Dose, demand and residual
Every milligram of chlorine you add either gets used up reacting with something in the water, or it stays behind as a residual you can measure. Three equations tie the terms together. All values are in mg/L, measured after the contact time.
Dose is the total chlorine requirement: what the feed pump puts in. To hold a target residual, you have to dose enough to cover the demand first.
Demand is chlorine consumed by iron, manganese, hydrogen sulfide, organics and ammonia. It changes with source water, temperature, pH and contact time, so you find it by test.
Free (HOCl + OCl⁻) is the strong disinfectant. Combined is chloramines from ammonia. A DPD total test reads both; the free test reads free only, so combined = total − free.
Breakpoint chlorination
Set your source water's immediate demand and ammonia, then drag across the chart or use the dose slider. The gap between the dashed 1:1 line and the total residual line is the demand at that dose.
Teaching model: monochloramine forms 1:1 up to a 5:1 Cl₂:NH₃-N weight ratio, then is destroyed by the breakpoint ratio (7.6:1 in theory, 8–10:1 in practice). Real curves are rounder and the hump is lower; run a breakpoint jar test on your own water.
Chlorine dosing calculator
Works out the dose, chlorine mass and product amount for a continuous feed or a one-time batch, such as shocking a cistern. It opens with example values; replace them with yours.