Chloramine Cl₂:NH₃-N Ratio Calculator
Chloramination lives or dies on one number: the chlorine-to-ammonia weight ratio. Enter your doses to see whether you’re making monochloramine — or drifting toward dichloramine taste and odor.
Cl₂:NH₃-N Weight Ratio
| Item | Value |
|---|
How the calculation works
Formulas
Cl₂:NH₃-N ratio = chlorine dose (mg/L as Cl₂) ÷ ammonia-N (mg/L as N)
Monochloramine reaction: NH₃ + HOCl → NH₂Cl + H₂O
Stoichiometric weight ratio = 71 / 14 = 5.07 : 1 (Cl₂ : NH₃-N)
Utility target: 4.5:1 – 5:1 · Breakpoint ≈ 7.6:1
Cl₂:NH₃-N ratio = chlorine dose (mg/L as Cl₂) ÷ ammonia-N (mg/L as N)
Monochloramine reaction: NH₃ + HOCl → NH₂Cl + H₂O
Stoichiometric weight ratio = 71 / 14 = 5.07 : 1 (Cl₂ : NH₃-N)
Utility target: 4.5:1 – 5:1 · Breakpoint ≈ 7.6:1
| Ratio zone | Meaning |
|---|---|
| Below 4.5 : 1 | Excess free ammonia; monochloramine forms but ammonia remains |
| 4.5 : 1 – 5 : 1 | Monochloramine zone — typical utility target |
| 5 : 1 – 7.6 : 1 | Dichloramine risk zone — taste and odor complaints likely |
| At / above 7.6 : 1 | Breakpoint region — free-chlorine chemistry dominates |
Notes & limitations
The 5.07:1 stoichiometric ratio, the 4.5–5:1 operating target, and the ~7.6:1 breakpoint are from AWWA M20 (Water Chlorination/Chloramination Practices and Principles) and White’s Handbook of Chlorination and Alternative Disinfectants.
- Both inputs must be on the same basis: chlorine as Cl₂ and ammonia as N. If your lab reports ammonia as NH₃, convert first (NH₃-N = NH₃ × 14/17).
- The zones assume near-neutral pH and adequate contact time. Low pH and short contact shift speciation toward dichloramine even inside the target band.
- Nitrification in the distribution system consumes ammonia and can drag the ratio upward over water age — monitor far-system residuals, not just plant effluent.
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