What does CT stand for?

CT stands for Concentration × Time. It is the product of the residual disinfectant concentration (mg/L) and the effective contact time (minutes, typically T10), giving units of mg·min/L.

What is T10 contact time?

T10 is the time it takes for 10% of the water to travel through a contact basin. It accounts for short-circuiting and is more conservative than the theoretical hydraulic retention time.

Why does temperature matter for CT?

Disinfection reactions slow at lower temperatures, so a higher CT is required to achieve the same log inactivation credit in cold water. EPA tables provide CT values at 0.5°C, 5°C, 10°C, 15°C, 20°C, and 25°C.

Why is Cryptosporidium harder to inactivate with chlorine?

Cryptosporidium oocysts are highly resistant to free chlorine. EPA LT2ESWTR requires ozone, chlorine dioxide, or UV for Cryptosporidium credit — free chlorine does not receive credit for Crypto inactivation at practical doses.

What is log inactivation?

Log inactivation is a logarithmic measure of pathogen reduction. 1-log = 90% reduction, 2-log = 99%, 3-log = 99.9%, 4-log = 99.99%, etc.

Which regulations require CT calculations?

In the US, the Surface Water Treatment Rule (SWTR), LT1ESWTR, and LT2ESWTR require CT calculations for surface water and groundwater-under-the-influence systems. WHO Guidelines for Drinking-water Quality also reference CT values.

Disinfection CT Value Calculator

Enter disinfectant concentration and contact time to compute CT value, then compare against EPA/WHO tables for Giardia, Crypto, and virus inactivation.

Inputs

Disinfectant type

Residual concentration (mg/L)

Measured residual at end of contact zone

T10 contact time (minutes)

Time for 10% of flow to exit the contact zone

Water temperature

pH (for free chlorine)

Higher pH reduces free chlorine disinfection efficiency

CT Result

— mg·min/L

Enter values above and click Calculate

Log Inactivation Credits

Based on EPA LT1ESWTR / SWTR reference CT tables. CT achieved vs. CT required at selected temperature and pH.

Calculate to see inactivation credits

EPA Reference CT Values

Showing — at —°C, pH —

Pathogen	1-log	2-log	3-log	4-log	Unit
Select inputs and calculate

Source: US EPA Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems. "N/A" = this disinfectant does not receive credit for this pathogen under the referenced rule.

Summary

CT value (concentration × time) is the fundamental measure used in water treatment to verify adequate disinfection. This calculator multiplies your measured residual disinfectant concentration by the contact time to produce the CT value in mg·min/L, then compares it against EPA and WHO reference values for common pathogens including Giardia lamblia, Cryptosporidium parvum, and enteric viruses at selected temperatures.

How it works

Enter the residual disinfectant concentration in mg/L measured at the end of the contact zone.
Enter the T10 contact time in minutes (the time for 10% of the water to travel through the contact zone).
The calculator multiplies concentration × time to compute the CT value (mg·min/L).
Select your disinfectant type (free chlorine, chloramine, chlorine dioxide, or ozone) and water temperature.
Results are compared against EPA LT1ESWTR / LT2ESWTR reference tables with explicitly labeled 1-, 2-, 3-, and 4-log columns.
Achieved log inactivation credits are displayed for Giardia, Cryptosporidium, and viruses.

Use cases

Verify disinfection compliance at a small community water system.
Train operators on the relationship between chlorine dose, contact time, and log inactivation.
Compare free chlorine vs. chloramines efficacy at different temperatures.
Estimate CT requirements before designing a contact chamber.
Cross-check CT values against WHO drinking water guidelines.
Quickly assess whether a system meets 3-log Giardia inactivation under LT1ESWTR.
Evaluate ozone CT requirements for Cryptosporidium credit.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu