand	1×10⁻⁴ to 1×10⁻⁵	0.0001 – 0.00001
Sandy silt / loam	1×10⁻⁵ to 1×10⁻⁶	0.00001 – 0.000001
Clay (generally unsuitable)	< 1×10⁻⁶	< 0.000001

Site parameters (apply in both modes)

Long-term Infiltration Reduction Factor (default 0.5) *

0.5 = 50% reduction (default assumes clogging over time). Range 0.3–1.0. Use 1.0 for newly constructed systems with regular maintenance.

Groundwater Level Below Soakaway Base (m) *

Minimum 1.0 m required per EA guidance. Enter depth below soakaway invert.

Step 3: Design Rainfall Parameters

Note: Postcode lookup was not completed. Please go back to Step 1 and look up your postcode to use auto-estimation mode, or select another rainfall method below.

Enter M5-60 value (5-year return period, 60-minute rainfall), M5-60/M5-2day ratio, and select return period. Rainfall for other durations will be calculated using Table 1 and Table 2 factors.

M5-60 Rainfall (mm) *

M5-60/M5-2day Ratio *

Region

England & Wales

Scotland & Northern Ireland

Return Period (years) *

Climate Change Factor (%) *

EA epoch

EA band

Management catchment (EA allowance auto-fill)

EA allowance for this catchment (%)

Select your catchment to auto-fill the EA peak-rainfall allowance; the tool then checks that your Climate Change Factor is at least that allowance. Guidance: pick the epoch spanning the design life (2070s for a longer-lived scheme); upper-end band for essential/vulnerable development or where the LPA requires. The catchment dataset has only central (50th) and upper (95th) percentiles. Cross-check on the EA peak rainfall map.

FEH Rainfall Data Entry
Enter design rainfall depths for each storm duration from FEH Web Service. These are your site-specific design rainfall values for the chosen return period. Climate change allowance is applied automatically.

Return Period (years) *

Climate Change Allowance (%) *

Region

England & Wales

Scotland & Northern Ireland

Enter design rainfall depth (mm) for each duration:

Duration	Rainfall Depth (mm)
5 min
10 min
15 min
30 min
60 min (1 hr)
120 min (2 hr)
240 min (4 hr)
360 min (6 hr)
600 min (10 hr)
1440 min (24 hr)

Step 4: Catchment Area

Define the drainage catchment area. Choose between total area entry or detailed breakdown by surface type.

Add each surface type separately. The runoff coefficient (C_v) adjusts the effective contributing area. Values based on CIRIA C753 Table 24.1 and industry practice.

Effective Catchment Area: 0.00 m²

Step 5: Soakaway Design

Select soakaway type and enter dimensions. Three types are available: rectangular void, precast ring system, or trench.

Soakaway Type *

Rectangular Void Dimensions

Dimension Mode

Crate Module snaps to crate multiples

Custom Dimensions enter any values

Standard crate module: 1.0m (L) × 0.5m (W) × 0.4m (D). Dimensions snap to nearest crate multiple on calculation.

Length (m) * (step: 1.0m) *

Width (m) * (step: 0.5m) *

Depth (m) * (step: 0.4m) *

Void Ratio

0.95: Geocellular Crates

0.30: Clean Gravel

0.35: Coarse Gravel

0.40: No-fines Concrete

Custom void ratio

Use custom crate module size (default: 1.0m x 0.5m x 0.4m)

Step 6: Design Results & Verification

Key Assumptions

Uniform soil permeability assumed throughout infiltration depth.
Groundwater table assumed to be >1m below soakaway invert.
No contribution from surrounding impermeable surfaces unless specified.
Climate change allowance applied as percentage uplift on rainfall.

Refer to the Help panel (?) for full standards references and glossary.

Engineer's Notes Optional: included in PDF report