CT Disinfection Value Calculator

How it is calculated

CT is a product, CT = C × T, where C is the residual disinfectant concentration in mg/L at the end of the contact chamber and T is the contact time in minutes. The result is in mg·min/L and is shown to one decimal; if CT is below 1 it keeps three significant digits (0.5 mg/L for 0.25 min shows 0.125) and fractional minutes are shown to two decimals instead of being rounded. Both values must be above zero or there is no result. The tool only multiplies; it does not look up the CT required for your disinfectant, water temperature, pH or target log-inactivation, so you must compare the number with the table that applies under your own rules. Contact time should be the effective time the water spends in the basin, not simply volume divided by flow, because short-circuiting lets part of the water leave sooner.

Common mistakes

• Using the dosed concentration instead of the residual at the end of the contact time. Chlorine is consumed on the way, so the residual leaving the basin is lower than what was applied, and CT counts the lower figure.
• Using the theoretical detention time (volume ÷ flow) as T. Real basins short-circuit, so the effective time is shorter than the theoretical one.
• Entering hours or seconds. The tool expects minutes, so 0.75 hours must be entered as 45.
• Comparing CT with a requirement without matching temperature and pH. Required CT depends on both, so a value that passes in warm water can fall short in cold water.
• Entering the concentration of a different compound (total chlorine instead of free chlorine, or chlorine dioxide). Required CT values are tied to the specific disinfectant.