Operator math, worked
How to calculate chlorine dose, demand, and residual
When you add chlorine to water, some gets used up killing organisms and reacting with material (the demand) and some is left over (the residual). The dose is the total you add. Knowing any two lets you find the third.
Last reviewed 2026-08-04
The formula
dose = demand + residual (all in mg/L)
The steps
Know the relationship
The chlorine you add (dose) equals what gets used up (demand) plus what is left (residual).
Rearrange for what you need
Demand = dose minus residual. Residual = dose minus demand. Just move the pieces.
Worked example
You dose 3.0 mg/L of chlorine and measure a 0.5 mg/L residual. What was the demand?
- Demand = dose - residual
- Demand = 3.0 - 0.5 = 2.5 mg/L
Answer: 2.5 mg/L of demand
Common questions
- What is chlorine demand?
- The chlorine used up reacting with organisms and material before any residual is left.
- Why keep a residual?
- A measurable residual proves disinfectant is still working out to the customer.
- What units?
- Milligrams per liter (mg/L) for all three.