Hydraulic retention (detention) time for tanks and treatment basins from volume and flow rate, in hours and days.
Result
How to use this tool
Enter tank volume and its unit.
Enter inflow rate and its unit.
Read detention time in minutes, hours and days.
Read how many tank volumes pass per day.
Formula & worked example
A 100 m³ basin at 10 m³/h has a detention time of 10 hours (0.417 days).
Limitations — read this before you rely on it
Theoretical completely-mixed value at steady flow; real effective retention is lower with short-circuiting or variable flow.
Frequently asked questions
The average time water spends in a tank: volume divided by flow rate, also called hydraulic retention time (HRT).
Biological and settling processes need enough contact time; too short a detention time means incomplete treatment.
When inflow shortcuts to the outlet, some water leaves much faster than the average — baffles and good inlet design reduce it.
US gallons here (1 US gal = 3.78541 L); imperial gallons are 4.54609 L and would give a different answer.
Only through volume for this average; shape affects mixing and short-circuiting, not the theoretical retention time.