Free Chlorine vs Total Chlorine Explained
Quick Answer
Free chlorine (FC) is the active portion that actually sanitizes. Total chlorine (TC) equals free plus combined chlorine (CC). Combined chlorine forms when FC reacts with nitrogen compounds and is ineffective as a sanitizer. If TC minus FC exceeds 0.5 ppm, shock to break down the combined chlorine.
- Free chlorine (FC) is the only portion that actively kills bacteria and algae
- Combined chlorine (CC) = TC − FC; above 0.5 ppm indicates a chloramine problem
- The chlorine smell in pools is almost always combined chlorine, not excess free chlorine
- Breakpoint shock at 10× CC reading destroys chloramines
Most test kits measure both free chlorine (FC) and total chlorine (TC). Subtract FC from TC to get combined chlorine (CC). Ideal pools have CC near zero.
The three forms of chlorine
| Form | What it is | Target |
|---|---|---|
| Free chlorine (FC) | Active hypochlorous acid; kills pathogens | 1–3 ppm (pools), 3–5 ppm (spas) |
| Combined chlorine (CC) | Chloramines from FC reacting with ammonia/nitrogen | < 0.5 ppm |
| Total chlorine (TC) | FC + CC; total chlorine in the water | TC = FC when CC is near zero |
Why combined chlorine matters
Chloramines are the real cause of eye irritation, skin rash, and the distinctive "chlorine smell" at heavily-used pools. A strong chlorine odor is actually a warning that combined chlorine is too high — not that there is too much free chlorine.
When to interpret your readings
- FC = 2 ppm, TC = 2 ppm — Perfect. CC = 0. No action needed.
- FC = 1 ppm, TC = 2 ppm — CC = 1 ppm. Needs breakpoint shock (10 ppm FC).
- FC = 0, TC = 1 ppm — All chlorine is combined; no free chlorine. Shock immediately.
How to eliminate combined chlorine
Raise FC to at least 10× the CC reading in a single dose. This is breakpoint chlorination — it destroys chloramines chemically, rather than diluting or masking them. The Pool Shock Calculator does not read a combined-chlorine value or calculate a breakpoint dose automatically: it offers flat FC-increase presets (5, 10, 15, or 20 ppm) for a chosen chlorine product. To use it for breakpoint chlorination, first calculate your own target (10 × your CC reading, added to your current FC), then select the closest calculator preset, or apply the shock dose formula directly for an exact figure using a non-stabilized product such as liquid chlorine or calcium hypochlorite.
Calculator
Pool Shock Calculator · Full Chemical Calculator
Reference: Pool Chlorine Levels Chart
Related Pool Chemistry Guides
Related in this topic
- Free Chlorine Vs Total Chlorine
- Liquid Chlorine Vs Tablets
- Pool Shock Vs Chlorine
- Salt Water Pool Vs Chlorine Pool
- Saltwater Pool Vs Chlorine
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Tools
Hub guide
- Typical range: 1–3 ppm chlorine
- Recommended pH: 7.2–7.6
- Test water regularly
WaterBalanceTools provides practical calculators and guides for pool and hot tub water chemistry. These tools are designed to help maintain safe chlorine, pH, and total alkalinity within a healthy water balance.