How to Raise Free Chlorine in a Pool

Here is how to increase free chlorine in a pool: measure the water first, add a chlorine product such as liquid chlorine, calcium hypochlorite, dichlor, or trichlor at the dose printed on that product’s label, run the pump so it mixes, following your pump manual’s electrical warnings, and then measure again. If you have a salt system, the fix is usually to raise the chlorinator’s output as its manual describes. The harder part is figuring out why free chlorine dropped, so the reading does not slide again by tomorrow.

Chemical safety first: Pool chemicals can cause burns and toxic fumes. Read and follow the label on every product, and dose only as the label directs for your pool’s volume. Never mix different pool chemicals with each other, and never combine them in one bucket. Wear gloves and eye protection when handling them, and store each product in its original container, secured away from children and pets. This guide is general information, not medical advice. If someone is exposed to a chemical or feels ill, follow the first aid directions on the label and call a poison center at 1-800-222-1222; call 911 for emergencies.

What Is Free Chlorine in a Pool?

Free chlorine is the chlorine that is still available to kill germs. The CDC glossary of pool water terms defines it as the chlorine in water not combined with other constituents, and therefore able to serve as an effective disinfectant. Once chlorine combines with organic material, such as the body waste swimmers bring into the water, it becomes combined chlorine, which the same glossary describes as chlorine that is no longer an effective disinfectant.

That is why a pool can smell strongly of “chlorine” and still be short on free chlorine. The odor most people notice comes from chloramines, which the CDC’s chloramines and pool operation page, written for pool operators, describes as a type of combined chlorine that forms in water and then off-gasses into the air above it.

Free Chlorine vs Total Chlorine in a Pool

Total chlorine is free chlorine plus combined chlorine. The same CDC chloramines page gives the math: combined chlorine equals total chlorine minus free chlorine. A water analysis kit that reports both numbers lets you see which problem you have:

• Free chlorine low, total chlorine about the same: The pool simply needs more chlorine. Add it per the product label.
• Free chlorine low, total chlorine noticeably higher: A chunk of your chlorine is tied up as combined chlorine. Bring free chlorine up only as your product label directs. If the label gives a separate higher-dose treatment, follow its wording exactly, or take a water sample to a pool store and ask what applies to your pool.
• Both readings high: You do not need to raise anything. Stop adding chlorine, keep the pump running while following your pump manual’s electrical warnings, and measure again before anyone swims, using the range on your product label. If you need to lower chlorine sooner, use only a product labeled for that job and follow its directions. If combined chlorine stays high, check your product label for directions, or take a water sample to a pool store for analysis.

The CDC chloramines page notes that most city, county, and state health departments limit combined chlorine in the water to 0.4 ppm or less. Those limits are set by health departments for the pools they regulate. The CDC does not set that limit for home pools: its guidance for home pool owners does not include a combined chlorine limit.

What Free Chlorine Level to Aim For

The CDC’s guidance for home pool and hot tub owners recommends pH 7.0 to 7.8 and a chlorine concentration of at least 1 ppm in pools. If you use cyanuric acid (stabilizer) or chlorine products that contain it, the CDC recommends at least 2 ppm in pools, with the same pH range. Its guidelines for keeping a pool safe describe the typical range for chlorine as 1 to 4 ppm.

Those are minimums for disinfection, not targets for any one product. Your chlorine product’s label and your equipment manual may give their own ranges, and those directions apply to that product.

How to Measure Free Chlorine Accurately

You cannot raise free chlorine with any precision if the reading is off. The CDC’s home pool guidance says that, for the most accurate results, pool owners should use a DPD kit to measure chlorine and pH. Strips are also an option, but the CDC calls them less accurate than a DPD kit. Either way, follow the kit maker’s directions exactly, including sample size, reagent order, and how long to wait before reading the color.

Pick a kit that reports free chlorine separately from total chlorine. A strip or reagent kit that shows only “chlorine” cannot tell you whether you have a supply problem or a combined chlorine problem. The CDC recommends checking chlorine and pH at least twice per day, and more often when the pool is being used by a number of people. For a family pool, a quick check in the morning and again before an afternoon swim fits that guidance.

