Free chlorine vs total chlorine: what's the difference?
Your test kit gives you two chlorine numbers. One matters a lot more than the other — and the gap between them is a warning sign most pool owners miss.
The three types of chlorine in your pool
When you add chlorine to your pool, it doesn't stay in one form. It splits into three categories:
| Type | What It Is | Does It Sanitize? |
|---|---|---|
| Free Chlorine (FC) | Active chlorine available to kill bacteria, algae, and contaminants | Yes — this is your sanitizer |
| Combined Chlorine (CC) | Chlorine that has already reacted with contaminants — "used up" | No — it's spent |
| Total Chlorine (TC) | Free + Combined. Everything in the water that registers as chlorine | Partially — it includes both |
The relationship is simple:
CC = TC − FC
Why free chlorine is the number that matters
Free chlorine is the only chlorine actually protecting your pool. It's the active ingredient — the chlorine molecules that haven't reacted with anything yet and are available to kill bacteria, destroy algae, and oxidize contaminants.
When someone says "my chlorine is 3 ppm," they usually mean free chlorine. And that's the right number to focus on. Most pools should maintain FC between 2–4 ppm for everyday sanitization (the ideal range depends on your CYA level).
What combined chlorine tells you
Combined chlorine (also called chloramines) is what's left after free chlorine does its job. When FC reacts with nitrogen-based contaminants — sweat, urine, body oils, sunscreen — it forms chloramines.
Here's what most people don't realize: that "chlorine smell" at pools isn't from too much chlorine. It's from chloramines. A well-sanitized pool actually doesn't smell like much.
The chloramine smell myth
If your pool smells strongly of chlorine, it usually means you need more chlorine, not less. The smell comes from combined chlorine (chloramines), which means your free chlorine is getting used up faster than you're adding it.
Combined chlorine should ideally be 0.5 ppm or less. If it creeps above that, it means contaminants are building up faster than your free chlorine can handle them.
When the gap is a problem
If your TC is significantly higher than your FC, you have a combined chlorine problem. Here's how to read it:
| CC Level | Status | What To Do |
|---|---|---|
| 0 – 0.5 ppm | Normal | Nothing — your pool is handling the load fine |
| 0.5 – 1.0 ppm | Elevated | Keep an eye on it. May need a larger chlorine dose soon |
| Above 1.0 ppm | High | Time to shock — you need to break down those chloramines |
What to do about high combined chlorine
The fix for high CC is breakpoint chlorination — commonly called "shocking" the pool. You add enough free chlorine to overwhelm and destroy the chloramines.
The threshold is roughly 10x your combined chlorine level. So if your CC is 1.5 ppm, you'd need to raise your FC to about 15 ppm to reach breakpoint. This is a large dose, so:
- Shock in the evening (sunlight burns off chlorine)
- Run the pump continuously during and after
- Don't swim until FC drops back below 5 ppm
- Retest the next day to confirm CC has dropped
Why partial shocking doesn't work
Adding a little extra chlorine when CC is high can actually make things worse. You need to fully reach breakpoint to destroy chloramines. A half-dose creates more combined chlorine without breaking it down — you end up wasting chlorine and still smelling chloramines.
How test kits measure chlorine
Most home test kits and strips work like this:
- DPD test kits — DPD-1 reagent measures free chlorine. DPD-3 reagent (added after) measures total chlorine. You subtract to get combined chlorine.
- Test strips — Usually show FC and TC directly. Some only show FC or TC (not both) — check what your strips measure.
- FAS-DPD drop test — The most accurate method. Uses a titration process to measure FC and CC separately. Preferred by serious pool owners and the TFP community.
Whichever method you use, always test for both FC and TC so you can calculate the difference.
Quick reference
- Free chlorine (FC) — Track this closely. It's your active sanitizer. Keep it at 2–4 ppm (or higher relative to your CYA level).
- Total chlorine (TC) — Should be very close to your FC reading. If it's not, you have combined chlorine.
- Combined chlorine (CC) — The difference between TC and FC. Keep it below 0.5 ppm. Above 1.0 ppm, it's time to shock.
Tracking both numbers over time is the easiest way to catch chloramine buildup before it becomes a visible (or smellable) problem.
Track FC, TC, and CC together
PoolChem Tracker logs your free chlorine and total chlorine, calculates combined chlorine automatically, and flags when CC is getting too high — so you know when it's time to shock before your pool tells you with that smell.
