Understanding Labeled Chlorine %

In our last article, we introduced the concept of the Mole, a unit of measurement that allows us to weigh atoms, ions, and more complex molecules. This article expands on that lesson to demonstrate how to read product percentages listed on chemical labels. In particular, we're focused on Chlorine %.

Covered in this article:

• Decoding chemical product percentages
  • Retail vs. professional-grade chemicals 
  • Active ingredients in pool chemicals
  • Inactive ingredients in pool chemicals
• Common misconceptions about chemical product percentages
• Chlorine product percentages
  • • Trichloro-s-triazinetrione (trichlor)
    • Dichloro-s-triazinetrione dihydrate (dichlor)
    • Calcium Hypochlorite (cal hypo)
    • Sodium Hypochlorite (liquid chlorine, or bleach)
• Choosing the right chemicals for your water

Decoding chemical product percentages

Product labels can be misleading. This is especially true for swimming pool chemicals like various chlorine products. At a glance, it may seem like 99% trichlor is stronger than 68% calcium hypochlorite, or that 99% sodium dichlor is WAY stronger than 12.5% sodium hypochlorite (liquid chlorine). But are they?

When you look at the label of a pool chemical, you will often see a percentage (or several percentages) of listed ingredients. These numbers usually represent a weight percentage of the listed ingredient compared to the total weight of the product. But in the case of chlorine, sometimes products are listed as a percentage of available chlorine. This can confuse consumers, so be sure to read the label carefully.

The listed active ingredient is responsible for the chemical's intended purpose, such as sanitizing pool water or adjusting the pH level. We first need to know the active ingredient's molecular weight (in grams-per-mole, or g/mol) to decode the product's labeled percentages. 

Below is a list of common pool chemicals and their molar weights, which we will use to decode common pool product percentages later in this article.¹ 

The percentage listed on the label only indicates the concentration of the active ingredient, not its overall effectiveness. Factors like the quality of the ingredients, formulation, and manufacturing process also play a significant role in determining a chemical's strength. Therefore, it's crucial to consider other factors besides the percentage when choosing the right chemicals for your swimming pool(s).

Retail vs. Professional Grade Chemicals

Before going further into this, we have noticed a difference in product concentrations between retail and professional-grade products. By "retail", we mean products you can find in a pool store or a big-box store like Walmart, Lowe's or Home Depot. By professional grade, or "trade grade" we mean products that are sold to pool professionals through wholesale distributors.

Yes, many pool retailers sell professional-grade products too, but the only way to know for sure is to look at the product percentages of active ingredients...especially for chlorine.²

Retail chlorine products, for instance, often contain inactive filler ingredients that trade-grade products usually do not have. These fillers reduce the chlorine weight percentage listed on the label, and the inactive ingredients are usually not listed. We'll show examples later in this article when we show each chlorine type.

It should be noted that chlorine concentration percentages impact the amount and costs of the chemicals required to have the same effect in water. Lower % chlorine will not be less effective if you use a proportionate dose to account for the percentage.  For example, if one chlorine product is half the strength of the other, using twice as much of the half-strength chlorine will have the same effect in the pool.

Active ingredients in pool chemicals

Active ingredients in pool chemicals are the substances that directly contribute to the desired chemical action in the water. For example, the weight % is based on the weight of the entire chlorine product compound (i.e. trichlor, cal hypo, bleach) relative to the molecular weight of elemental chlorine (Cl). Available chlorine % is relative to molecular chlorine (Cl₂).

It was conventionally derived that chlorine gas (Cl₂) introduced 100% of available chlorine to a body of water, even though only half of that is in the oxidation form and the other half in the reduced form of Cl. In other words, only half of Cl₂ will be in the form of free chlorine. Given this convention, we need to multiply the % of chlorine by a factor of 2 for the other product presentations. We'll cover this with examples later in this article. The reaction of Cl₂ in water is the following:

Cl₂ + H₂O → HOCl + HCl
Molecular chlorine + water → Hypochlorous acid + Hydrochloric acid

The HOCl then dissociates with its slower-and-weaker counterpart, Hypochlorite Ion (OCl^-).

HOCl ⇌ H⁺ + OCl^-
Hypochlorous acid ⇌ Hydrogen ion + Hypochlorite ion

These two substances (HOCl and OCl^-) make up free available chlorine (FAC).

