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 the weight percentage (w/w) of the listed ingredient relative to the total weight of the product. Chlorine products, however, introduce an additional labeling convention called available chlorine, which is not a measurement of the physical amount of chlorine present in the product. Instead, available chlorine expresses the product's oxidizing capacity as an equivalent mass of chlorine gas (Cl₂). Because these are two different measurements, chlorine labels can be confusing unless you know which percentage is being reported.

The listed active ingredient is responsible for the product's intended purpose, such as sanitizing pool water or adjusting pH. Understanding the active ingredient's molecular weight (g/mol) allows us to calculate two different quantities: (1) the actual mass percentage of chlorine atoms contained in the compound, and (2) the compound's theoretical available chlorine, expressed as a Cl₂-equivalent. These are different measurements and should not be confused.

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 active-ingredient percentage printed on the label only indicates how much of that ingredient is present by weight. It does not necessarily indicate how much chlorine the product physically contains, nor does it directly indicate its available-chlorine value. These quantities must be determined separately.

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 chlorine products, three different percentages are commonly encountered throughout this article:

•  Chlorine by weight: the percentage of the compound's molecular weight that consists of chlorine atoms.

• Available chlorine: the compound's theoretical oxidizing capacity expressed as the equivalent mass of chlorine gas (Cl₂).

• Trade %: The active ingredient percentage found specifically in liquid chlorine or liquid sodium hypochlorite

Although these percentages are related through molecular stoichiometry, they describe different physical quantities and should never be used interchangeably.

By convention, chlorine gas (Cl₂) is assigned an available chlorine value of 100%. All other chlorine compounds are therefore compared against chlorine gas by expressing their oxidizing capacity as an equivalent mass of Cl₂. This convention is used throughout the water treatment industry and is the basis for available-chlorine labeling. 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 apply molecular stoichiometry to each chlorine product. For every compound, we will calculate two different values:

1. The percentage of elemental chlorine contained in the molecule (chlorine by weight).

2. The theoretical available chlorine, expressed as the equivalent mass of chlorine gas (Cl₂).

Finally, we will compare these theoretical values with the concentrations declared on commercial product labels. Keep in mind that the calculated values represent ideal molecular stoichiometry, whereas commercial labels represent the registered formulation of the product.

The available chlorine of a pure compound can be calculated directly from its molecular formula by determining how many moles of Cl₂-equivalent oxidizing capacity are produced per mole of compound. Converting those moles into mass using the molecular weight of Cl₂ allows us to express available chlorine as a weight percentage.

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 what will convert to 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.

Available chlorine is not calculated from the physical amount of chlorine present in the molecule. Instead, it is based on oxidizing capacity. By convention, chlorine gas (Cl₂) is assigned an available chlorine value of 100%, so other chlorine compounds are expressed as the equivalent mass of Cl₂ that would provide the same oxidizing power.

One mole of trichlor produces three moles of hypochlorous acid (HOCl):

C₃Cl₃N₃O₃ + 3H₂O → C₃H₃N₃O₃ + 3HOCl

Because each mole of HOCl has the same oxidizing capacity as one mole of Cl₂, one mole of trichlor is equivalent to three moles of Cl₂.

Therefore:

3 mol Cl₂ × 70.906 g/mol = 212.7 g Cl₂ equivalent

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

Why is trichlor only 45.8% chlorine but over 90% available chlorine?

Although only 45.8% of trichlor's molecular weight consists of chlorine atoms, available chlorine is not based on the physical mass of chlorine contained in the molecule. Instead, available chlorine measures oxidizing capacity relative to chlorine gas (Cl₂).

Because one mole of trichlor produces three moles of hypochlorous acid—and each mole of HOCl is equivalent in oxidizing power to one mole of Cl₂—the available chlorine is calculated using the equivalent mass of three moles of chlorine gas rather than the mass of the chlorine atoms alone.

Consequently,

45.8% and 91.5% are both correct. They simply describe two different properties of the same molecule.

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 convert to the equivalent 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%.

Unlike trichlor and dichlor, calcium hypochlorite products are commonly marketed using either active-ingredient percentage or available-chlorine percentage. As a result, two products containing essentially the same chemistry may display different numbers on the label depending on which convention the manufacturer chooses.

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 once again adopt the equivalent Cl₂, just like we 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.

Pure calcium hypochlorite has a theoretical available chlorine of approximately 99.2% based on molecular stoichiometry. Commercial products contain less than 100% calcium hypochlorite, so their available chlorine is reduced proportionally. Product labels may declare available chlorine values that differ slightly from theoretical calculations because they represent registered commercial formulations rather than pure compounds.

Sodium hypochlorite (liquid chlorine)

Liquid chlorine introduces a third labeling convention: Trade %. Unlike dry chlorinating compounds, which are typically sold as nearly pure solids, sodium hypochlorite is manufactured and sold as an aqueous solution. Because the product is already dissolved in water, the pool industry commonly refers to its concentration using a Trade %, which provides a convenient way to estimate dosing.

One of the reasons this convention became so popular is because it translates almost directly into free chlorine added to a swimming pool:

1 gallon of X% liquid chlorine added to 10,000 gallons of water increases free chlorine by approximately X ppm.

For example:

• 1 gallon of 12.5% liquid chlorine raises free chlorine by approximately 12.5 ppm in 10,000 gallons.
• 1 gallon of 10% liquid chlorine raises free chlorine by approximately 10 ppm in 10,000 gallons.

This relationship exists because one gallon added to 10,000 gallons represents approximately a 1:10,000 dilution, making the percentage conveniently convert into parts per million.

Household bleach typically contains 5–6% sodium hypochlorite, roughly half the concentration of commercial pool chlorine. Chemically, however, both products use the same active ingredient: sodium hypochlorite (NaOCl).

Sodium Hypochlorite by Molecular Weight and Available Chlorine

Like the dry chlorines discussed earlier, we can analyze sodium hypochlorite using molecular stoichiometry. Sodium hypochlorite (NaOCl) has a molecular weight of 74.44 g/mol. Elemental chlorine (Cl) has an atomic weight of 35.45 g/mol.

35.493 ÷ 74.44 = 0.476 = 47.6% chlorine (by weight) in pure sodium hypochlorite.

For available chlorine %, one mole of sodium hypochlorite provides the oxidizing capacity of one mole of chlorine gas (Cl₂). Therefore,

(35.493 x 2) ÷ 74.44 = 0.952 = 95.2% available chlorine in pure sodium hypochlorite.

Pure sodium hypochlorite has a theoretical available chlorine of 95.2%.This means that 100 grams of pure sodium hypochlorite provides the same oxidizing capacity as approximately 95.2 grams of chlorine gas (Cl₂). Notice that this value is not the same as the percentage printed on a commercial jug of liquid chlorine.

Commercial products contain water and excess sodium hydroxide (NaOH), which stabilize the hypochlorite and significantly extend its shelf life. As a result, a product labeled as 12.5% sodium hypochlorite contains approximately 12.5% NaOCl by weight, which means not 95.2% available chlorine.

Using the same molecular relationship we established above:

(0.952 x 0.125) = 0.119 =11.9% available chlorine in 12.5% sodium hypochlorite.

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  The product label shown in this article declares 48.6% available chlorine. The slight difference reflects the commercial formulation and registered product specification rather than the ideal molecular calculation.

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.