Sponge City Storage Calculator

Plan resilient drainage with practical sponge storage sizing. Enter inputs, view results, and export reports. Use it for streets, parks, campuses, and developments today.

Inputs

Used in downloads and report headers.
Contributing area draining to the sponge facility.
Often set to a capture depth or design storm.
Choose the method you prefer for estimating runoff.
Typical range 0.2–0.95 depending on surfaces.
%
Converts to C using separate impervious/pervious coefficients.
Imp
Perv
Defaults: impervious 0.95, pervious 0.20.
%
Percent of runoff volume you aim to manage.
%
Covers uncertainty, clogging, and aging performance.
Depth of storage layer (aggregate, voids, chambers).
%
Typical 25–45% for aggregate storage layers.
hr
Many guidelines target 24–72 hours.
mm/hr
Use conservative field-tested rates where possible.
L/s
Set to 0 for pure infiltration storage.
Auto mode estimates footprint from storage depth and porosity.
Used to compute required storage depth and drawdown.

Example data

Use this dataset to sanity-check typical results.

Scenario Catchment Rainfall C Target Depth Porosity Infiltration Outlet Allowed drawdown
Park retrofit 1.0 ha 25 mm 0.75 80% 0.60 m 35% 10 mm/hr 5 L/s 24 hr
Street segment 3,500 m² 18 mm 0.85 70% 0.45 m 30% 6 mm/hr 2 L/s 36 hr
Campus plaza 2.5 acre 1.0 in 0.65 90% 2.0 ft 40% 12 mm/hr 0 L/s 48 hr

Tip: If you use manual footprint mode, add the built footprint and check the “depth required” result.

Formula used

This calculator estimates the design storage volume needed to manage a target fraction of stormwater runoff, while crediting infiltration and controlled outlet discharge over an allowed drawdown time.

  • Captured runoff volume: Vrunoff = A × P × C × f
  • Infiltration credit: Vinfil = Afp × i × t
  • Outlet credit: Vout = q × t
  • Net storage: Vnet = max(0, Vrunoff − Vinfil − Vout)
  • Design storage: Vdesign = Vnet × (1 + SF)
  • Footprint from depth and porosity: Areq = Vdesign ÷ (D × n)

Where A is catchment area, P is rainfall depth, C is runoff coefficient, f is target capture fraction, Afp is facility footprint, i is infiltration rate, q is outlet flow, t is allowed time, D is storage depth, and n is effective porosity.

How to use this calculator

  1. Enter your contributing catchment area and design rainfall depth.
  2. Select a runoff method. Use C if known, or estimate from imperviousness.
  3. Set the target capture and a safety factor for uncertainty.
  4. Provide storage layer depth and effective porosity for the system.
  5. Enter infiltration rate, outlet flow, and allowed drawdown time.
  6. Choose auto footprint sizing for early concepts, or manual footprint for as-built checks.
  7. Click Calculate. Review drawdown status and export CSV/PDF for records.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.