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How to Raise Free Chlorine Levels in a Pool

By Matt Giovanisci | Updated: July 19, 2022

If your free chlorine level is lower than 1 part per million (or ppm), then you need to take action to raise your chlorine levels. Chlorine levels at or below 1 ppm mean your pool water isn’t sanitized, which can lead to algae growth and bacteria build-up.

But what if you’ve added chlorine tablets or granules and the levels won’t change? How do you raise free chlorine in a pool if you’ve already added chlorine?

If your chlorine levels are low (1 ppm) or you’re not getting a chlorine reading at all (0 ppm), shock your pool water with a chlorine-based shock.

But if your chlorine levels continue to remain low, even after shocking, you may have a high chlorine demand issue.

Keep reading for a step-by-step guide on what causes low chlorine levels and how to raise free chlorine levels in your pool.

What is Free Chlorine?

Free chlorine is the amount of chlorine that’s available to sanitize your pool water. In other words, it’s the chlorine that hasn’t been used up and is still free to kill contaminants. If there isn’t enough free chlorine in your water, bacteria and other harmful microorganisms can build up. Your free chlorine levels should be between 1 and 3 ppm.

On the other hand, combined chlorine (a.k.a. chloramines) is chlorine that’s already been used up to fight contaminants in the water. Your combined chlorine level should be no more than 0.5 ppm. If the amount of combined chlorine gets too high, your chlorine will stop working effectively and it will produce that classic chlorine smell.

Finally, total chlorine is the sum of free chlorine and combined chlorine. If the total chlorine levels are the same as the free chlorine levels (like 3 ppm), it means none of the chlorine has been used up (combined chlorine) and it’s available to work as a sanitizer.

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What Causes Low Free Chlorine Levels?

The first step in raising the free chlorine in your pool is knowing what caused it. And there are several factors that can contribute to sudden or chronically low free chlorine levels.

High Chlorine Demand

Chlorine can only sanitize so much. If you have a high bather load or a lot of debris in the water, you’ll need more chlorine to sanitize the water effectively. The more contaminants you have to fight in the water, the more chlorine your pool water will demand.

Chlorine demand is most common when your pool water sits untreated and stagnant for the winter. Heavy rainfall can also lead to chlorine demand, especially if you experience runoff from your yard.

If you just added chlorine or a chlorine-based shock to your pool and you still have a low chlorine level, there’s a chlorine demand problem.

In order to cure chlorine demand, triple shock your pool with calcium hypochlorite or cal-hypo shock. Add three pounds of shock for every 10,000 gallons of water in your pool.

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Use the entire contents of the bag when opened. If any granules settle to the bottom of the pool use brush to disperse them. Add the right dosage of this product during evening hours while the filter pump is running.

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Heavy Pool Use

The more swimmers you have in the pool and the more often you use it, the more contaminants your chlorine will need to fight. Things like sunscreen, dirt and even the oil from your skin can cause your chlorine to work over time.

Shock your pool weekly and especially after parties to keep your free chlorine levels high and your water sanitized.

Sunlight and Low Cyanuric Acid Levels

If you’re using unstabilized chlorine (like liquid chlorine or bleach) to sanitize your water, your chlorine levels will drop in the sunlight. That’s because the sun’s rays will break down your chlorine if it doesn’t have any stabilizer (a.k.a. cyanuric acid).

Test your cyanuric acid levels regularly and use a chlorine stabilizer to keep your chlorine from breaking down in the sunlight.

Very High Cyanuric Acid Levels

When your CYA levels are too high, it can hamper your chlorine’s effectiveness. High CYA will weaken your chlorine’s ability to sanitize your water. If you use 3-Inch chlorine tablets (trichlor pucks), it will consistently raise your cyanuric acid.

Regularly test your CYA levels and make sure that it stays within the correct range of 30 and 50 ppm.

Increased Organic Contaminants

A heavy rain storm can increase the contaminants in your pool water, And that can quickly use up your free chlorine as it binds to everything in an effort to sanitize the water.

Be sure to clean debris from your water after a storm and shock your pool, especially if you’ve experienced runoff.

Refilling the Pool with Fresh Water

If you’ve just added fresh water to your pool, it could disrupt your water chemistry. More fresh water means less sanitized, balanced water with the proper amount of pool chemicals.

Not Adding Enough Chlorine

One of the simplest reasons behind low free chlorine levels is that there’s not enough chlorine regularly added to the water. If you’re adding chlorine tablets to your skimmer but your pump shuts off, that chlorine won’t get into the water. And if you use a chlorine floater, make sure you have enough chlorine in it to cover your surface area.

Not Enough Salt in Your Salt Water Pool

If you use a saltwater generator and your chlorine levels are low, your pool may not have enough salt. Your salt cell needs a certain level of salinity to function.

