How Much Chlorine to Add to a Pool

  • By: John Parker
  • Last Updated: September 30, 2022

With spring coming around, swimming season will soon be upon us. Although beaches are a common getaway, many of us love swimming in pools. But, when the pool is murky, smelly, or dirty, well, the healthy swimming environment we were looking forward to definitely goes down the drain. 

Before you resort to avoiding the swimming pool entirely, rest assured that you can restore its “shine” by cleaning it with chlorine. But how much should you add?

It can be difficult to figure out at first, but once you learn how to do it, you’ll never worry about adding chlorine again. We’re going to show you how to determine the amount of chlorine to add to your swimming pool.

How Much Chlorine to Add to a Pool
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Table of Contents

Determining the amount of chlorine to add

You need to deposit the correct amount of chlorine into the swimming pool water to make sure it’s properly sanitized and that it doesn’t cause adverse reactions to swimmers. Remember, there is such a thing as too much chlorine!

Getting that balance right relies on two things: knowing the volume of the pool and testing the water. 

Calculating the volume of a swimming pool

If you already know the volume of your pool, obviously you can skip this section.

Knowing how many gallons your pool holds, whether it is an in-ground pool or an above ground pool. is something that you will need to know quite often when maintaining your pool, not just when you want to chlorine to the pool.

When calculating the volume of your pool (pool size), you are going to need all or some of these values:

  • Area (A)
  • Width (W)
  • Depth (D)
  • Height (H)
  • Length (L)
  • Volume (V)
  • Diameter (d)
  • Radius (r)
  • Pi (3.14)

When calculating the cubic volume of your swimming pool, you will need to know the depth and surface area. To get the most accurate calculations, you may need to separate your pool into sections based on depth. This means you calculate the shallow end separately from the deeper end.

One cubic foot of water consists of 7.5 gallons of water. So after you’ve determined the cubic footage of the swimming pool, the next step is to multiply that number by 7.5. The result of this calculation is the volume of your pool in gallons (how much water it can hold).

Since pools can be of different shapes, calculating the volume will be based on your pool’s geometry. Most pools are rectangular or squared, so we’ll start there. For any shaped swimming pool, though, you must first determine if the depth is the same throughout.

Rectangular pool (or squared)

If the depth is consistent, then you need to measure the length and width of your pool. Multiply this figure by the depth to get the cubic feet. After that, you should multiply the result by 7.5 to get the pool’s volume in gallons.

This is the formula you should be working with: V = L x W x D x 7.5

If the bottom of your pool has a gradual slope, then you need to use the average depth of the pool to determine its volume. You can get this measurement by adding the depth of the shallow end to the depth of the deep end and then dividing that by two. The length, width, and depth of the surface area of the pool should be added together.

For pools that have variable depths and drop-offs, you need to treat the shallow ends and deep ends as separate pools. So you would need to calculate the volume for both individually and then add the numbers together to get the cubic volume for the entire pool. Multiply that by 7.5 to get the volume in gallons.

Round swimming pool

Calculating the volume of a swimming pool

To get the volume of a round pool, you would multiply the radius squared (r2) by Pi (3.14), then by the diameter. You can find the diameter by measuring the length or width of the pool (should be the same distance since it’s round). You would divide the diameter by two to get the radius. 

To get the radius squared, just multiply it by itself once. The complete formula for the volume in gallons should look like this: V = 3.14 x r2 x d x 7.5

Irregular swimming pools

Not all pools can fit into a single, plane shape. But they can be broken down into several simple shapes, and that’s the best way to measure these complex pools.

You would need to identify the different round, square, and rectangular sections in the pool. Then you would calculate the volume based on the shape of each section. Once you have all the numbers, you would just add them up for the final result.

It will take some complex calculations to get everything right. If the calculations become overwhelming, you can try to use an online calculator to see if that works for you.

Testing your pool’s chlorine level

Testing your pool's chlorine level

Now that you have the volume for your swimming pool, we can move on to the next step. Before you add any chlorine, you need to know how much is already in the pool water. Again, we don’t want too much chlorine in there, right?

