Detention Time Calculator

Work out detention time instantly with clear inputs, formula shown and shareable results.

About Detention Time Calculator

Theoretical detention time is volume divided by flow, but real tanks short-circuit. The baffling factor converts theoretical time to T10, the time in which 10% of the water has passed through — 0.1 for an unbaffled tank, 0.5-0.7 with baffling and 0.7 or better for a serpentine contact tank. Disinfection credit uses CT, residual times T10.

Formulas

Detention time

Theoretical = volume x 24 / flow; T10 = theoretical x baffling factor

CT value

CT = disinfectant residual x T10 in minutes

Worked example

With the default values below, Detention Time Calculator returns Effective Contact Time of 2.0160. Change any input above and every figure here is recalculated in your browser.

Inputs used

Tank Volumeminimum 0 m3: 1,200 m3
Flow Rateminimum 0 m3/day: 10,000 m3/day
Baffling Factoraccepts 0.01–1: 0.7
Disinfectant Residualminimum 0 mg/L: 1 mg/L

Results

Effective Contact Time: 2.0160
Theoretical Detention Time: 2.8800
T10 In Minutes: 120.96
CT Value Achieved: 120.96

Step by step

1Values used
2Detention time
3CT value
4Effective Contact Time= 2.0160
5Theoretical Detention Time= 2.8800
6T10 In Minutes= 120.96
7CT Value Achieved= 120.96

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Disinfection CT credit is a regulated compliance calculation. Baffling factors must be justified by tracer study or the applicable regulatory default.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Disinfection CT credit is a regulated compliance calculation. Baffling factors must be justified by tracer study or the applicable regulatory default.

Frequently Asked Questions

Why use T10 rather than mean detention time?

Because disinfection must protect the fastest-moving fraction of water, not the average. Using mean time would credit the plant for contact that a tenth of the water never receives.

How is the baffling factor determined?

By a tracer study, which measures the actual residence time distribution. Regulatory guidance provides default factors by tank configuration where no tracer study exists.