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Detention Time Calculator
Detention time is how long water stays in a tank at a given flow. It shows up on nearly every water and wastewater operator exam, and the usual trap is a units mismatch.
Worked example
Inputs: Tank volume = 150000 gallons, Flow = 1.2 MGD.
- Detention time = volume ÷ flow (units must be compatible)
- Flow = 1.2 MGD = 1,200,000 gal/day ÷ 24 = 50,000 gal/hr
- Detention = 150,000 gal ÷ 50,000 gal/hr
- = 3.000 hours (180 minutes)
Gallons ÷ gallons-per-hour leaves hours. If you divide by gallons-per-DAY you get days — a factor-of-24 error.
When operators use this
Operators use detention time to size clarifiers, contact basins, and chlorine contact chambers, and to check whether a unit is getting enough contact time to do its job.
Frequently asked questions
What is detention time?
The average time a volume of water spends in a tank, found by dividing the tank volume by the flow rate through it.
Why do I keep getting the wrong answer?
Almost always a units problem. Flow in MGD is per day, so you have to convert to gallons per hour before dividing into a tank volume in gallons, or your answer is off by a factor of 24.
Is this calculator free?
Yes. No account, no email, no ads. It runs entirely in your browser.