Ideally, you want the concentration of chlorine to be between 1 ppm (parts per million) and 3 ppm. 2-3 ppm is considered a neutral level.

When testing how much chlorine is in your pool water, you must be aware of the different types. The three types of chlorine that are important are free chlorine, combined chlorine and total chlorine.

Combined chlorine is chlorine that has already sanitized and has combined with contaminants in the water. Free chlorine has yet to bind with any contaminants so it is available to sanitize. See my post Free chlorine vs total chlorine vs combined chlorine for more details.

There are test kits that give you the value of free chlorine and total chlorine (sum of combined and free available chlorine) present in the water. These are usually more expensive. 

Here are the different ways or equipment that can be used to test the chlorine concentration in the water:

  • Chlorine test strips
  • Chlorine test kits
  • Electronic chlorine testers
  • Automatic chlorinator and tester

If you’ve recently added chlorine, you should wait 24 hours before testing. Most test strips and kits are easy to use, so just follow the instructions that come with them.

Chlorine test strips

Test strips are probably the simplest way to test the water in terms of the technology used. They usually come with a transparent, cylindrical container with color charts on the outside. After matching the chart with what’s in the container, there’s usually reference material included to explain what it means. 

The instructions that come with the strips will also inform you of how much water you should collect to perform the test. After you collect the water from your swimming pool, you will usually have to leave it for some time for the strips to work.

Now, the shade may not always be an exact match with what’s on the chart, but it’s usually close enough. The color will reference the ppm, and that will help when you’re adding chlorine.

JNW Direct Pool and Spa Test Strips - Quick and Accurate Pool Test Strips - 7-1 Pool Test Kit - 100 Bromine, pH, Hardness and Chlorine Test Strips - with Ebook and App - 100 Water Test Strips

I use these JNW Direct Pool and Spa Test Strips

Chlorine test kits

These are more complex than test strips but are also more accurate and reliable. By sampling a large amount of water from your swimming pool, test kits can provide a more accurate measurement of how much chlorine is in the water.

Included in each kit is:

  • A guide/manual
  • Test tube to hold samples
  • Reagents
  • Saturation index tool

It is important that you follow the instructions exactly as directed or you won’t get accurate results. Test kits also come with color graphs to help decipher the chlorine levels.

Poolmaster Water Chemistry Case (22270) Premiere Collection 5-Way Swimming Pool & Spa Test Kit, Small, Neutral

Taylor K-1003 Safety Plus Swimming Pool Chlorine Bromine pH Alkalinity Test Kit

Electronic chlorine testers

For those who are color blind or have trouble with the color charts, electronic testers are a good alternative. It’s a bit faster and simpler than the previous methods, but it needs to be calibrated before use. As a handheld device, you would use it to sample the water and then wait for the results as it measures the exact chlorine levels.

There are some testers on the market that can run continuously. You can also get a tester that can provide data on other things as well.

AquaChek TruTest Digital Test Strip Reader For Pool and Spa Water Testing - Tests for Free Chlorine, Free Bromine, pH, and Total Alkalinity - Quick & Accurate Results Includes 25 TruTest Strips (Blue)

AquaChek Aqua Chek Trutest Digital Reader

So, finally, how much chlorine should you add?

Once you’ve tested the chlorine levels and determined the pool’s volume, you can then calculate how much chlorine to add to pool water.

To increase the chlorine content by 1 ppm, you would have to multiply the gallon volume of your pool by 0.00013. For example, if your pool holds 10,000 gallons of water, then you would multiply 10,000 by 0.00013 to get 1.3 ounces of chlorine.

The above calculation works for chlorine powder, granules or tablets. If you’re using liquid chlorine, then you need to read the label to calculate how much liquid chlorine to add as the chlorine levels vary depending on the brand.

