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Detention time

Reviewed by the Water Cert Prep editorial team. Worked from public plant-operation physics, never a copyrighted study manual. Last verified: 2026-07-24.

How do you calculate detention time?

Divide the tank volume by the flow through it, keeping units consistent. Volume in gallons over flow in gallons per day gives detention time in days; multiply by 24 for hours or by 1,440 for minutes. A 100,000-gallon tank at 500,000 gallons per day has a 0.2-day, or 4.8-hour, detention time.

Detention time = tank volume / flow. Keep the units consistent: volume in gallons over flow in gallons per day gives days, so multiply by 24 for hours. A common exam form is detention time (hours) = volume (gallons) x 24 / flow (gallons per day).

Water enters one side of the basin and takes time to cross it. That crossing time, at the flow the basin actually carries, is detention time.
Detention time diagram A 100,000-gallon basin fed at 500,000 gallons per day: volume divided by flow gives 0.2 day, or 4.8 hours, of detention time. Inlet, Q Volume, V 100,000 gal Outlet DT (hr) = (V × 24) ÷ Q = (100,000 × 24) ÷ 500,000 = 4.8 hr

The formula, worked

Formula - detention time
DT (hr) = ( Volume (gal) × 24 ) ÷ Flow (gal/day)
Worked example
Tank holds 100,000 gal; plant flow is 0.5 MGD = 500,000 gal/dayDays: 100,000 ÷ 500,000 = 0.2 dayTo hours: 0.2 × 24
= 4.8 hour detention time

Common slip Leaving the answer in days when the exam wants hours (or minutes), and mixing MGD with gallons per day. Convert the flow to plain gallons per unit time before you divide, and read which unit the question asks for.

Where this shows up

Detention time sets whether a sedimentation basin settles solids and whether a chlorine contact tank meets its CT target. The CT-disinfection drill uses detention time (adjusted by a baffling factor) as the contact-time input. Verify it in the process calculator.

Process calculator Practice test All math domains

Problem set: 5 worked problems

Each problem is original, authored from the underlying non-copyrightable relationship, with the full worked solution and the exact slip that produces each wrong answer. Work the problem before opening the solution. For a timed, randomized run with a per-topic score, use the free practice test.

Watch for Detention time is volume divided by flow, but the exam changes the unit it wants. Keep volume in gallons and flow in gallons per unit time, then convert: multiply days by 24 for hours or by 1,440 for minutes. Read which unit is asked before you answer.

  1. A tank holds 200,000 gallons and the plant flow is 1.2 MGD. What is the detention time in hours?

    • A 0.17 hour
    • B 96 hours
    • C 6.0 hours
    • D 4.0 hours
    Show the worked solution

    Correct answer: D. 4.0 hours

    Flow 1.2 MGD = 1,200,000 gallons per day. Detention in days = 200,000 / 1,200,000 = 0.1667 day; times 24 = 4.0 hours. Leaving it in days gives 0.17; forgetting to convert MGD to gallons distorts the scale.

    Source: Public relationship: detention time = volume / flow, days x 24 = hours

  2. A chlorine contact tank holds 50,000 gallons and flow is 500 gallons per minute. What is the detention time in minutes?

    • A 10 minutes
    • B 60 minutes
    • C 250 minutes
    • D 100 minutes
    Show the worked solution

    Correct answer: D. 100 minutes

    With flow already in gallons per minute, detention time in minutes is just 50,000 / 500 = 100 minutes. No day conversion is needed because both are per-minute. This 100-minute figure is the input a CT calculation adjusts by a baffling factor.

    Source: Public relationship: detention time = volume / flow (consistent units)

  3. A sedimentation basin holds 300,000 gallons and treats 2.0 MGD. Detention time in hours is:

    • A 2.4 hours
    • B 150 hours
    • C 0.15 hour
    • D 3.6 hours
    Show the worked solution

    Correct answer: D. 3.6 hours

    300,000 / 2,000,000 = 0.15 day; 0.15 x 24 = 3.6 hours. A 3.6-hour detention is typical for a sedimentation basin. Reporting 0.15 leaves the answer in days.

    Source: Public relationship: detention time = volume / flow, days x 24 = hours

  4. You need a 2.5-hour detention time at a flow of 1.0 MGD. What tank volume is required?

    • A 2,500,000 gallons
    • B 41,700 gallons
    • C 60,000 gallons
    • D 104,000 gallons
    Show the worked solution

    Correct answer: D. 104,000 gallons

    Rearrange to volume = detention x flow. 2.5 hours = 2.5 / 24 = 0.1042 day; 0.1042 x 1,000,000 = about 104,000 gallons. Forgetting to convert hours to days (multiplying 2.5 x 1,000,000 and dividing wrong) throws the scale off.

    Source: Public relationship: volume = detention time (days) x flow (gal/day)

  5. A basin holds 80,000 gallons and flow is 400 gallons per minute. What is the detention time in hours?

    • A 200 hours
    • B 5.0 hours
    • C 3.0 hours
    • D 3.33 hours
    Show the worked solution

    Correct answer: D. 3.33 hours

    Detention in minutes = 80,000 / 400 = 200 minutes; 200 / 60 = 3.33 hours. Leaving it at 200 and calling that hours is the trap; convert minutes to hours with 60.

    Source: Public relationship: detention time = volume / flow, minutes / 60 = hours

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