Water and wastewater · Worked step by step
What is the detention time in minutes for 5,000 gallons at 250 GPM?
If math has never been your thing, you are in the right place. Try it on paper first, then we write it out with you, one small step at a time, and explain every step in plain words so you never lose the thread.
The problem
A tank holds 5,000 gallons and the flow through it is 250 gallons per minute. What is the detention time in minutes?
Grab a pen and paper first
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Press play and I’ll write it out with you, one small step at a time.
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The written version, step by step
- 1
Write down what you know
volume = 5,000 gal • flow = 250 GPM
- The tank holds 5,000 gallons.
- 250 gallons flow through every minute.
- Because the flow is per minute, our answer will be in minutes.
Quick trick: Peek at the flow's time unit first. GPM means the answer is minutes.
- 2
Divide volume by flow
time = 5,000 ÷ 250 • time = 20 minutes
- Detention time = volume ÷ flow, so 5,000 ÷ 250.
- How many 250s fit in 5,000? 250 × 20 = 5,000.
- So the answer is 20 minutes.
Quick trick: Stuck on 5,000 ÷ 250? Cross off a zero from each: 500 ÷ 25 = 20.
Remember this for good
Detention time = volume ÷ flow. Gallons ÷ GPM gives minutes.
Same rule as always: volume on top, flow on the bottom. The flow was per minute, so the answer comes out in minutes. The time unit always matches the flow's time unit.
Solve it fast next time
- Detention time = volume ÷ flow.
- Gallons ÷ GPM = minutes. Gallons ÷ GPH = hours.
- Match the answer's unit to the flow's time unit, every time.
The answer: 20 minutes.