Operator math, worked
How to calculate detention time
Detention time is how long water stays inside a tank or basin. It matters because settling and disinfection both need enough time. You find it by dividing the tank volume by the flow rate, in matching units.
Last reviewed 2026-08-04
The formula
detention time = tank volume / flow rate
The steps
Match the units
Use the same volume and flow units. Gallons with gallons per minute gives minutes; gallons with gallons per hour gives hours.
Divide
Divide the tank volume by the flow rate. The answer is the time the water spends in the tank.
Worked example
A tank holds 50,000 gallons and the flow is 100 gallons per minute. What is the detention time?
- 50,000 / 100 = 500 minutes
- In hours: 500 / 60 = about 8.3 hours
Answer: 500 minutes, or about 8.3 hours
Common questions
- Why does detention time matter?
- Settling and disinfection both need enough time in the tank to work.
- What lowers detention time?
- A higher flow rate or a smaller tank.
- How do I get hours from minutes?
- Divide the minutes by 60.