How to Increase Free Chlorine in a Pool: Step by Step

1. Measure free chlorine, total chlorine, and pH. Write the numbers down so you can compare after treatment.
2. Check the pH. The CDC notes that as pH goes up, chlorine’s ability to kill germs decreases, especially above 8.0. If pH is outside 7.0 to 7.8, adjust it with a pH product according to its label, as a separate step, and wait the time that label gives before adding chlorine. Do not add a pH product and a chlorine product at the same time or in the same container. If your pH product is an acid, keep it apart from chlorine: a CDC report on pool chemical injuries says never to mix chlorine products with acid, because this could create toxic gases.
3. Choose one chlorine product. See the comparison below. Use one product at a time, and never mix two kinds of chlorine: one chlorinating tablet label on file with the EPA says not to mix that product with any other kind of chlorine, as an explosion could result.
4. Read the label dose for your pool’s volume. Labels list how much to use for a given number of gallons and for routine use versus higher-dose treatment. Use the label figure; do not estimate or scale it yourself.
5. Add it the way the label says. Some products go straight into the pool, some must be pre-dissolved, and tablets go into a floater or feeder. Run the pump so the water circulates. If you run a pump on an above ground pool, follow your pump manual’s electrical warnings. Bestway’s Flowclear pump manual, for example, warns of a risk of electric shock and says the pump cannot be used while people are inside the pool, says to make sure your hands are completely dry when plugging in the filter pump, says not to use an extension cord to connect the unit, and says to keep the ground-fault circuit-interrupter (GFCI) dry and off the ground. For an inground pool, follow the electrical warnings in the manual for your own pool’s pump.
6. Wait, then measure again. Use the waiting time on the label before swimming and before re-checking. Do not swim until that waiting time has passed and a new reading falls within the range the label recommends. If free chlorine is still below the CDC minimum, repeat only as the label allows.
7. Keep a log. Recording readings and doses over a week shows how fast your pool uses chlorine, which helps you spot the cause.

Which Chlorine Product to Use

All of these raise free chlorine. They differ in how fast they work, what else they add to the water, and how they are handled:

• Liquid chlorine (sodium hypochlorite): Mixes in quickly and does not add stabilizer, which makes it a common choice for a fast bump in free chlorine. Containers are heavy, so plan where you will pour, and store it as the label directs.
• Calcium hypochlorite (granules or “shock”): A dry product that also raises free chlorine quickly. If the label calls for pre-dissolving it in a bucket of water before adding, follow that step exactly. CDC injury-prevention guidance says to predissolve only when the product label directs it, and then to add the pool chemical to water, never water to the pool chemical.
• Dichlor (stabilized granules): Dissolves fast and contains cyanuric acid. The CDC names dichlor and trichlor as examples of chlorine products with cyanuric acid.
• Trichlor (tablets or sticks): Dissolves slowly in a floater, skimmer, or inline feeder, so it maintains chlorine rather than raising it quickly. It also adds cyanuric acid. Tablets placed in the skimmer basket should follow the label and your equipment maker’s guidance.
• Salt chlorine generator: Makes chlorine from dissolved salt. To raise free chlorine, increase the output setting or run time, or use the boost or “super chlorinate” mode if your unit has one, as described in its manual.

Stabilizer is meant to slow chlorine loss in sunlight. Because it is not used up the way chlorine is, its level can climb when you rely on stabilized products for a long stretch. The CDC notes that disinfection times are longer in the presence of cyanuric acid, and its recommended minimum is 2 ppm, rather than 1 ppm, in pools that use it. If you rely on chlorine tablets all summer, keep an eye on cyanuric acid as part of your routine water analysis.

