ppm, FAC and Total Available Chlorine: What the Numbers Mean

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Disinfectant labels, generator manuals and research papers all talk in “ppm.” Test strips add abbreviations like FAC. Those numbers help only if you know what they include—and what they leave out.

ppm is a concentration unit

ppm means parts per million. For dilute water solutions it is commonly treated as roughly equivalent to milligrams per liter (mg/L). A reading of 200 ppm free chlorine means about 200 mg of free chlorine per liter of solution, not “200% strength” and not a guarantee of kill on every surface.

Always pair the number with what was measured (free vs total), pH, and how it will be used.

Free chlorine / FAC: HOCl plus OCl⁻

ATSDR explains a common misconception about chlorinated water: elemental chlorine gas (Cl₂) is not what usually remains in the water. Hypochlorous acid and hypochlorite are among the chemicals that do the work, and that “free chlorine” usually means the amount of hypochlorous acid and hypochlorite in the water (ATSDR Chlorine ToxFAQs).

In disinfection talk, free available chlorine (FAC) is that same free-chlorine idea: undissociated HOCl plus hypochlorite ion (OCl⁻). ATSDR’s toxicological profile defines free chlorine as molecular chlorine, hypochlorous acid and hypochlorite—and notes that because molecular chlorine is usually absent, the term usually means hypochlorous acid and hypochlorite (ATSDR Toxicological Profile for Chlorine, Table 4-2). CDC attributes chlorine’s microbicidal activity “largely to undissociated hypochlorous acid (HOCl)” and notes that efficacy falls as pH rises and HOCl converts to OCl⁻ (CDC 2008, p. 41).

So an FAC strip answers “how much free chlorine is here?” It does not answer “how much of that is HOCl versus OCl⁻?”

Why pH still matters when the strip looks fine

Len and colleagues measured HOCl and OCl⁻ separately with UV spectrophotometry. HOCl peaked near pH 4, kill tracked HOCl (r = 0.95), and the authors cautioned that standard titration methods cannot show how much chlorine is present as HOCl (Len et al., 2000). Free-chlorine tests report HOCl plus OCl⁻; they don’t show the split.

This site’s working target for HOCl solutions is pH 5–6. Speciation measurements in saline, from a stabilized-HOCl developer, place HOCl dominance roughly at pH 3–6, with dissolved chlorine becoming important below about pH 3.5 (Wang et al., 2007). ATSDR’s speciation figure shows molecular chlorine (Cl₂) forming as pH drops; this site treats pH 4 as a conservative line (ATSDR Toxicological Profile for Chlorine, ch. 4). Measure pH separately whenever the free-chlorine number matters.

Total available chlorine is a broader bucket

Regulatory text sometimes says total available chlorine, not FAC. ATSDR defines total chlorine as free chlorine plus combined chlorine (chloramines), and “available chlorine” as a measure of oxidizing strength (ATSDR Toxicological Profile for Chlorine, Table 4-2).

For example, EPA’s tolerance exemption for hypochlorous-acid chemicals in antimicrobial formulations used on food-contact surfaces says that, when ready for use, “the end-use concentration of all hypochlorous acid chemicals in the solution is not to exceed 200 ppm determined as total available chlorine,” with adequate draining before contact with food (40 CFR 180.940(a)). That is an exemption from the residue-tolerance requirement for ingredients of antimicrobial pesticide formulations used on food-contact surfaces in public eating places, dairy-processing equipment, and food-processing equipment and utensils. It does not make generator output or any unregistered solution a food-contact sanitizer; a solution above 200 ppm is outside its limit; and its “total available chlorine” wording is not the same as “FAC.”

Water-treatment practice distinguishes free chlorine from combined chlorine (chloramines) and from total chlorine (ATSDR Toxicological Profile for Chlorine, Table 4-2). Follow the instructions for your strip or kit: range, wait time, and whether the pad reads free or total.

High-range testing with a dilution

Many free-chlorine test strips top out well below 500 ppm; check the highest block on your strip’s color chart. Stronger solutions can read “off-scale.”

HOCL LLC, which funds this site, gives this dilution method in its product guidance:

1. Mix 1 part HOCl solution with 4 parts water (a 1:5 dilution by volume).
2. Test the diluted sample with the strip.
3. Multiply the strip reading by 5. Example: a 100 ppm reading on the diluted sample means about 500 ppm in the original.

This is not a government method, and strip brands have not all been validated for it. Multiplying by 5 also multiplies any reading error, so treat the result as approximate. Use distilled water to dilute, measure volumes carefully, and check pH on the undiluted solution. If your strip maker publishes high-range instructions, follow those.

A note on drinking water

EPA limits chlorine in drinking water to 4 ppm (4.0 mg/L) (EPA National Primary Drinking Water Regulations).

Practical checklist

• Read ppm as concentration, then ask free or total?
• Use FAC/free-chlorine strips for free chlorine; don’t treat the number as “percent HOCl.”
• Measure pH separately; aim for 5–6 for HOCl work on this site.
• For readings above your strip’s range, use the strip maker’s method or the 1:5 dilution above.
• Match any regulatory or label limit to the exact phrase it uses (FAC vs total available chlorine).
• Free chlorine falls over time, faster in open containers and at lower pH, so test a stored batch before you rely on it (Len et al., 2002).

Bottom line

ppm tells you how much; FAC tells you free chlorine; pH tells you how that free chlorine splits between HOCl and OCl⁻. Strips without pH leave half the story unread.

What this doesn’t mean: A strip reading is not an efficacy claim for any machine or labeled product, and the 1:5 dilution method is HOCL LLC guidance, not a government method.