CT Disinfection Value Calculator

About this calculator

The CT value is the core metric water treatment operators use to prove a disinfectant has done its job: it’s simply the disinfectant concentration (C, typically in mg/L) multiplied by the contact time (T, in minutes) the water spends in contact with it. Regulators, most notably the U.S. EPA under the Surface Water Treatment Rule, publish required CT tables for achieving a target log-inactivation of pathogens like Giardia cysts and viruses, varying by disinfectant type, water temperature, and pH.

A water plant must hit or exceed the CT value specified for its disinfectant and target organism to certify the water is safe, so operators calculate CT continuously from chlorine (or ozone, chloramine) residual readings and the hydraulic contact time through the clearwell or pipeline — this calculator reproduces that basic C × T multiplication from your concentration and contact time inputs.