Swimming Pool Chemistry 101

Written by Michael Dean
February 7, 2024

swimming pool chemicals

A lot more goes into owning a pool than filling up a hole with water and calling it a day! Your pool water requires chemical treatments to keep it clean and healthy. That’s where knowledge of swimming pool chemistry comes into play. From the chemicals that go into the pool to keep the water clean and prevent algae buildup to understanding how to keep your pool at the proper pH, a lot goes into balanced water chemistry.

Below, I’ll explain the critical information you need to know to maintain your pool’s chemistry throughout the year. So, let’s get straight to it with some basic terminology and the recommended chemical levels for your pool chemistry.


Main Takeaways

  • The basic swimming pool chemistry terms you should know are pH, alkalinity, and calcium hardness.
  • The different types of sanitizers for a pool include chlorine, bromine, biguanide, minerals, and ozone.
  • You should shock your pool if your free chlorine is less than the total chlorine and/or if you smell a strong chlorine smell coming from your pool.
  • Other chemicals you may need are clarifier/flocculant, algaecide, and metal sequestrant.

Ideal Swimming Pool Chemistry Levels

Free Chlorine (Standard Chlorine Pool)2-3 ppm (parts per million)
Chlorine (Saltwater Pool)1-3 ppm
Chlorine (Mineral Pool).5 ppm
pH7.2-7.8 (optimal level is 7.4)
Alkalinity80-120 ppm
Calcium Hardness175-275 ppm
Cyanuric Acid30-40 ppm
Salt (Saltwater System)2,700-3,400 ppm
Bromine (Alternative Sanitizer)3-5 ppm
Biguanide (Alternative Sanitizer)30-50 ppm

Basic Swimming Pool Chemistry Terms

pH

Pool sanitizers simply won’t work correctly if your pool’s pH level is out of whack. Chlorine and other sanitizers only remain effective within a specific pH range. Too high, and they won’t be able to kill germs like they’re supposed to. Too low, and they could cause harmful reactions that damage your pool.

pH is also vital for health reasons. Eye and skin irritation are common side effects of a pool’s pH level being too high or low. Ideally, pH levels should stay between 7.2 and 7.8, or the close-to-neutral range.

pH scale

So how do you maintain the proper pH level for your pool? The best thing you can do for your pool is to test its chemical and pH levels routinely (every day and before and after each use of the pool is best).

If pH levels are too high, you need to add an acid, like hydrochloric acid or muriatic acid, to bring the number down. Low pH levels need an alkaline boost with a substance like sodium carbonate to get the water within the ideal range. You can use test strips to get a quick reading and my calculator to figure out how many chemicals you need to adjust your pool’s pH level.

Alkalinity

Alkalinity is essentially your pool’s ability to resist a change in the pH level. The higher the pH level of your pool’s water, the more alkaline it is. The ideal pH range leans slightly toward being more alkaline than acidic, which keeps it comfortable on swimmers’ skin and eyes. Balancing your alkalinity along with your pH level is a great way to ensure your pool stays safe for swimmers. However, too much alkalinity can cause excess calcium deposits, or scale, to form in the pool, which leads to murky, unsightly, and unsafe water. You can read my article on removing calcium deposits from your pool for more. Shoot for 80 ppm to 120 ppm. If your alkalinity is below 80 ppm, consider shocking your pool (more on that below) or adding sodium bicarbonate (baking soda). Check out my alkalinity calculator to help.

It’s crucial to check not just the pH of your water but also its total alkalinity – they have an important relationship in your pool. There are testing strips available that test only the alkalinity of the water rather than the overall pH. It’s best to use them in addition to pH testing strips.

Your pool chemicals, when properly maintained, can help control the pH level of the water. Sometimes, you’ll need the help of another substance to lower or raise the pH to the proper range. Muriatic acid (a milder version of hydrochloric acid) helps reduce extremely high pH levels. While slower, sodium bisulfate is also an alternative to lower pH.

Calcium Hardness

You have probably heard someone describe water as “hard” or “soft.” But how can a liquid be “hard”? Water hardness is a measurement of how many minerals, such as calcium or magnesium, are in the water. “Hard” water contains a lot of minerals. Well water and tap water in certain regions may be “hard.” Calcium hardness is an important chemistry level to track in a pool, as high or low levels can cause problems.

