What is the difference between free chlorine and total chlorine?

Free chlorine (FC) is the active sanitizer in your pool — the chlorine that hasn't yet reacted with anything and is available to kill bacteria and algae. Total chlorine (TC) is the sum of free chlorine plus combined chlorine. Combined chlorine is chlorine that has already reacted with contaminants and is no longer sanitizing. TC minus FC equals combined chlorine.

Which is more important, free chlorine or total chlorine?

Free chlorine is the more important number because it's the only chlorine actively protecting your pool. Total chlorine only matters because subtracting FC from TC gives you combined chlorine, which tells you whether you need to shock. Maintain FC at 2-4 ppm in low-CYA water (or higher relative to your CYA level).

Why is my total chlorine higher than free chlorine?

Total chlorine is always equal to or higher than free chlorine because TC = FC + combined chlorine. A small gap (under 0.5 ppm) is normal. A gap of 0.5 ppm or more means combined chlorine is building up — chloramines are accumulating faster than your FC can destroy them. That's when you need to shock the pool.

What does combined chlorine mean?

Combined chlorine (CC) is chlorine that has already reacted with nitrogen-based contaminants — sweat, urine, sunscreen, body oils — to form chloramines. It still registers as chlorine on a test kit, but it's spent and no longer sanitizes. Chloramines produce the harsh chlorine smell and eye irritation people associate with pools. Above 0.5 ppm CC, it's time to shock.

How do I lower combined chlorine?

The fix is breakpoint chlorination — commonly called shocking the pool. You raise free chlorine high enough to overwhelm and destroy the chloramines. A common rule of thumb is to raise FC to roughly 10x your combined chlorine level. Partial doses make things worse: they create more chloramines without breaking them down.

Part of: Pool Chlorine Series

Free chlorine vs total chlorine: the gap is what matters

Q: What if my free chlorine equals my total chlorine?

If FC equals TC, combined chlorine is zero — that's ideal. It means all the chlorine in your pool is still active and none has been consumed by contaminants. A zero or near-zero gap means your sanitizer demand has been low and your pool is in great shape chlorine-wise.

Q: What should my free chlorine level be?

Your FC target depends on your CYA level. The minimum is CYA × 7.5%. At CYA 30 ppm, target 3–5 ppm FC. At CYA 50 ppm, target 5–7 ppm. At CYA 80 ppm, target 7–9 ppm. If you don't know your CYA, test it — it determines what your FC reading actually means.

Q: Does a strong chlorine smell mean too much chlorine?

No — it usually means the opposite. That sharp pool smell is chloramines (combined chlorine), not free chlorine. Free chlorine itself is essentially odorless at normal levels. When combined chlorine builds up, the chloramines cause the harsh smell and eye irritation. The fix is more free chlorine via a shock dose, not less.

Q: My free chlorine reads 0 but total chlorine is still high — what does that mean?

This means all your active chlorine has been consumed and converted to combined chlorine. Your pool has no sanitizer left. Shock immediately with a large dose of liquid chlorine, run the pump continuously, and retest in 4–6 hours. A pool with FC at zero can develop an algae bloom within hours.

Q: Is it safe to swim when combined chlorine is elevated?

At CC 0.5–1.0 ppm, expect eye and skin irritation, especially for sensitive swimmers. Above 1.0 ppm CC, swimming is not advisable — chloramines at this level are a respiratory irritant. Shock the pool and wait until CC drops below 0.5 ppm and FC returns to the normal range before swimming.

8 min read · Updated July 7, 2026 · By Jeffrey Thompson

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.

FC + CC = TC — same total reading, completely different situations

What is free chlorine?

Free chlorine (FC) is the active sanitizer in your pool — the chlorine that hasn't reacted with anything yet and is available to kill bacteria, destroy algae, and oxidize contaminants. It's the number your test kit shows first, and it's the only chlorine reading that tells you whether your pool is actually protected. When FC drops too low relative to your CYA level, your pool becomes vulnerable within hours.

The "ppm" unit (parts per million) means milligrams of chlorine per liter of water. At 3 ppm FC, there are 3 milligrams of active chlorine per liter — tiny, but enough to kill most bacteria within seconds at the right pH. What "enough" means changes with your CYA level: at CYA 50, you need at least 4 ppm FC to have the same sanitizing power as 2 ppm FC at CYA 20. This is why knowing your CYA is the prerequisite to interpreting any FC reading.

