Bioretention / Rain Garden Calculator

Site & Design Parameters

Fields marked * are required

Project Name * ?

Site Reference *

Contributing Areas

Contributing Impermeable Area * ?

m²

Contributing Pervious Area * ?

m²

Design Storm Event (Attenuation)

Design Storm Return Period * ?

For attenuation design (typically 1–2 year)

Design Storm Duration * ?

minutes (critical for hydrograph)

Rainfall & Storm Design

Design Rainfall Depth (Attenuation) * ?

mm (base design rainfall, before climate uplift)

Climate Change Allowance (EA May 2022) * ?

Source: EA May 2022 Climate Change Allowances Table 3 (peak rainfall intensity, surface water). Choose by time horizon + band: 2050s central 20 % / higher 25 % / upper 35 %, 2080s central 25 % / higher 30 % / upper 40 %, for 3.3 % AEP (30-year) sizing. For 1 % AEP (100-year) use the higher column (e.g. 2080s upper = 50 %). Check EA guidance for your river-basin district.

% uplift to rainfall depth

Adjusted Rainfall Depth (with Climate Change) * ?

mm (used for attenuation storage calculations)

Design Rainfall Intensity * ?

mm/hr

Design Rainfall Depth (for Treatment) * ?

mm (typical 10–15mm, for water quality)

Site Ground Conditions

Soil Infiltration Rate (at base of bioretention) * ?

Infiltration classification per BS 3882

Depth to Seasonal High Water Table * ?

metres below ground surface

Bioretention System Base Depth (below ground surface) * ?

metres (system depth)

Filter Media & System Configuration

Filter Media Specification

Filter Media Depth * ?

mm (range 300–800)

Filter Media Type *

Infiltration per CIRIA C753

Filter Media Permeability *

mm/hr (auto-set from type, or custom)

Filter Media Porosity * ?

(range 0.15–0.40)

Ponding & Storage Layers

Maximum Ponding Depth * ?

mm (range 50–300)

Gravel Storage Layer Depth * ?

mm (range 100–400)

Gravel Porosity *

(typical 0.35–0.45)

System Type & Overflow

System Type * ?

Choose based on infiltration rates and GW depth

S2 Interception Ratio Limit ?

Imp:Bio area ratio limit. Leave blank for auto (25:1 infiltrating, 5:1 lined)

Overflow Type *

Method of handling flows exceeding design capacity

Calculate & Review

Quick Results

Design Flow & Area Calculations

Design Inflow Calculation

Runoff coefficient (impermeable) 1.0

Design rainfall intensity — mm/hr

Total contributing area — m²

Design inflow rate — m³/s

Filter Area Sizing (Darcy's Law)

Formula: A = Q / k

Design flow — m³/s

Filter media conductivity — m/s

Required filter area — m²

Storage Volume Calculation

Ponding volume — m³

Filter media storage — m³

Gravel layer storage — m³

Total active storage volume — m³

Drawdown & Drainage Time

Total storage volume — m³

Drainage rate (k × A) — m³/s

Drawdown time — hours

48-hour limit (Standard S3) —

Suggested Dimensions:
If rectangular bioretention with L:W = 2:1, dimensions would be approximately —
Adjust L and W to match site layout; required area is fixed.

Compliance & Design Summary

Standards Compliance Table

Standard	Requirement	Result	Status
S2	Interception: impermeable area ≤ ratio × bioretention area	—	—
S3	Attenuation: adequate storage volume (with climate change uplift)	—	—
S3	Drawdown time ≤ 48 hours	—	—
S3	Groundwater depth ≥ 1.0m below lowest point	—	—
S3	Filter area × design storm volume check	—	—
S3	Unsaturated zone clearance ≥ 1.0m (EA May 2022 guidance)	—	—
S4	TSS mitigation factor ≥ 0.7	0.7	NOTED
S4	Metals mitigation factor ≥ 0.6	0.6	NOTED
S4	Hydrocarbons mitigation factor ≥ 0.6	0.6	NOTED
S7	Maintenance access: sides ≥1.5m apart (safe access)	Review design	CHECK

Pollutant Removal Efficiency (CIRIA C753)

Typical Removal Rates for Bioretention

Pollutant Parameter	Removal Rate
Total Suspended Solids (TSS)	85–95%
Total Phosphorus (TP)	50–80%
Total Nitrogen (TN)	30–50%
Heavy Metals (Cd, Cu, Pb, Zn)	80–95%
Hydrocarbons (PAH, TPH)	>90%

Removal rates depend on filter media depth, residence time, and vegetation type. Rates increase with design life maturation. Reference: CIRIA C753 Table 26.3

Half-Drain Time Analysis

Half-Drain Time

24.0

hours Pass

Max Allowable

48.0

hours (Defra S3)

Design Summary

Bioretention Sizing Summary

Required vegetated area — m²

Suggested length × width (L:W = 2:1) —

Maximum surface ponding — mm

Total active storage volume — m³

Drawdown time (48-hour max) — hours

System type —

Underdrain diameter —

Overflow type —

Key Assumptions & Notes

Runoff coefficient C = 1.0 for impermeable areas (full generation)
Water quality mitigation factors per CIRIA C753 §18
Drawdown via filter media and underdrain (if lined); infiltration to base soil (if unlined)
Design assumes level ponding; sloped surfaces reduce effective depth
Infiltration rate must be verified by field testing (constant head permeameter test)
If infiltration rate <1×10⁻⁶ m/s or GW <1m, system must be lined with impermeable membrane
Maintenance access: ensure minimum 1.5m width on all sides (S7 requirement)
Vegetation establishment: select plants tolerant of temporary inundation (40–48-hour max wet period)

Export Report

Engineer's Notes Optional, included in PDF report