HOCl is the killing form of chlorine, which effectively kills bacteria, algae, and other microorganisms. It also oxidizes non-living contaminants like metals, nitrogen compounds, and non-living organics and oils.

Bromine products are similar to chlorine in that they have a percentage of the active ingredient, which will dissolve in the water and create Hypobromous acid (HOBr), the killing form of bromine.

If we look beyond the pool industry, we will see active ingredient percentages on the label of any household cleaning product. It's likely to have a low percentage of an active ingredient. For example, a very popular cleaning product label reads:

ACTIVE INGREDIENT:
Alkyl (C12 40%, C14 50%, C16 10%) dimethyl benzyl ammonium chloride...................0.3%
OTHER INGREDIENTS..................99.7%

One might look at that and think..."Wait...just 0.3% active ingredient?! What a ripoff!" But hold on...this substance is potent. It does not take much to effectively disinfect surfaces in the home.³  This product we're referring to is one of the most popular household cleaning products ever, and that did not happen by mistake.

Another example of a product with a seemingly low active ingredient percentage is Hydrogen Peroxide (H₂O₂). The hydrogen peroxide product we can buy at any pharmacy is just 3% H₂O₂. That means 97% of it is inactive ingredients...usually distilled water. Pure hydrogen peroxide is so strong it would oxidize more than we want.⁴ In this low concentration, we can disinfect wounds and gargle it like mouthwash. 

For non-chlorine swimming pools that use biguanides, a hydrogen peroxide shock is also used as the primary oxidizer.⁵ This concentration of hydrogen peroxide is 27%!! That's 9x stronger than household hydrogen peroxide.  Same substance, different concentration. This concentration is more economical for swimming pools, since the pools will dilute it rapidly. But the point remains the same. Using 27% hydrogen peroxide for the same purposes we use the 3% household product would be unsafe and painful.

Inactive ingredients in pool chemicals

While active ingredients play a crucial role in pool chemicals, there are also inactive ingredients present in the product. Inactive ingredients are substances added to the formulation for various purposes, such as stabilizing the active ingredient, enhancing solubility, or improving shelf life.

As mentioned before, the main difference between retail and trade-grade pool chemicals is the amount of inactive ingredients. Retail products tend to have more inactive ingredients and a lower concentration of active ingredients.⁶

Common inactive ingredients in pool chemicals include pH adjusters, buffering agents, and scale inhibitors. Oftentimes inactive ingredients are just distilled water or salts. Understanding the role of inactive ingredients can help you make informed decisions when selecting pool chemicals. Sometimes they are just fillers, and other times, inactive ingredients are necessary to to stabilize the product so it is usable.

Common misconceptions about chemical product percentages

One common misconception about chemical product percentages is that a higher percentage automatically translates to a stronger and more effective product. As if using a higher percentage of a chemical will always yield better results. But this is not always the case.

While it's true that increasing the concentration can enhance the chemical's effectiveness to a certain extent, there is a limit beyond which adding more chemicals may not provide any additional benefits. Additionally, using excessive amounts of chemicals is not recommended in general. We recommend only using appropriate amounts of chemicals to keep the water as minimalist and simplified as possible.

It's important to follow the manufacturer's guidelines and recommendations for product usage and dosing. This ensures you achieve the desired results without overdosing or underdosing the chemicals. Our free Orenda Calculator™ gives precise doses for all the essential pool chemicals.

Chlorine Percentages

Now let's show the math for each type of chlorine on the market to illustrate actual chlorine percentages, both by weight % and available chlorine %. We refer to the molecular weights of known substances, which we have listed in the graphic above. On that graphic, we have included each chlorine product. You can find these molar weights online, as they are public information.

Each chlorine product's available chlorine percentage is based on the molar weight of elemental chlorine relative to the molar weight of the product itself. So we need to know these values to plug into the math formulas below.

Trichloro-s-triazinetrione (Trichlor)

Most trichlor products are labeled 99% Trichloro-s-triazinetrione (by weight). Some retail versions are less concentrated, like 53.5% (by weight). These products are mixed with inactive ingredients that are not disclosed on the label (they're called "other ingredients").

We have occasionally seen a few concentrations in between 53.5 and 99%. Regardless of the percentage, you can figure out any trichlor's chlorine percentage following the same math, multiplied by the % listed on the label.

