Pool Chemistry for Beginners: The Only 5 Numbers That Matter

Pool chemistry sounds complicated, but you really only need to understand five numbers. Here's what each one means, why it matters, and what your target should be.

1. Free Chlorine (FC): 1-4 ppm

Free chlorine is your pool's sanitizer. It kills bacteria, destroys algae, and breaks down organic contaminants. Without adequate free chlorine, your pool becomes a breeding ground for things you don't want to swim in. The ideal range is 1-4 ppm. Below 1 ppm and your water isn't safe. Above 5 ppm and it can irritate skin and eyes. Chlorine works best when the other four numbers are in range, especially pH.

2. pH: 7.4 - 7.6

pH measures how acidic or basic your water is, on a scale from 0 to 14. For pool water, 7.4-7.6 is the sweet spot. This range is comfortable for swimmers (your eyes are about 7.5) and allows chlorine to work effectively. At pH 7.2, about 63% of your chlorine is active. At pH 8.0, only about 21% is. That's a massive difference. Keeping pH in range means your chlorine works harder so you use less of it.

3. Total Alkalinity (TA): 80-120 ppm

Total alkalinity is pH's bodyguard. It acts as a buffer that prevents pH from swinging wildly up and down. When TA is in the right range (80-120 ppm), your pH stays stable. When TA is too low, small amounts of acid or base will cause big pH swings. When TA is too high, pH becomes stubborn and hard to adjust. Always adjust TA before trying to dial in pH.

4. Cyanuric Acid (CYA / Stabilizer): 30-50 ppm

Cyanuric acid protects chlorine from the sun's UV rays. Without it, sunlight can destroy 90% of your free chlorine in just a couple of hours. With CYA at 30-50 ppm, your chlorine lasts much longer. But there's a catch: too much CYA suppresses chlorine effectiveness. Above 80 ppm, your chlorine starts to struggle even at normal levels. The only way to lower CYA is to partially drain and refill. Indoor pools don't need CYA since there's no UV exposure.

5. Calcium Hardness (CH): 200-400 ppm

Calcium hardness measures the dissolved calcium in your water. Water naturally wants a certain level of calcium. If your water is too soft (low calcium), it will pull calcium from your pool surfaces: plaster, grout, tile, and equipment. This is called aggressive water and it causes etching and damage. If calcium is too high, it deposits white scale on surfaces and inside equipment. The target is 200-400 ppm for plaster pools and 175-225 ppm for vinyl or fiberglass.

The Order Matters

When adjusting chemicals, do it in this order: total alkalinity first, then pH, then calcium, then stabilizer, then chlorine. Each adjustment affects the next, so working in order saves you time and chemicals.

Start Simple

You don't need to test all five every time. For routine checks, focus on chlorine and pH (the two that change fastest). Test alkalinity, CYA, and calcium weekly or biweekly. Pool Clarity handles the math for you. Enter whatever readings you have and get personalized instructions for your specific pool.