How to Calculate Pool Chlorine Demand: A Pro's Guide to CYA

I received a call last summer from a client in a panic. “Arend, my pool is turning green, but I put three chlorine pucks in the feeder yesterday! The test strip says the chlorine level is high!” I drove over, took one look at his crystal-clear test water, and knew the problem wasn’t a lack of chlorine. It was too much of something else.

Your pool’s chlorine demand isn’t simply a calculation of sun exposure plus bather load. The single most important—and most misunderstood—factor is your cyanuric acid (CYA) level. Get this right, and you’ll have a sparkling, safe pool while saving money on chemicals. Get it wrong, and you’ll fight a losing battle against algae all season, just as that client did.

What Really Drives Pool Chlorine Demand?

Most people think chlorine is simply “used up” by two things: sunlight, or UV radiation, and contaminants such as sweat, sunscreen, leaves, and algae. They’re right, but that’s only half the story. The real driver of how much chlorine you use is how efficiently it works. Your water chemistry—and specifically your CYA level—controls that efficiency.

First, let’s define the terms.

• Free Chlorine (FC): This is the “good” chlorine. It is the active, available sanitizer that kills germs, algae, and bacteria. This is the number you should care about most.
• Combined Chlorine (CC): This is the “bad” chlorine. It is chlorine that has combined with nitrogen-containing contaminants, such as sweat and urine. It creates that “pool smell” and is a poor sanitizer. High CC levels—above 0.5 ppm—indicate that your FC is struggling to keep up.
• Total Chlorine (TC): This is simply FC + CC.
• Cyanuric Acid (CYA): This is your chlorine stabilizer, or “conditioner.” Its job is to bond with free chlorine and protect it from rapid degradation by the sun’s UV rays.

The FC-to-CYA ratio is the key to managing pool chlorine demand because CYA protects free chlorine from rapid UV degradation.

How Does Cyanuric Acid Affect Pool Chlorine?

Think of CYA as sunscreen for your chlorine. Without it, the sun will obliterate your FC within hours. During a new pool startup in Austin, Texas, we once filled the pool and added chlorine before adding any stabilizer. To see what would happen, we tested the FC at noon and again at 2 p.m. We measured a staggering chlorine loss of over 90% in two hours under full sun with no CYA. Your chlorine doesn’t stand a chance without a baseline level of CYA.

An outdoor pool needs some CYA. We typically aim for a starting level of 30–50 ppm, or parts per million.

But here's the part that trips everyone up: CYA doesn't just protect chlorine; it also slows it down. The bond between a CYA molecule and a chlorine molecule is strong. When chlorine is bonded to CYA, it's protected from the sun, but it's also much slower to act as a sanitizer. It's like your chlorine is wearing handcuffs—it's still there, but it can't react quickly to kill new algae or bacteria that enter the pool.

This leads to the paradox of high CYA. If your CYA level creeps too high (say, over 90 ppm), your chlorine becomes so slow and ineffective that algae can grow faster than the chlorine can kill it. This is 'chlorine lock.' The test strip might show a high FC level, but that chlorine is so bound up by CYA that it's functionally useless. That was my client's exact problem. His CYA was over 150 ppm.

What is the ideal fc/cya ratio for a pool?

Because of the 'handcuff' effect, the absolute ppm of Free Chlorine in your pool is almost meaningless by itself. What really matters is the ratio of Free Chlorine to Cyanuric Acid.

As a firm rule on all our jobs, we maintain an FC level that is at least 7.5% of the CYA level. This is the industry-accepted minimum for effective sanitation.

Let’s see how that plays out:

Look at that table. If your CYA is 90 ppm, you need an FC of almost 7 ppm just to keep the water sanitized. If you let it creep to 110 ppm, you need over 8 ppm of FC! Most homeowners are aiming for 2-4 ppm, completely unaware that their high CYA has made that level totally inadequate.

This ratio is the core of calculating your true chlorine demand. You aren't just replacing what the sun and swimmers use; you are maintaining a specific FC level that is a percentage of your CYA level.