Test your pool salinity every month to make sure your salt levels are within range. And be sure to regularly inspect your salt water generator to make sure it’s working properly.

How to Raise Free Chlorine in a Pool

If you’ve not cleaned your pool yet and there’s a lot of debris, your free chlorine level is likely low because it’s busy working on cleaning all the contaminants from the debris. Clean your pool and remove debris, then test the chlorine level.

Then, test your water. You’ll want to make sure your pH, alkalinity, and cyanuric acid levels are within range. If you need help balancing your water chemistry, be sure to check out our guides on pH and cyanuric acid.

Once your pool is free of debris and your other levels are balanced, it’s time to shock your water.

When you shock your pool, you’re adding enough chlorine to reach a “chlorine breakpoint.” This raises your pool’s free chlorine levels to 10 times or more over the normal amount.

There are multiple types of chlorine pool shock and each one can affect the other pool chemicals in the water, like your pH levels. Keep in mind that using a non-chlorine shock will decrease your combined chlorine but it will not raise your free chlorine level. That’s because the oxidation in a non-chlorine shock helps break up contaminents without adding chlorine.

  • For Traditional Chlorine Pool Owners: Use Cal-Hypo Shock. Cal-hypo is the strongest type of chlorine shock available and will help quickly raise your free chlorine levels. Because cal-hypo shock is unstabilized chlorine, you’ll need to add it to your pool at night to avoid the sun’s UV rays destroying your shock. Then wait at least 8 hours before swimming again. But if you suspect you have high chlorine demand (lots of contaminants and a reading of 0 ppm), you’ll need to triple shock your pool.
  • For Salt Water Pool Owners: Use Sodium Dichlor Shock. Dichlor shock will help raise your chlorine without causing scale buildup on your saltwater generator. But it will also raise your cyanuric acid (CYA) levels. So if you’re already dealing with high CYA levels, opt for liquid chlorine.

What if Your Free Chlorine Levels Don’t Change After Shocking?

There are several reasons why your free chlorine levels are still low after shocking your water:

  • Other pool chemicals are out of balance: Without the right amount of pH or cyanuric acid in your water, your chlorine levels can fluctuate. So make sure these are balanced before shocking your pool.
  • High contaminant load: You may have a build-up of bacteria or algae that requires a more aggressive shock treatment. If your pool looks dusty on the bottom or clouds up when you brush it, it’s likely mustard algae.

What if Your Free Chlorine Levels are Constantly Low?

If you find that you’re consistently getting low chlorine readings, check your cyanuric acid level. Low CYA means that your chlorine is susceptible to the sun’s UV rays. So your chlorine will burn up faster.

On the other hand, you may have high chlorine demand. Your chlorine’s effectiveness is finite. So if there are lots of swimmers or debris and organic contaminants in the water, more chlorine is required to properly sanitize the water. At a certain point, your chlorine gets used up faster than you can add it and your levels remain low. Again, shocking your pool can help fix this.

On the other hand, if you have constantly low chlorine levels in a saltwater pool, you likely need to add more pool salt or clean or replace your salt cell.

Frequently Asked Questions About How to Raise Free Chlorine in a Pool

Need more help figuring out how to raise free chlorine in a pool? Here are some common questions and answers.

What do I do if my free chlorine is too low?

If your levels are low, start by simply adding more chlorine. Then use test strips or a liquid testing kit to determine the current level. Finally, add chlorine granules to raise the levels quickly. If that doesn’t work, shock your pool.

How much shock do I need to raise free chlorine?

When shocking your pool, the goal is to significantly and quickly raise your free chlorine levels above your combined chlorine levels. So if you’re using cal-hypo shock, use 1 pound bag per 10,000 gallons of water.

Does chlorine-free shock raise free chlorine?

Non-chlorine shock will not add chlorine to your pool, and therefore, will not raise free chlorine levels. However, non-chlorine shocks will remove chloramines, a.k.a. combined chlorine.

What is more important: your free chlorine or total chlorine levels?

Both are important, but what matters is that your free chlorine levels are always higher than your combined chlorine levels. If the free chlorine level is low and the combined level is high, then you don’t have enough available chlorine to adequately sanitize your pool. Total chlorine accounts for both free and combined chlorine.

Is it safe to swim in a pool with low chlorine?

Technically, you could swim in a freshly filled pool with low chlorine. But contaminants will build up quickly in the water. So continuously using a swimming pool with a low level of chlorine could cause illness and eye or skin irritation. It’s always better to balance pool chemicals and swim in a pool with adequate chlorine.

Need More Pool Maintenance Help?

Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1995. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.
A Cheat Sheet For Perfectly Balanced Pool Water and Crystal Clear Swimming
Download this FREE printable cheat sheet to keep your pool clean and clear without any fuss. You’ll never worry about your pool again with this easy-to-use guide.
Click Here For The FREE Cheat Sheet