Pool water chemistry basics (2026): pH, chlorine, alkalinity

The five numbers that matter

You do not need a chemistry degree. You need to know five values, what range they should sit in, and what to add when one drifts.

Test	Target range	Why it matters
Free chlorine	1 to 3 ppm	Kills bacteria and algae
pH	7.4 to 7.6	Comfort, equipment life, chlorine effectiveness
Total alkalinity	80 to 120 ppm	Stops pH from bouncing
Calcium hardness	200 to 400 ppm	Protects plaster and equipment
Cyanuric acid (CYA)	30 to 50 ppm	Protects chlorine from sunlight

Test free chlorine and pH twice a week in season. Test the rest once a month.

Free chlorine

Chlorine is the sanitizer doing the actual work. Drop below 1 ppm in summer and you can grow algae overnight. Common sources are tablets in a floater or feeder (slow), liquid chlorine or cal-hypo (fast), and salt cells that generate chlorine on the fly.

If chlorine is low, add liquid chlorine in the evening so sunlight does not burn it off. A gallon of 10 percent liquid chlorine raises 10,000 gallons by about 10 ppm.

pH and total alkalinity

pH controls comfort and protects everything chlorine touches. Below 7.2 you get itchy eyes and corroded heaters. Above 7.8 chlorine works at half strength and you get scale.

Alkalinity is pH's shock absorber. With alkalinity in range, pH stops bouncing every time it rains. Raise alkalinity with sodium bicarbonate (baking soda). Lower pH with muriatic acid or dry acid.

Cyanuric acid (CYA) is the trap

CYA acts like sunscreen for your chlorine. You need 30 to 50 ppm or your chlorine evaporates. But CYA does not break down on its own. Stabilized chlorine tablets keep adding it. Once you pass 80 ppm, your chlorine effectively stops working and the only fix is to drain part of the pool and refill.

This is the single most common reason a green pool will not clear. Test CYA once a month.

Calcium hardness

Too low (under 150) and the water pulls calcium out of your plaster. Too high (over 500) and you get cloudy water and scale on the tile line. Most tap water comes in around 150 to 250 already. You almost never need to lower it (you would have to drain). You raise it with calcium chloride.

When to call a pro

Call when CYA is over 100 ppm, when total dissolved solids climb above 3,000 ppm, or when you have tried the obvious fix twice and the number will not move. A water test at the local pool store is free and catches things a strip will miss.

Frequently asked questions

What is the most important pool chemistry number?

Free chlorine. If chlorine is in range and the water is clear, everything else has time. If chlorine is at zero, you can grow algae in 24 hours regardless of how perfect the other numbers are.

Can I just use test strips?

Strips are fine for routine checks. For decisions (how much acid to add, whether to shock, why CYA seems off), a Taylor K-2006 drop kit gives you numbers you can trust. Most serious owners keep both.

How much liquid chlorine do I need to add?

One gallon of 10 percent liquid chlorine raises 10,000 gallons by roughly 10 ppm. For a 20,000 gallon pool, half a gallon raises chlorine by about 2.5 ppm. Add in the evening so sun does not burn it off.

Why does my pH keep climbing?

Saltwater pools and aerated water (waterfalls, fountains, returns pointed up) drive pH up naturally. Add muriatic acid in small doses (one cup at a time for a 20,000 gallon pool) and retest after a few hours of run time.

What happens if my CYA is too high?

Chlorine becomes much less effective. At 100 ppm CYA you may need 5 to 8 ppm chlorine to do the work of 1 ppm at 30 ppm CYA. The fix is to drain and refill part of the pool. There is no chemical that removes CYA in residential use.

How long after adding chemicals can I swim?

Liquid chlorine: wait until it drops back into the 1 to 3 ppm range, usually 30 to 60 minutes with the pump running. Acid: 15 minutes with the pump on. Shock: wait until chlorine is back under 5 ppm.

Do I need to test in winter?

In a closed pool, no. In an open pool running year round, yes, just less often. Once a week for chlorine and pH is plenty in cool weather.

Pool water chemistry basics: pH, chlorine, and alkalinity