Trichlor (C₃Cl₃N₃O₃) molar weight = 232.41 g/mol
Chlorine (Cl) = 35.453 g/mol

Trichlor has 3 chlorine atoms attached to it (Cl), so the total weight of chlorine in trichlor is:
(3 x 35.453) = 106.359

Divide the total chlorine weight into the total molecular weight of trichlor to get the weight percentage of chlorine in trichlor:
106.359 ÷ 232.41 = 0.458 = 45.8% chlorine (by weight) in trichlor.

Think about that for a moment. When you hold a typical 3-inch trichlor tablet in your hand, only 45.8% of that tablet is chlorine. The rest is cyanuric acid (CYA) and some salt. Trichlor is more CYA than chlorine (by weight).

Too much Trichlor use can lead to CYA overstabilization; a problem severe enough that limiting CYA is our Fourth Pillar of the Orenda Program.

For available chlorine, we need to double the chlorines because it takes a Cl₂ to dissolve in water to create free available chlorine (Cl₂ + H₂O → HOCl + HCl). So we multiply the weight of chlorine by 2:
106.359 x 2 = 212.72.

Then divide this total Cl₂ weight into the total molecular weight of trichlor to get the available chlorine percentage:
212.72 ÷ 232.41 = 0.915 = 91.5% available chlorine in pure trichlor. But trichlor products are not pure.

So we need to multiply this available chlorine percentage by the product weight percentage listed on the label, and we get various answers, depending on the product concentration:

0.915 x 0.99 = 90.6% available chlorine in 99% trichloro-s-triazinetrione
0.915 x 0.535 = 48.9% available chlorine in 53.5% trichloro-s-triazinetrione⁷

This same math can be applied to other chlorine products.

Sodium dichloro-s-triazinetrione dihydrate (Dichlor)

Like Trichlor, sodium dichlor is usually labeled as 99% sodium dichloro-s-triazinetrione dihydrate. There are actually two types of dichlor: anhydrous and dihydrate. Anhydrous is more hazardous and not sold in the pool industry. The dichlor on the market is dihydrate, which contains two water molecules to stabilize the product and make it less reactive––particularly in a fire.⁸

Sodium dichlor dihydrate (C₃H₄Cl₂N₃NaO₅) molar weight = 255.98 g/mol
Chlorine (Cl) = 35.453 g/mol

Dichlor has 2 chlorine atoms attached to it (Cl), so the total weight of chlorine in dichlor is:
(2 x 35.453) = 70.91

Divide the total chlorine weight into the total molecular weight of dichlor to get the weight percentage of chlorine in dichlor:
70.91 ÷ 255.98 = 0.277 = 27.7% chlorine (by weight) in sodium dichlor dihydrate.

For available chlorine, we need to double the chlorines to get Cl₂, like we just did in the trichlor equation above. Then we divide that into the total molecular weight of sodium dichlor dihydrate to get the available chlorine percentage:
(70.91 x 2) ÷ 255.98 = 0.554 = 55.4% available chlorine in pure sodium dichlor dihydrate.

Then we multiply this by the product percentage listed on the label, which is usually 99%:
(0.554 x 0.99) = 0.548 = 54.8% available chlorine in 99% sodium dichloro-s-triazinetrione dihydrate.⁹

Like trichlor, sodium dichlor has more CYA than chlorine. By weight, sodium dichlor's inactive ingredients consist of:

• CYA (minus 3 hydrogens that were replaced with chlorine and sodium, so (129.07 - 3) ÷ 255.98 = 49.2%),
• two water molecules ((2 x 18.02) ÷ 255.98 = 14.1%), and
• one sodium atom (22.99 ÷ 255.98 = 9.0%). 

Calcium hypochlorite (cal hypo)

Trade-grade cal hypo products are available in two concentrations. To make matters more confusing, both of these concentrations can be labeled one of two ways, which leaves customers with at least four different percentages seen on packaging: 65 or 68%, and 70% or 73%. This is because some products are labeled as their available chlorine percentage, and others list their weight percentage. The larger two (68% and 73%) are the weight percentages. The available chlorine percentages are the smaller two (65% and 70%).

Then there are also retail cal hypo shock products that are less concentrated, with various other ingredients that are not listed on the label. Because of the higher pH of cal hypo, these are usually phosphate-based scale inhibitors. For example:

For easier math, we'll stick with the trade-grade cal hypo products.

