What is “chlorine demand”?

One of the things pool and spa owners struggle with is the fact that they can’t just add X amount of Y chemicals each week and maintain good water chemistry. What’s with all the testing? Why do I have to figure out how much of this and that to add? Why isn’t it the chlorine demand the same week after week?

Blue Today, Green Tomorrow

Today’s chemical products make it easier than ever to measure and use the right amount of water care products. But there are still fluctuations in how much is required. That’s because a number of factors determine how much chlorine and other chemicals are needed to do the job of sanitation at any given time.

For example, this week you may use your pool every day. Last week rain or work obligations kept you out of your pool most days. Next week you may have company and several more bodies in your pool. And the week after that, when your kids are away at grandma’s, it will be different yet again. Similarly, there could be rainfall, sunlight, wind, or what’s blooming or pollinating nearby that affect your water. There are almost unlimited factors to determine how much chlorine it takes to sanitize your pool or spa on any given day.

Defining “Chlorine Demand”

According to the Department of Civil Engineering at Sacramento State, chlorine demand is “the difference between the amount of chlorine added to water or wastewater and the amount of residual chlorine remaining after a given contact time. Chlorine demand may change with dosage, time, temperature, pH, and nature and amount of the impurities in the water.” In other words, how much chlorine is left available to work after the water is sanitized? 

If you’ve ever been to a public or hotel pool that smelled of chlorine, it may have worried you. You probably thought that they’d used too much chlorine. In fact, the smell was probably caused by too little chlorine. The deficit in chlorine demand leads to a powerful chlorine odor that isn’t present when the chlorine balance is right.

Passing Pool Chemistry 101

The amount of chlorine you need today is likely not the same as last week or next week. Your at-home testing strips will test for the most common problems with water chemistry. But only the ALEX computerized water tests we conduct in our stores gives a full report of all potential problems. We recommend that you bring us a water sample at least every few weeks during the swim season. We will make sure your pool water is problem-free.

In the pictures, you can see the difference just 12 hours makes after the chlorine demand is rectified. You’d be embarrassed for anyone to see your pool green like that. And you certainly wouldn’t want your family and friends jumping in there to swim! The the breathtaking blue is exactly what you had in mind when you invested in a pool to begin with. So, let’s get your water back to being blissfully beautiful.

Come in to Parnell Pool & Spa today, and get that water chemistry thoroughly tested. Let’s make your pool look like the “after” picture here, instead of the mean, green “before”. In the meantime, check out our pool selections online. We have everything you need to keep your pool sparkling and clean all summer long.

Let's Get Started

Request a quote, schedule service, or just let us know what water care questions are on your mind.

Contact Us

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.


(910) 425-6138

4206 Cumberland Rd
Fayetteville, NC 28306

Mon-Fri: 9am to 5pm
Saturday: 9am to 3pm
Sunday: Closed


(910) 738-8110

1380 Linkhaw Rd
Lumberton, NC 28358

Mon-Fri: 9am to 5pm
Saturday: 9am to 1pm
Sunday: Closed

Follow Us

Scroll to Top