Free chlorine vs total chlorine
Free chlorine (FC) is the chlorine still able to sanitize, and total chlorine (TC) is FC plus the used-up part, combined chlorine (CC).
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
Combined chlorine is the part bound to sweat and urine. Dose by free chlorine, and treat a gap of 0.5 ppm or more between TC and FC as a signal to shock.
Definitions
| Term | What it is | What you want |
|---|---|---|
| Free chlorine (FC) | Hypochlorous acid and hypochlorite: the active sanitizer, plus the reserve that stabilizer holds and releases | In your target range, which depends on stabilizer (CYA) |
| Combined chlorine (CC) | Chloramines: chlorine that has reacted with ammonia and nitrogen compounds from sweat, urine, and other swimmer waste | 0 to under 0.5 ppm |
| Total chlorine (TC) | FC + CC, everything the test detects | Equal to FC, or within 0.5 ppm of it |
Taylor Technologies describes combined chlorine as an ineffective sanitizer that causes irritation and odor. That is why the "strong chlorine smell" at a busy pool usually means too little free chlorine, not too much. More on that in why a pool smells like chlorine.
How much free chlorine you need is a separate question: it rises with your stabilizer level. At CYA 40 our minimum is 3 ppm, with a target of 3–5 ppm. See your numbers on the pool chlorine levels page.
Combined chlorine = TC − FC (calculator)
Enter both readings. The calculator subtracts FC from TC and flags combined chlorine of 0.5 ppm or more. At that level, shock the pool with the amount on your product's label.
Use only a product labeled for shocking pools. Shock labels usually give an amount per 10,000 gallons, so a 20,000-gallon pool needs twice the listed amount. When a drop kit, store test, or digital meter shows CC of 0.5 ppm or more, the full pool chemical calculator adds a shock step based on your product's label. The goal is CC at 0.5 ppm or less on a retest the next day. If it's still higher, repeat only as the label directs, and call a pool professional if a few days of that don't clear it. Step-by-step instructions are in how to shock a pool.
No swimming while chlorine is above the swim limit on your label (4 ppm on many labels) after a shock. Never mix chlorine with acid or with another chlorine product; add each separately with the pump running. Wear gloves and eye protection. Poison Control: 1-800-222-1222.
Which one to dose by
Always dose by free chlorine. Combined chlorine is already used up, so counting it would make you add too little. A pool reading TC 4 and FC 2 does not have "4 ppm of chlorine"; it has 2 ppm of working chlorine and 2 ppm of chloramines, and needs a shock.
Use total chlorine for one thing: finding combined chlorine. If your test kit or strip only reads total chlorine, you can't tell a clean pool at 3 ppm from a pool with 1 ppm free and 2 ppm combined. That is the main reason to get a kit that reads both.
What it means when TC is much higher
A gap between TC and FC means something is reacting with your chlorine faster than the chlorine can break it down. Common causes:
- Heavy swimmer load: a party, a day of kids swimming, or lots of sunscreen and sweat. The fix is a shock the same evening.
- Chlorine running too low: when free chlorine drops below the minimum for your CYA, chloramines build up instead of being broken down. Keeping FC in range prevents most of it.
- Fresh fill from a utility that uses chloramine: some public water systems disinfect with chloramine, which reads as combined chlorine. A shock after filling clears it.
- Non-chlorine shock (MPS): potassium monopersulfate residue reads as combined chlorine in DPD tests (PHTA and AQUA tech notes). Test before adding MPS, or wait until the next day.
- A slow, persistent gap: PHTA describes combined chlorine that won't drop below 0.4 ppm as a sign that water quality is degrading. If shocks don't clear it, check circulation, filter condition, and stabilizer level, and see chlorine not holding.
The opposite case, TC slightly lower than FC, is test error: the two readings come from different steps or pads. Treat it as CC = 0.
Testing: strips vs DPD vs FAS-DPD
| Method | How it reads | Good for | Limits |
|---|---|---|---|
| Test strips | FC and TC pads matched to color blocks | Quick daily FC check | Steps are coarse, so a 0.5 ppm gap is hard to see; pads bleach out at high chlorine |
| DPD color-match kit | Pink color compared with a scale; FC first, then TC after an extra reagent | Routine FC and TC at normal levels | Color matching is subjective; very high chlorine can bleach the color out (Taylor) |
| FAS-DPD drop kit | Count drops until pink turns clear; FC first, then CC in the same sample | Measuring CC precisely; checking FC before and after a shock | Takes a few minutes; reads 0.2 or 0.5 ppm per drop, up to about 20 ppm (Taylor) |
The CDC recommends a DPD test kit over strips for home pools because strips are less accurate. If you care about combined chlorine, a FAS-DPD kit is the practical choice. Tips for getting the most out of strips are in how to read pool test strips, and every target is in the pool chemistry chart.
FAQ
What is free chlorine?
Free chlorine is the chlorine in your water that is still available to kill germs: hypochlorous acid and hypochlorite, including the reserve held by stabilizer. It is the number you dose by, and its target rises with your stabilizer (CYA) level.
Is free chlorine dangerous?
Not at normal pool levels. The CDC recommends at least 1 ppm in pools (at least 3 ppm in hot tubs). Our calculators advise no swimming above 10 ppm, the upper limit in the CDC's Model Aquatic Health Code. They also tell you to wait whenever chlorine is above the swim limit on your label (4 ppm unless you enter your own). Concentrated chlorine products are dangerous: handle them with gloves and goggles and never mix them with acid.
How do you get free chlorine?
By adding any chlorine product: liquid chlorine, bleach, trichlor tablets, cal-hypo, or dichlor, or by making it from salt with a chlorine generator. All of them form hypochlorous acid in water. They differ in strength and side effects, such as added stabilizer or calcium.
Sources
- Taylor Technologies, How FAS-DPD tests for free and combined chlorine values: taylortechnologies.com
- Taylor Technologies, K-1005 instruction manual and treatment tables: PDF
- CDC, Home pool and hot tub water treatment and testing: cdc.gov
- CDC, Model Aquatic Health Code 2024, Annex: cdc.gov (PDF)
- AQUA Tech Notes (PHTA), Potassium monopersulfate non-chlorine shock: PDF