Although many people prefer to swim in what they’d describe as “soft” pool water, water that’s too soft may not have enough calcium in it. Calcium hardness is essential to maintain water cleanliness and optimal pool plumbing and component operation.

Ideally, you want your pool’s calcium hardness levels to stay in the range of 175 to 275 ppm. If you have a plaster pool, aim for 200-275 ppm. For fiberglass or vinyl liner pools, target 175-225 ppm. High calcium hardness can cloud your water with calcium deposits, clog your filter, and leave buildup on piping or pool equipment and accessories. Low calcium hardness leads to the breakdown of pool concrete, grout, plaster, and other materials used to keep your pool working and looking great for years to come.

Your pool’s pH level can affect calcium hardness, but so can the water you use to fill it. Some areas have an exceptionally high calcium content in tap or well water. No matter what water you use, it’s necessary to test calcium hardness and pH and adjust if necessary. Calcium chloride is used to get your calcium hardness levels in your pool within range. You can use my calcium hardness calculator to determine how much to add.

Types of Pool Sanitizers

To keep your pool clean, contaminant-free, and healthy to swim in, you need to use sanitizing chemicals. Although most people have heard of using chlorine for sanitization, a few other chemicals can also do the trick.

Chlorine

First, let’s talk about chlorine, the most popular type of pool sanitizer. Chlorine comes in 2 forms, tablets or granules.

  • Tablets tend to allow for the best distribution of chlorine in your pool. They dissolve much slower in pool water, so the effects of the chlorine are generally longer than with granules. Tablets can be put in the skimmer, in a chlorine floater, or into an automatic chlorinator.
  • Granules are beneficial for shock treatments when you need a quick method to disperse chlorine. Granules have a much lower concentration of chlorine compared to tablets, so they dissolve in water much faster. Because chlorine granules dissolve so fast, the chlorine is less likely to be evenly distributed, leaving pockets of super-chlorinated or under-chlorinated water.

The ideal chlorine level is between 2-3 parts per million (ppm) for a standard chlorinated pool. Your pool water won’t be totally clean if it’s below that level, and anything higher than that will require you to use a chlorine neutralizer to lower your chlorine levels. You can use my swimming pool chlorine calculator to help you figure everything out.

If you have a saltwater pool, your chlorine level can drop slightly lower but should be between 1-3 ppm. Mineral pools require much less chlorine, at only .5 ppm.

Stabilized vs. Unstabilized Chlorine

There are two types of chlorine to choose from: stabilized and unstabilized. Unstabilized chlorine has no cyanuric acid (also known as chlorine stabilizer) in it to keep it from burning off in the sun. This makes it an excellent choice for indoor pools or shock treatments when you don’t need the chlorine to hang around the pool for a while. If you’re shopping for chlorine and it doesn’t say “unstabilized” on the packaging, look at the ingredients list. Anything with calcium hypochlorite, sodium hypochlorite, or lithium hypochlorite is unstabilized.

Stabilized chlorine, on the other hand, includes cyanuric acid, which allows it to resist the sun’s rays and stay in the pool longer, which is ideal for outdoor pools. The problem with cyanuric acid is that it can affect how chlorine works, making it take longer to kill bacteria and potentially causing it to be ineffective. Once again, if you’re looking at chlorine and it doesn’t say “stabilized” on the label, look for trichlor or dichlor in the ingredients list – that means it is stabilized.

If you want to add cyanuric acid, use my pool stabilizer calculator to figure out how much to add.

Chloramines

Chloramines are byproducts of chlorine mixed with bodily fluids that linger in your pool as chlorine dissipates. When your pool gets dirty with debris, sweat, oil, or urine, chlorine molecules bind with the contaminant, cleaning the water. But, when the chlorine binds with the contaminant, chloramines are formed. This byproduct does not help to sanitize your pool water and can be harmful and irritating to you and anyone else who swims in your pool. Chloramines can cause red, painful eyes, dry, itchy skin, and even digestive problems.