Calcium hypochlorite (CaCl₂O₂, or Ca(ClO)₂) molar weight = 142.98 g/mol
Chlorine (Cl) = 35.453 g/mol

Cal hypo has 2 chlorine atoms attached to it (Cl), so the total weight of chlorine in cal hypo is:
(2 x 35.453) = 70.91

Divide the total chlorine weight into the total molecular weight of cal hypo to get the weight percentage of chlorine in cal hypo:
70.91 ÷ 142.98 = 0.496 = 49.6% chlorine (by weight) in pure cal hypo.

Then we need to multiply this by the listed weight percentage on the product label to find out the actual weight percentage of chlorine in the product:
0.496 x 0.73 = 0.362 = 36.2% chlorine (by weight) in 73% cal hypo
0.496 x 0.68 = 0.337 = 33.7% chlorine (by weight) in 68% cal hypo
0.496 x 0.5644 = 0.279 = 27.9% chlorine (by weight) in 56.44% cal hypo

Now that we know the weight percentages, let's figure out the available chlorine percentages. 

For available chlorine, we need to double the chlorines to get Cl₂, like we just did in the trichlor and dichlor equations above. Then we divide that into the total molecular weight of calcium hypochlorite to get the available chlorine percentage:
(70.91 x 2) ÷ 142.98 = 0.992 = 99.2% available chlorine in pure calcium hypochlorite.

Then we multiply this by the weight percentage of the product, if the weight percentage is listed on the label. Of course, if the available chlorine percentage is listed on the label, the answer is already known.
(0.992 x 0.68) = 0.674 = 67.4% available chlorine in 68% cal hypo. 
(0.992 x 0.73) = 0.724 =72.4% available chlorine in 73% cal hypo.

But wait...you might be recognizing that we just mentioned the percentages of 65-68% and 70-73%. Why the discrepancy in the available chlorine percent? Why is it 67.4% instead of 65% for available chlorine and 72.4% instead of 70%? The reason comes from the EPA. Cal hypo is classified as a hazardous material (NFPA Class 3 Oxidizer), so manufacturers tend to be conservative with this listed percentage in case of variance in concentrations.¹⁰ 

Sodium hypochlorite (liquid chlorine)

Liquid chlorine introduces a third type of percentage: trade %. Unlike solid chemicals, which need to dissolve into water, sodium hypochlorite is already a liquid. So, a trade percentage is used as a volume percentage for an easier conversion into parts per million (ppm). The conversion from trade % into ppm looks like this:

1 gallon of (X Trade %) sodium hypochlorite added to 10,000 gallons of water results in (X ppm) of free chlorine.

Why 1 gallon compared to 10,000 gallons of water? Because regardless of the trade %, 1 gallon into 10,000 gallons is one ten-thousandth. The percentage is divided by 100, so the chlorine in the product is one-millionth, or 1 part per million. The math works out like this:

1 gallon of 12.5% = 12.5 ÷ 100 ÷ 10,000 = (12.5 ÷ 1,000,000) = 12.5 ppm. 
1 gallon of 10% = 10 ÷ 100 ÷ 10,000 = (10 ÷ 1,000,000) = 10 ppm.

Household bleach is usually between 5 and 6% (depending on how it is labeled), which is about half the strength of pool chlorine. So when we are asked what the difference is between bleach and liquid chlorine, there isn't one, except pool chlorine is about twice as concentrated. It's the same chemical otherwise.

Looking at the trade percentage alone, 10% and 12.5% look very low compared to the dry chlorines we have listed above. But this is not comparing apples to apples. For that, we need to know the available chlorine % in pure sodium hypochlorite. And since it's a liquid, we need to know its density, which is public information. The density of 12.5% liquid chlorine is 1.16 g/mol.

Chlorine (Cl) = 35.493 g/mol
Sodium hypochlorite (NaOCl) = 74.44 g/mol
12.5% NaOCl density = 1.16 g/mol

Divide the total chlorine weight into the total molecular weight of sodium hypochlorite to get the weight percentage of chlorine in sodium hypochlorite:
35.493 ÷ 74.44 = 0.476 = 47.6% chlorine (by weight) in pure sodium hypochlorite.

For available chlorine %, we multiply the chlorine by 2 (because it's relative to Cl₂)
(35.493 x 2) ÷ 74.44 = 0.952 = 95.2% available chlorine in pure sodium hypochlorite.

