Calculators / Detention Time
Detention Time calculator
Detention time tells you how long, on average, water stays in a tank. Exams give you tank dimensions and a flow, so you calculate the volume in gallons first.
Detention time (hr) = Tank volume (gal) ÷ Flow (gal/hr) Rectangular volume = L × W × D × 7.48 gal/ft³
Worked example
A rectangular primary clarifier is 100 ft long, 30 ft wide and 12 ft deep. Flow is 2,000,000 gpd. What is the detention time in hours?
- Volume = 100 × 30 × 12 × 7.48 = 269,280 gal
- Flow = 2,000,000 gpd ÷ 24 = 83,333.3 gal/hr
- Detention time = 269,280 ÷ 83,333.3 = 3.23 hours
Answer: 3.23 hours
Common mistakes
- Leaving flow in gpd. Divide by 24 to get gal/hr, or multiply your answer by 24 for hours.
- Forgetting 7.48 gal/ft³ and answering in cubic feet.
Check your exam's reference sheet for its exact constants. Some use 3.14 instead of 3.1416, which changes answers in the last digit.