In a properly managed pool, you won’t be able to smell the tell-tale pool smell from chloramines that most people think is chlorine. When you treat your pool with enough chlorine and keep it properly maintained and sanitized, you shouldn’t smell the strong odor of chloramines. Calcium hypochlorite can also help eliminate these (not to be confused with sodium hypochlorite).

The best way to remove chloramines from your pool is to super chlorinate the water by shocking it. This effectively removes the combined chlorine molecules, replacing them with a new dose of free chlorine.

Bromine

Bromine is a popular alternative to chlorine for its ability to disinfect water while remaining stable at high temperatures. If you prefer warmer pool water, bromine can be an excellent option. It is commonly used in hot tubs for this reason.

Like chlorine, bromine produces byproducts called bromamines when it combines with ammonia. Bromamines will reduce the effectiveness of bromine as they multiply, so again, proper treatment is key.

Bromine tablets are the most popular form of treatment, although it is also available in liquid form.

The ideal bromine level is between 3-5 ppm.

Biguanide

Some pool owners choose biguanide to sanitize their pools. This substance is chlorine and bromine-free, so many consider it a safer alternative to chemicals. Biguanide pools are excellent for swimmers with sensitivities to chlorine. Biguanide attaches itself to the cell membranes of intruding pool cells and eventually kills them, causing them to get picked up by the pool filter.

Biguanide pools require a lot of maintenance and upkeep, so they are not a good option for new pool owners. It’s especially important to be diligent about cleaning your pool filter when using biguanide, as it’s notorious for creating filter clogs.

The ideal biguanide level is between 30 ppm and 50 ppm.

Minerals

Mineral sanitizers use naturally-occurring metals to keep pools clean. Most mineral sanitizers use silver, copper, zinc, or a mixture of the three.

While mineral sanitizers have cleanliness and comfort benefits – they’re known for making pool water feel soft – they don’t have the same power as chlorine or bromine. Therefore, a low level of supplementation may be necessary to keep your pool sanitized and buildup-free.

You’ll still need chlorine in a mineral pool, but much less than in chlorine or saltwater systems. If you use a mineral sanitizing system, the ideal chlorine level is 0.5 ppm.

Ozone

Ozone has become increasingly popular over the years as a pool sanitizer to reduce your pool’s reliance on chlorine. It is applied at very low concentrations through your pool circulation system. To learn more, read my complete guide on pool ozone generators.

The Why and How Behind Shocking Your Pool

Your regular pool maintenance routine can balance its chemicals and pH, but your pool water might need a little boost occasionally, especially after long periods of swimming. Every time you, another person, or a pet enters the pool, dead skin, hair, sweat, and other particles end up in the water.

To keep pool water as fresh as possible despite these unwanted additives, you need to “shock” it regularly. Shocking is, in its simplest form, adding extra chlorine or other sanitizers to the pool to assist the sanitization process.

How do you know when your pool needs a shock? You test the chlorine levels! Find a chlorine test kit that shows you the reading for both free chlorine and total chlorine.

Total chlorine is the full amount of chlorine in the pool, while free chlorine is the amount available to neutralize those other contaminants. If your free chlorine is dangerous below the recommended levels of 1 to 3 ppm, then your water could benefit from a shock treatment to boost the chlorine’s ability to do its job.

You can also use your nose to detect when shocking might be necessary. Remember how I said smelling chlorine is usually a sign that you don’t have enough in your pool? That applies here. If you smell a strong chlorine smell, you’re smelling chloramines, which means you need to add chlorine as a shock treatment to get rid of contaminants in the water.

Check out my complete guide on shocking your pool for a step-by-step process on how to do it. Oh, and make sure you shock your pool at night too! The sun eats up chlorine, so you’ll get the best shocking effect in the evening and overnight.

Other Pool Chemicals You Need

There are a few other chemicals besides your traditional pool sanitizers that you should consider keeping stocked to use when you need them. You’ll also need to add chemicals in the correct order.

Salt

Saltwater pool systems use a salt cell to turn sodium chloride into chlorine. So, instead of directly adding chlorine to the pool, you add salt, and the salt chlorine generator converts it into chlorine. You won’t need to add salt to your pool often, as salt does not dissipate from the water. You should add salt when draining and refilling your pool water or after heavy rains.