How to add chlorine to pool

When using powdered chlorine or chlorine granules you should first mix them in a bucket of pool water. For safety’s sake, always add the water first, then add the chlorine to the water. Then mix it until it is completely dissolved. If you add quite a substantial amount you will need to mix it in stages using a number of buckets of water.

Both for the chlorine you have mixed in a bucket and liquid chlorine, walk around the pool’s edge pouring the mixed chlorine to the pool as you go.

You should run the pool pump and pool filter for a few hours at least after adding the chlorine to ensure it is spread evenly.

If using chlorine tablets then you can put them in a floating chlorine dispenser which will allow water in and dispense chlorine to your pool slowly as it floats. You can also put chlorine tablets in your skimmer basket – see Can you put chlorine tablets in a skimmer basket?

Pool Maintenance Course

When I first bought my house with a swimming pool, I knew nothing about cleaning and maintaining it. I was recommended Swim University’s Pool Care Handbook and video course so I bought it and have never regretted it.

It was probably the best money I spent that year as I have saved thousands by doing it myself.

Liquid chlorine, chlorine tablets, or chlorine granules?

how much chlorine to add to pool

Now that you know how to calculate how much chlorine to put in your pool, and have figured out different ways to determine the chlorine level of your swimming pool, the next question becomes which form of chlorine should you use. As a pool owner, that really depends on your personal preference. 

For example, chlorine tablets and granules are much lighter to carry around than hauling the equivalent amount of liquid chlorine back and forth. And with a floating chlorine dispenser, or adding the chlorine tablets to your pool skimmer, you do get a sort of uniformity to your chlorine level.

Not to mention, chlorine tablets dissolve slowly, more so than chlorine granules, and so keep the chlorine level in your swimming pool constant. Determining how many tablets you need, though, is tricky.

However, liquid chlorine really doesn’t involve a lot more work than pouring the liquid in. Since liquid chlorine is relatively weak you might be surprised at how much you actually need.

How often should you add chlorine?

Knowing when to add chlorine is based on several factors.

Pool usage

The more you use the pool, the more frequently you will have to maintain it. If it’s used just once per week, then it can be tested once per week. However, if it’s used more frequently than that, then you should test the chlorine levels 2-3 times per week. You should also add chlorine based on your swimming pool water contamination level.

Debris

Insects, dirt, and leaves can get trapped in your pool quite easily. Once there, they tend to exhaust the amount of active chlorine present in the water since the chlorine reacts to them. If you want to keep leaves and other external debris out of your pool, install a pool cover. 

Sunlight

Sunlight also plays a role in how often you add chlorine. The UV rays break down the chlorine and can cause the levels to drastically drop. This can be fixed by using pool covers to shield your pool from the sun or an easier way is to use stabilized chlorine that contains cyanuric acid to protect it from the UV rays.

Of course, if it’s an indoor pool, then the UV problem is pretty much solved on its own.

Handling chlorine safely

Before you put anything into your pool, you need to first understand the nature of this chemical and how it affects what it comes into contact with. Chlorine is the most popular pool sanitizer and is great for keeping your pool safe and clean because it can kill germs and prevent outbreaks. However, the chemical itself is very corrosive.

This means you need to handle it with care and follow proper procedures when using it. 

Tips to follow

  • Always inspect the label and instructions that come with your pool cleaning products.
  • When it comes to protecting yourself, use personal protective equipment (PPE) whenever you’re handling chemicals. The most common injuries that occur when handling chlorine are inflammation of the eyes, face, hands, and lungs. To prevent these injuries you should use chemical-resistant gloves, goggles, and only open chemical containers in wide, open spaces.
  • Opening chemical containers indoors (especially in small, enclosed spaces) will enable the chemical to concentrate in the air. This increases the likelihood of lung injury. When you open the containers outside, the chemicals can safely disperse into the fresh air and tend to waft away from you.

Working out how much chlorine to add to your pool can seem daunting. It could be one of the harder things to figure out since it involves so many calculations. However, once you’ve determined the size of your pool and the current chlorine levels, things get easier, and you’ll be able to enjoy the swimming season a whole lot more!

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