What is total chlorine?

Total chlorine (TC) is the sum of all chlorine in the water: free chlorine plus combined chlorine (chloramines). TC on its own doesn't tell you whether your pool is safe — a TC of 3 ppm could mean FC 3.0 and CC 0.0 (great) or FC 1.8 and CC 1.2 (needs shocking). The gap between TC and FC is what matters: CC = TC − FC. Keep that gap below 0.5 ppm.

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:

Total Chlorine = Free Chlorine + Combined Chlorine
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.

The gap between FC and TC is one of the earliest warning signs in pool chemistry. PoolChem Tracker tracks both readings over time and flags when combined chlorine starts climbing — before it becomes a shocking situation. Track it →

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 30 ppm: Target FC 3–5 ppm, minimum 2.25 ppm
CYA 50 ppm: Target FC 5–7 ppm, minimum 3.75 ppm
CYA 80 ppm: Target FC 7–9 ppm, minimum 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 (CC) tells you about your FC

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

What does it mean when total chlorine is higher than free chlorine?

When TC is higher than FC, the difference is combined chlorine (CC = TC − FC). A small gap of 0.2 ppm or less is normal — some chlorine is always reacting with trace contaminants. Once the gap exceeds 0.5 ppm, chloramines are accumulating: you'll notice a stronger pool smell, eye irritation, and declining sanitizer effectiveness. Above 1.0 ppm CC, the pool needs to be shocked. The table below maps the gap to action.

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.

Free chlorine is low but total chlorine reads fine — what does that mean?

This is the most common chlorine misreading. If TC is 3 ppm but FC is only 1 ppm, your combined chlorine is 2 ppm — well above the 0.5 ppm action threshold. The pool appears to "have chlorine" but most of it is spent chloramines that no longer sanitize. TC looks acceptable while your pool is actually under-protected.

It happens when a heavy bather load, rain, or organic contamination consumes FC faster than it's replaced — the chloramines stay in the water and keep TC elevated even as FC falls. Always calculate the gap (CC = TC − FC) rather than relying on TC alone. A TC of 3 ppm with FC of 1 ppm is a pool that needs to be shocked; a TC of 3 ppm with FC of 3 ppm is a pool in great shape.

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 to test free chlorine vs total chlorine

Getting accurate FC and TC readings requires the right test method — and knowing what you're actually measuring with each one.

DPD liquid test kits

DPD is the most common colorimetric method for home pools. The kit uses two reagents: DPD-1 turns the water pink in proportion to free chlorine only. DPD-3 (added to the same sample afterward) reacts with combined chlorine too, shifting the color to represent total chlorine. You read both colors against a reference chart — or count drops for a more precise titration. Subtract FC from TC to get CC.

Watch out for OTO kits — the type that turns the sample yellow. OTO measures total chlorine only and can't separate FC from CC. If your liquid kit produces a yellow color (not pink), you're not getting an FC reading.

Test strips

Test strips show FC and TC on separate pads — but check the label before trusting the result. Some entry-level strips show only one "chlorine" reading without specifying free or total. If it doesn't label both separately, you can't calculate CC. Strips are accurate to roughly ±20–30%, which is fine for confirming a reading is in the right ballpark but not for detecting a 0.5 ppm CC problem early.

FAS-DPD drop test (most accurate)

The FAS-DPD titration method measures FC and CC separately by counting drops of ferrous ammonium sulfate reagent until a color endpoint disappears — each drop equals a precise ppm increment. This is the method recommended for high-CYA pools where a 1 ppm error in FC matters. The Taylor K-2006 is the standard kit. More effort than strips, but the only method that reliably catches CC below 0.5 ppm. See our test strips vs liquid test kits comparison for help choosing.

Interpreting what you get

Once you have FC and TC, the math is CC = TC − FC. Here's what each result means:

FC reading	TC reading	CC (gap)	What it means
3.0 ppm	3.2 ppm	0.2 ppm	Healthy — negligible combined chlorine
2.0 ppm	2.8 ppm	0.8 ppm	Elevated — dose up, monitor closely
1.5 ppm	3.5 ppm	2.0 ppm	Shock needed — pool smells, eye irritation
0 ppm	2.0 ppm	2.0 ppm	No sanitizer left — shock immediately

Always test for both FC and TC so you can calculate the difference — TC alone tells you nothing useful about whether your pool is protected.

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.