The ideal salt levels in a saltwater pool are between 2,700 and 3,400 ppm.

Cyanuric Acid Reducer

Cyanuric acid stabilizes the chlorine levels in your pool and prevents the chlorine from dissipating in the sun. However, adding too much cyanuric acid to your pool is common, which can lower your pH levels and cause your chlorine to be less effective. Cyanuric acid reducers are one of only two ways to lower your cyanuric acid level (the other one is to partially drain and refill the pool).

Clarifier and Flocculant

When you have a cloudy pool on your hands, flocculant and clarifier can take care of the problem by acting as coagulants to bind the clumps of yuck together. Clarifier tends to take more time than flocculant to do its thing before you see results.

Adding flocculant (aluminum sulfate is an example) to your pool will generally clear up the problem within a few hours. Unfortunately, it’ll also leave clumps at the bottom of the pool that you’ll need to vacuum before swimming. On the other hand, clarifier creates clusters the pool filter can take care of. You can read my complete guide on how to clear cloudy pool water fast for more tips on this process.

Algaecide

If you notice algae growing on the sides of your pool, something is causing it to be there. Sometimes, algaecide can help the problem, but not necessarily get rid of it. Pool algae can be different colors – green, black, or yellow – and each requires a different treatment in addition to the use of algaecide.

The best way to get rid of algae in your pool, along with cleaning it off manually, is to shock the pool. However, algaecide can help the process along. For more, read my complete guide on how to clear a green pool.

Metal Sequestrant

Some water used to fill pools may contain high levels of metals. It’s not usually a problem until it starts staining your pool, giving it an unsightly rust-colored hue. A metal sequestrant does precisely what its name suggests: it sequesters, or separates, the metals from the water by attaching itself to them so they won’t stain your pool surfaces.

Get My Free Pool Care Checklist

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Testing Your Pool Water

Testing your swimming pool water regularly is incredibly important to maintaining a clean and safe pool. As I mentioned above, you can pick up a testing kit and test your pool water every week or so to see if you need to make any adjustments. If you don’t have a good testing kit, head to my recommendations for the best pool water testing kits. I also walk through how to test your pool water in a separate article.

You can also use testing strips to test your pool water. These are affordable and easy to use but less accurate than liquid test kits. You also won’t be able to use testing strips for every chemical.

Safety Tips for Handling Chemicals

Pool chemicals can be incredibly damaging to your eyes and skin if you don’t handle them properly. Some can even cause digestive or respiratory problems. The following tips can help you stay safe while treating your pool:

  • Read the instructions on chemical packaging before using it. Every package has detailed safety information about handling the product safely and what to do in case of accidental touch or ingestion. Only use chemicals that are in their original containers with readable labels.
  • Keep all pool chemicals stored safely away and out of reach from children or pets. Read my articles on pool chemical storage and expiring pool chemicals for more tips.
  • Never smoke near or when applying pool chemicals. Keep all chemicals away from any open flame.
  • Always wear elbow-length gloves and safety goggles when handling any pool chemicals. After using chemicals, be sure to wash your hands and arms thoroughly.
  • Use appropriate tools when applicable for applying chemicals to the pool.
  • Add chemicals to water rather than adding water to the chemicals. Never mix chemicals (this includes new and old parts of the same product).
  • Never add chemicals through your pool skimmer unless specified otherwise.
    • Be diligent about cleaning up chemical spills if they happen, as they can destroy other materials and cause a safety hazard to animals and people if they’re accidentally touched or consumed.
  • Contact your local waste department to find out how to dispose of pool chemicals you no longer need. They should never get poured down a drain or end up in your trash bin.

I know that’s a lot of information to digest, but understanding how pool chemistry works can save you a lot of headaches. Keeping your pool in proper chemical ranges will lead to safer swimming water and a better lifespan for your pool and its many elements.

After reading this guide, you should have more awareness of how to maintain your pool year after year. With a pool testing kit in hand and a stock of all the chemicals and balancers you’ll need to have a safe, clean pool, you can handle the task like a real pool professional.

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