Disinfection CT Calculator – Free Calculator

Understanding the disinfection CT calculator — concept and principle

CT is the product of disinfectant concentration (C) and contact time (T). Water treatment plants use it to show that chlorine or another disinfectant has had enough "dose" to inactivate pathogens such as Giardia and viruses before water reaches the first customer. You calculate the CT actually achieved and compare it with the CT required by regulatory tables.

CT is the area under the chlorine-time box; double the time or the dose to double it.

How it is calculated

CT = C × T₁₀, in mg·min/L.
C is the residual disinfectant concentration (mg/L) measured at the outlet of the contact basin, not the dose added.
T₁₀ is the time in minutes within which only 10% of the water has passed through the basin, usually found by a tracer test or estimated as the theoretical detention time (volume ÷ peak flow) multiplied by a baffling factor.

The principle

Disinfection follows, to a first approximation, the Chick–Watson law: the logarithm of the surviving fraction of microorganisms falls in proportion to concentration times time. Doubling the residual or the contact time gives roughly the same kill. Using T₁₀ instead of average time is deliberately conservative: real basins short-circuit, and T₁₀ ensures at least 90% of the water receives the calculated CT. Required CT depends on the organism, the disinfectant, temperature (cold water needs more) and, for free chlorine, pH (higher pH needs more).

Worked example

Inputs: residual free chlorine C = 1 mg/L, T₁₀ = 30 min.
CT = 1 × 30 = 30 mg·min/L.
If a regulatory table required, for example, 60 mg·min/L for your conditions, the ratio of achieved to required CT would be 30 ÷ 60 = 0.5, meaning the target is not met.

Each point is the calculator re-run with only “Residual chlorine (mg/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Residual chlorine (mg/L)	CT (mg·min/L)
0.25	7.5
0.5	15
0.75	22.5
1	30
1.25	37.5
1.5	45
2	60

How to read the result

CT means nothing alone; compare it with the required CT for your pathogen, log-inactivation target, disinfectant, pH and temperature. A ratio of achieved to required CT of 1.0 or more meets the target. Viruses are generally inactivated by free chlorine at much lower CT than Giardia, while Cryptosporidium is highly resistant to chlorine and is normally controlled by filtration, UV or ozone. Chloramines need far higher CT than free chlorine for the same effect.

Common mistakes

Using the chlorine dose instead of the residual remaining at the basin outlet.
Using the average detention time instead of T₁₀, which overstates CT.
Calculating with average flow instead of peak hourly flow.

Keep in mind: This tool only multiplies two numbers. Compliance with drinking-water rules depends on official CT tables, tracer studies and local regulations, checked with the authority and a qualified engineer.

Frequently asked questions

What is a baffling factor?

It is the ratio T₁₀ ÷ theoretical detention time. US EPA guidance gives typical values from 0.1 for an unbaffled tank to 1.0 for pipe (plug) flow; a tracer test gives the real value.

Why does pH matter for chlorine?

Free chlorine exists as hypochlorous acid (HOCl) and hypochlorite (OCl⁻). HOCl is the stronger disinfectant and dominates below about pH 7.5, so disinfection is more effective at lower pH.

Sources: US EPA, Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources (SWTR Guidance Manual), 1991 · MWH, Water Treatment: Principles and Design, Wiley

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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