Pure sodium hypochlorite degrades quickly, so it is manufactured with excess sodium hydroxide (NaOH, or caustic soda) and water. This dilutes and stabilizes the product to give it a longer shelf life.  We'll cover the chlorine manufacturing process in a future article.

If you need help with a more specific pool volume dose, our free Orenda Calculator™ gives precise dosing of all these chlorine types, not just liquid chlorine.

Choosing the right pool chemicals for your water

Choosing the right pool chemicals for your water involves considering various factors beyond product percentages. It's crucial to assess the specific needs of your pool, such as water type, size, usage, and existing issues.

So what chlorine type is best for your pool? That depends on your tap water, climate (mainly the rainfall/dilution and temperature range), and which chlorine byproducts you are most comfortable managing.

Sodium hypochlorite leaves behind salt; cal hypo leaves calcium and salt; trichlor and dichlor leave cyanuric acid and salt. The Orenda Calculator™ results page shows you each outcome if you are increasing chlorine. Slide the toggle for chlorine type and see how much that prescribed chlorine will leave behind in your water.

In this article, we have focused on chlorine, but the same type of math can be used for any chemicals if you know their molecular weights (g/mol).  We hope this article helps you understand chlorine percentages better.

¹  Thankfully, we all benefit from centuries of scientists who have preceded us. Molecular weights of the entire Periodic Table of Elements and almost every chemical compound known to mankind are public information. Search any chemical on PubChem or any number of other websites to find the substance's molar weight (expressed in g/mol).

²  All Orenda products are professional-grade. We do not manufacture weaker strengths of our products.

³  As an aside, this chemical we listed is a common ammonia-based sanitizer. Variations of this product are used to disinfect floors, surfaces, and many other applications. Any time you see a word that looks like "ammonia", avoid using it in and around swimming pools. It's a nitrogen-based product. Chlorinated water will oxidize the ammonia and create chloramines (aka combined chlorine).  This is especially true for indoor pools that use ammonia-based deck cleaners. Wet swimmers walking on the deck will create chloramines on the deck, which contributes to indoor air quality problems in a big way.  Also be aware of products like algaecides that also contain ammonia, and words like "quat", which stands for quaternary ammonia. All of these products will contribute to combined chlorine levels in the water, which should be avoided.

⁴  According to WebMD, hydrogen peroxide can actually harm tissue and delay healing because it is a non-selective oxidizer. It can oxidize skin, blood, etc.

⁵  27% hydrogen peroxide is a very powerful oxidizer and should be used with caution. Hydrogen peroxide is also incompatible with chlorine and bromine pools. It neutralizes chlorine completely. And in bromine pools, sure, it can recharge bromide ions into Hypobromous acid (HOBr), but it also creates harmful bromates. So don't use hydrogen peroxide in any pool except a biguanide pool.

⁶  As an aside, Orenda products do not come in lower-concentrated versions. We only manufacture professional-grade products.

⁷  This result differs slightly from the photographed label of 53.5% trichlor, which shows 48.6% available chlorine, not 48.9%. We are unsure why this is, but we know manufacturers sometimes tend to be more conservative with their percentages. Perhaps this is just to be on the safe side with the EPA and other regulators. We do not know the other ingredients in the product shown, so we cannot speak to it.

⁸  Xu, H., Zhang, H. et.al. (2022). Thermal Hazard Evaluation of Sodium Dichloroisocyanurate via TG-MS, DSC and ARC. Process Safety and Environmental Protection. Vol. 166, pp. 68-77.

⁹  We have seen products that list available chlorine in sodium dichlor as 56%, and we do not know why, unless there is some anhydrous dichlor in the mix. Anhydrous dichlor is not sold in the pool business because it is more volatile and considered a "hazardous material", requiring special packaging for transportation and storage. Anhydrous dichlor is an NFPA class 2 oxidizer, whereas sodium dichlor dihydrate is a class 1 oxidizer (less volatile). For perspective, Cal hypo (< 50% by weight) is a class 2 oxidizer, and > 50% cal hypo is a class 3 oxidizer. 

¹⁰  We have heard the percentage is listed lower to comply with EPA registration in case of slight variance in concentrations. But we have not been able to confirm this. We presume there is sound legal reasoning for labeling the product this way, but we do not know exactly what it is.