Why Free Chlorine Keeps Dropping

If you raise free chlorine and it falls again within a day, something is using it up. The CDC lists things that reduce chlorine levels: pee, poop, sweat, and dirt from swimmers’ bodies, as well as sunlight. In a backyard setting, that can look like one of these:

• Strong sun on an unstabilized pool: An above ground pool in full sun that gets only unstabilized liquid chlorine may lose free chlorine faster on bright days. A reading in the late afternoon, compared with the morning reading, shows how much you are losing.
• Heavy use: A pool party or a week of kids in the water every afternoon adds a lot of body waste. In guidance written for pool operators, the CDC notes that even a quick rinse shower removes much of the body waste that helps form chloramines. Having swimmers rinse off before they get in is a simple way to apply that idea at home.
• Leaves and debris: An inground pool under oak or maple trees takes in organic material daily. Empty the skimmer basket and remove leaves from the floor before they break down.
• Algae starting: Cloudy or slightly green water with steady chlorine loss can point to algae. Brush the walls and treat according to the product label, or have a pool service take a look.
• Low circulation: Short pump run times or a clogged filter leave parts of the pool poorly treated. Check that the pump runs long enough for chlorine to reach the whole pool, and follow your pump manual’s electrical warnings whenever it runs.

Storing and Handling Chlorine Safely

The CDC’s pool chemical safety guidance, written for pool operators, calls for storing chemicals in original, manufacturer-labeled containers, protecting stored chemicals from mixing together or with other substances, protecting stored pool chemicals from getting wet, and storing them below 95°F and in conditions recommended by the manufacturer. For home owners, the CDC’s guidelines for keeping your pool safe and healthy start with reading and following all directions on product labels.

The chlorinating tablet label on file with the EPA cited above gives similar storage directions for that product: keep it dry in a tightly closed container when not in use, and store it in a cool, dry, well ventilated area away from heat or open flame. Other products carry their own storage directions, so read each label.

In practice, that means a dedicated shelf or cabinet for chlorine, away from acids and other pool chemicals, as each product’s label directs, with lids closed tight. A CDC report on pool chemical injuries advises opening one product container at a time and closing it before opening another. Never pour leftovers from one chemical into another container.

When to Call a Pro

Call a pool service company or your local pool supply store’s water analysis desk if free chlorine will not hold after you have treated per the label, if the water stays green or cloudy, or if readings from your strips and your DPD kit disagree. If chemicals are spilled, mixed by mistake, or give off fumes, move away to fresh air and follow the label’s directions. The first aid section of the EPA tablet label says that if the product is inhaled, move the person to fresh air, and if the person is not breathing, call 911 or an ambulance. For salt systems that will not produce chlorine, contact the equipment maker’s support line before replacing parts.

For more on keeping water balanced, browse the water care section.

FAQ

What Is Free Chlorine in a Pool?

It is the chlorine still available to disinfect, as opposed to chlorine that has already combined with contaminants. The CDC defines it as chlorine not combined with other constituents and therefore able to act as an effective disinfectant.

What Is the Difference Between Free Chlorine and Total Chlorine?

Total chlorine is free chlorine plus combined chlorine. Subtract free chlorine from total chlorine to get combined chlorine, the portion that is no longer disinfecting well.

How Fast Can I Raise Free Chlorine?

Unstabilized liquid chlorine and calcium hypochlorite act faster than slow-dissolving tablets. The product label gives the waiting time before swimming and before re-checking. Do not add extra product to speed things up.

Are Strips Good Enough to Check Free Chlorine?

Strips are an option, but the CDC calls them less accurate than a DPD kit. When you are deciding whether to add more chlorine, confirm the reading with a DPD kit.

Why Is My Free Chlorine Zero Right After I Add Chlorine?

A zero right after you add chlorine has two very different causes. Heavy organic load, algae, or a lot of combined chlorine can use up a normal dose. But the CDC notes that when a water sample holds more than 10 ppm of chlorine, the reading can bleach out and show a false low or 0 ppm result. Do not add more chlorine just because of one zero, and do not swim until you have a valid reading. First confirm the result: follow your test kit’s directions for diluting a sample if it gives them, or wait and measure again before anyone swims. A true zero in cloudy or green water means no free chlorine is left to disinfect the pool. In that case, keep everyone out of the water, follow your product label for the next dose, and have the water analyzed at a pool store or by a pool service.

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