Pool chemistry has a reputation for being complicated, but day to day you only need to understand six numbers. Two of them (free chlorine and pH) change every day; the other four change slowly and only need checking every week or two. Get them in range and your water stays clear, safe and comfortable on the skin.
The six numbers at a glance
| Level | Ideal range | Test | What it does |
|---|---|---|---|
| Free chlorine (FC) | At least 1 ppm; ~7.5% of CYA or more | Every 1–3 days | Kills germs and algae |
| pH | 7.2–7.8 | Every 1–3 days | Comfort, chlorine strength, equipment protection |
| Total alkalinity (TA) | 70–120 ppm | Weekly | Keeps pH stable |
| Stabilizer (CYA) | 30–50 ppm (salt: 60–80) | Every 2 weeks | Protects chlorine from the sun |
| Calcium hardness (CH) | 200–400 ppm (vinyl: 150–250) | Monthly | Protects plaster, prevents scale |
| Salt (salt pools) | Per your cell, often 3,200 ppm | Monthly | Raw material for the chlorine generator |
1. Free chlorine: the one that keeps you safe
Free chlorine is the chlorine that is still available to kill bacteria, viruses and algae. The CDC recommends at least 1 ppm in home pools. But the amount you really need depends on your stabilizer: cyanuric acid protects chlorine from sunlight and also slows it down. A practical rule is to keep free chlorine at about 7.5% of your CYA level or more. With 40 ppm of CYA, that means at least 3 ppm, and around 4–5 ppm is comfortable.
Your test kit may also show total chlorine. Total minus free is combined chlorine: chlorine that has already reacted with contaminants. More than 0.5 ppm of combined chlorine means it is time to shock. Calculate your daily dose with the chlorine calculator.
2. pH: comfort and chlorine strength
pH measures how acidic or basic the water is. Human eyes are most comfortable around 7.4–7.6. Below about 7.2 the water becomes corrosive to metal and plaster and stings the eyes; above 7.8 chlorine loses strength, water turns cloudy and scale builds up on tiles and in the heater.
In most pools pH slowly rises on its own, especially with waterfalls, spa spillovers or a salt cell, so expect to add acid regularly. Use the pH calculator, which adjusts the acid dose for your alkalinity.
3. Total alkalinity: pH's shock absorber
Alkalinity is the water's buffer. Low alkalinity lets pH swing wildly; high alkalinity pushes pH upward and makes it hard to bring down. If you are adding acid every few days, try lowering alkalinity toward 70–80 ppm: pH usually becomes much more stable. Baking soda raises alkalinity; acid plus aeration lowers it. The alkalinity calculator gives the exact amounts.
4. Stabilizer (CYA): sunscreen for chlorine
Without stabilizer, the sun can destroy most of an outdoor pool's chlorine in a few hours. With 30–50 ppm of CYA, chlorine lasts days. Too much, though, makes chlorine sluggish, and the only way to lower CYA is to replace water. Remember that trichlor tablets and dichlor add stabilizer every time you use them, so CYA creeps up all season. Track it with the CYA calculator.
5. Calcium hardness: protecting the surface
Plaster and concrete pools need enough calcium in the water, or the water dissolves it from the walls. Too much calcium does the opposite and leaves white scale. Vinyl and fiberglass pools are less sensitive. Calcium chloride raises it; only water replacement lowers it.
6. Salt (saltwater pools only)
A salt chlorine generator makes chlorine from dissolved salt. Keep salt in the range your manufacturer specifies: too low and the cell stops producing, too high and it may shut down or corrode metal. The salt calculator tells you how many bags to add.
How the numbers affect each other
- Adjust alkalinity before pH, because changing alkalinity also moves pH.
- Adjust pH before chlorine, because chlorine works much better at the right pH.
- Raising CYA raises the chlorine you need. Check your chlorine target every time CYA changes.
- Add one chemical at a time, with the pump running, and wait at least 30 minutes before the next one.
Which test kit should you use?
Test strips are quick but imprecise, especially for chlorine and CYA. A liquid drop-count kit (often called a FAS-DPD kit for chlorine) is more accurate and pays for itself in chemicals you don't waste. Whatever you use, store reagents out of the heat and replace them every year.
Sources and methodology
Last reviewed: November 2026. Doses are calculated from the chemistry of each product (molecular weights and available chlorine) and cross-checked against manufacturer and industry dosing charts. Recommended ranges follow CDC guidance for home pools and hot tubs.