Detention Time Calculator
How long water stays in a tank — hydraulic retention time — from the tank volume and the flow through it.
Detention time is volume divided by flow rate.
How the detention time calculator works
Detention time is volume divided by flow rate. A 100 m³ tank fed at 50 m³ per day has a two-day retention time.
It is the master variable in water and wastewater treatment: it sets how long a disinfectant has to act, how long particles have to settle, and how long micro-organisms have to work. Too short and the process fails; too long and the tank is wasted capital.
Formula: HRT = volume / flow rate
Worked examples
| Inputs | Detention time | Note |
|---|---|---|
| 100 m³ at 50 m³/day | 48 hours | 48 hours |
| A chlorine contact tank | 2 hours | 2 hours |
| A small flow | 13.8889 hours | 9.6 hours |
FAQFrequently asked questions
What is hydraulic retention time?
The average time water spends in a tank — volume divided by flow rate.
Why does it matter in water treatment?
It sets how long each process has to work. Chlorine contact needs a minimum; settling needs enough for particles to fall; biological treatment needs enough for the organisms.
Is every drop there for that long?
No — that is the average. Real tanks have short-circuiting and dead zones, so some water leaves far sooner. Baffles exist to reduce that.
What is the difference from sludge retention time?
HRT tracks the water; SRT tracks the solids, which are often deliberately held far longer by recycling. In activated sludge the two are decoupled on purpose.
How do I increase it?
A bigger tank or a lower flow. Since flow is usually fixed by demand, capacity is what gets designed.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.