Stormwater Detention Calculator
The on-site detention a development needs to hold back the extra runoff it creates — from the change in impervious area and the design storm — with the tank size and the time it takes to drain at a permitted outflow.
Paving a site sends more of each storm straight to the drains.
How the stormwater detention calculator works
Paving a site sends more of each storm straight to the drains. The extra volume is the site area times the increase in impervious fraction times the rainfall depth of the design storm (adjusted by how much of the rain runs off each surface). Detention stores that volume and releases it slowly at the pre-development rate, so the downstream system sees no more than it did before. Drawdown time is the volume divided by the permitted outflow.
Formula: volume = area × depth × (C_post − C_pre); drawdown = volume ÷ outflow
Worked examples
| Inputs | Detention volume | Note |
|---|---|---|
| 2,000 m² from 20% to 75% impervious, 50 mm storm | 35.8 m³ | 35.8 m³; drains in 2 hours |
| A 600 m² house lot, 30% to 60%, 40 mm | 4.7 m³ | 4.7 m³ |
| No change in paving | 0 m³ | none needed |
FAQFrequently asked questions
Detention or retention?
Detention holds water briefly and releases it slowly; retention keeps it (a rainwater tank, infiltration) so it never reaches the drain. Many councils accept a rainwater tank’s airspace as part of the detention volume.
What storm depth should I use?
The council’s on-site detention standard names it — often the 1-in-20-year or 1-in-100-year storm of a stated duration. Depths of 40–80 mm are typical for one-hour design storms in temperate cities; much more in the tropics.
Where do the 0.9 and 0.25 come from?
Runoff coefficients: impervious surfaces shed about 90% of a storm and pervious ones about 25% in a large event. The calculator blends them by the impervious share; a full design uses the local coefficients and a routed hydrograph.
How is the outflow limited?
By an orifice plate or a small-diameter outlet pipe sized for the permitted flow at the tank’s full head. The drawdown time should usually be under a day so the tank is empty for the next storm.
Where these figures come from
- UN-Habitat — Urban Indicators Guidelines — density, green space and public space benchmarks
- World Health Organization — Urban green spaces — the 9 m² per person minimum
- ITE — Parking Generation Manual — parking demand rates by land use
- US Census Bureau — Population and housing — the US national statistics agency
Last checked: September 2026. Default per-capita rates are mid-range figures from national statistical agencies and utility reports; every one is an input you should replace with the local figure.