Compared
Chlorine demand vs. chlorine residual
Chlorine demand and chlorine residual are two parts of the same dose. Demand is the chlorine used up reacting with material and organisms in the water. Residual is what is left over and still available to disinfect. The dose equals the demand plus the residual.
Last reviewed 2026-08-04
| Chlorine demand | Chlorine residual | |
|---|---|---|
| What it is | The chlorine used up | The chlorine left over |
| When | Consumed first, as chlorine reacts | What remains after demand is met |
| Why it matters | High demand means dirtier water needing more chlorine | A residual keeps protecting water out in the system |
| In the dose | Dose = demand + residual | Dose = demand + residual |
The bottom line
Demand is the chlorine consumed; residual is the chlorine left to keep working. Add them and you get the dose.
Common questions
- How are they related?
- The dose you add equals the demand (used up) plus the residual (left over).
- Which one disinfects downstream?
- The residual, since it is the chlorine still available in the water.
- What does high demand mean?
- More material and organisms are using up chlorine, so more dose is needed to leave a residual.