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.
Free chlorine (FC) is the active sanitizer that kills bacteria and algae. Total chlorine (TC) is FC plus combined chlorine — used-up chlorine that has already reacted with contaminants. The difference (CC = TC − FC) is your warning number: above 0.5 ppm, your pool needs to be shocked.
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
What if my free chlorine equals my total chlorine?
If FC = TC, then combined chlorine is zero. That's actually ideal — it means all the chlorine in your water is still active and none has been consumed by contaminants. Most pools run with a tiny gap (0–0.2 ppm CC) under normal conditions. A zero gap just means your sanitizer demand has been low recently.
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 — see the FC/CYA chart for exact targets).
What should my free chlorine level be?
The right FC target depends on your CYA (stabilizer) level. The minimum is CYA × 7.5% — below that, there isn't enough active chlorine to reliably sanitize. Quick reference:
- CYA 20–30 ppm: Target FC 2–4 ppm, minimum 1.5–2 ppm
- CYA 50 ppm: Target FC 4–6 ppm, minimum 4 ppm
- CYA 70–80 ppm: Target FC 6–8 ppm, minimum 5–6 ppm
If you don't know your CYA level, test it — it's the variable that makes every other chlorine number meaningful. The full FC/CYA chart has exact targets for every CYA value from 20 to 100 ppm.
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.
Does a chlorine smell mean there's too much chlorine in the pool?
No — it usually means the opposite. That sharp "pool smell" is chloramines, not free chlorine. Free chlorine itself is essentially odorless at normal levels. When combined chlorine builds up (typically from a heavy bather load, rain, or FC dropping too low), the chloramines become noticeable as that harsh chemical smell and the eye irritation people blame on chlorine. The fix is more free chlorine — specifically a shock dose to break down the chloramines — not less.
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 |
My free chlorine reads 0 but total chlorine is still high — what does that mean?
This means all of your active chlorine has been consumed and converted to combined chlorine. Your pool has no sanitizer left — just used-up chloramines. This can happen quickly during a heat wave, heavy rain, or a high bather load day. Shock immediately with a large dose of liquid chlorine, run the pump continuously, and retest in 4–6 hours. Don't wait: a pool with FC at zero is unprotected and algae can begin forming within hours.
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. If your chlorine is already too low, address that first before worrying about CC.
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
Is it safe to swim when combined chlorine is elevated?
At CC 0.5–1.0 ppm: expect some eye and skin irritation, especially for sensitive swimmers and children. At CC above 1.0 ppm: not advisable — chloramines at this level are a respiratory irritant and will cause significant eye redness. Shock the pool and wait until CC drops back below 0.5 ppm and FC is in the normal range before allowing swimming.
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. See our test strips vs liquid test kits comparison for help choosing.
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. Chlorine is just one piece of the puzzle — learn about all 5 numbers you should be tracking.
Track FC, TC, and CC together
PoolChem Tracker logs both chlorine readings, calculates combined chlorine automatically, and flags when CC is too high — so you know when to shock before the chloramine smell hits.
Pool Chlorine Series
Related reading
- Pool Chemistry for Beginners — the 5 numbers that matter
- How to Balance Pool Water in 4 Steps — correct order for adjustments
- What is LSI and Why Does Your Pool Need It? — the water balance number most miss
- Try PoolChem Tracker — calculate exact shock and chemical doses