Chlorine CT Calculator

Formula

CT = C × t where C = residual chlorine concentration (mg/L) and t = contact time (minutes).

Variables

• C — residual free chlorine at end of contact (mg/L)
• t — contact time (minutes)
• Temperature — affects CT requirements (higher temp = lower CT needed)
• pH — affects free chlorine spec (HOCl vs OCl⁻)

Example

2 mg/L residual chlorine, 10 minutes contact: CT = 20 mg·min/L. This exceeds the typical 15 mg·min/L for 4-log Giardia inactivation at 20°C, pH 7.

Application

Useful for preliminary disinfection system design, contact tank sizing and regulatory compliance checks in water treatment systems.

Limitations

Official CT tables (e.g. US EPA) account for temperature, pH, free vs combined chlorine and specific pathogen inactivation targets. This tool provides a simplified CT estimate. Full disinfection design requires a qualified water treatment engineer.