The 5 Numbers That Matter (and Their Target Ranges)

Your entire pool comes down to five measurements. Here’s what each does and where it should sit. (Philosophy first? Read Less Is More. Ready to dose? The chemical calculator does the math.)

Target ranges

pH won’t sit still, and that’s normal — it drifts up on its own and down when you add acid. If yours climbs fast, a water feature is often why: any fountain, bubbler, or spillover aerates the water and pushes pH up (a salt cell nudges it up too). See how fountains, bubblers & spillovers change your water.

These five aren’t independent — they’re one equilibrium. To see whether the combination is corrosive, balanced, or scaling (you can have every number “in range” and still be out of balance), read water balance, plainly and run the water-balance calculator.

The one relationship that matters most: FC and CYA

Free chlorine is not a fixed number. CYA (stabilizer) binds most of your chlorine into a protected reserve, so only a small fraction is active at any moment. The more CYA you have, the higher your FC must be to keep the same killing power. Maintain FC too low for your CYA and algae grows even though the reading “looks normal” — the single most common cause of a green pool with “fine” chemistry.

Rules of thumb: minimum FC ≈ 7.5% of CYA and SLAM level ≈ 40% of CYA. Your target sits between them — read it off the chart, because unlike the other two it isn’t a clean percentage (at CYA 30 the band is 4–6 ppm, or 13–20% of CYA; at CYA 100 it’s 11–13 ppm, or 11–13%). A quick way to hold the floor in your head: keep at least 1 ppm of free chlorine for every ~13 ppm of CYA — that’s the same 7.5%, just easier mental math. (Want the why — the hypochlorous-acid chemistry that makes this a ratio, not a fixed number? See how chlorine and CYA actually work.)

Your target isn’t a fixed dot, either. Chlorine demand rises with heat, sun, rain, and swimmers, so lean toward the top of the target range in the peak of summer and after a pool party, and ease toward the bottom as the water cools off. Warmer water burns chlorine faster: a pool that holds 6 ppm easily in spring can slide under its minimum during a 95°F week if you don’t feed it more. Test more often when it’s hot.

The FC/CYA chart

• Target FC band (read it off the chart)
• Minimum FC (≈ 7.5% of CYA)
• Below minimum — algae can grow

Liquid chlorineIdeal CYA 30–60

CYA	Min FC	Target FC
30	2	4–6
40	3	5–7
50	4	6–8
60	5	7–9
70*	5	8–10
80*	6	9–11
90*	7	10–12
100*	8	11–13

Salt-water chlorineIdeal CYA 60–90 · 70–80 sweet spot

CYA	Min FC	Target FC
60	3	4–9
70	3	5–10
80	4	6–11

* Outside the recommended CYA range — the water still sanitizes at these FC levels, but you'll burn more chlorine than you need to. Hold CYA inside the shaded band. All values in ppm. Shock (SLAM) levels scale the same way, at about 40% of CYA — the calculator below gives yours.

These FC levels are safe, not “over-chlorinated” — because CYA holds most of it in reserve, the active chlorine at target level is still well below a public pool’s active level at CYA 0. High CYA is what makes them necessary. (Salt pools run higher CYA, so they hold a wider target band; the chart tops out at the CYA levels each pool type should actually use.)

The borates exception, retired

You will read — you read it here until August 2026 — that maintaining 50 ppm of borates lets the minimum free chlorine ease from 7.5% of your CYA toward about 5%, because borates add a secondary check on algae.

We have retired that, because the check it rests on did not survive its own source. Trouble Free Pool’s wiki now states the algaestatic effect is overstated at the levels pools actually run: real inhibition takes 10–24 ppm of elemental boron, which is above 100 ppm of borates and closer to 200, against the 30–50 ppm you carry. TFP’s current borates page carries no reduced minimum at all, and Kim Skinner, whose service company has run borates since the 1960s, says the suggestion to hold lower chlorine on borates is “risky and problematic.”

So there is no exception. Hold the 7.5% minimum, borates or not — it’s the simple, safe, repeatable floor, and the chemical calculator uses it for every pool. Borates are still worth running for steadier pH and less scale inside a salt cell: borates in a pool: what they actually do (and what they don’t) has the whole picture, including what they cost by each route.

Why you can’t just “add more stabilizer”

CYA only goes up when you add it (or use stabilized chlorine like dichlor/trichlor tablets, which sneak it in). The only way down is draining and replacing water. So if your CYA climbs to 90+, your required FC climbs with it and never comes back down until you dilute. This is why tablets, left on autopilot, slowly wreck a pool: they push CYA up until your normal chlorine stops working. Keep CYA in range and everything else gets easier — and if yours has already climbed too high, see CYA too high: why your chlorine quit and how to lower it (and why “chlorine lock” is a myth).

None of these five numbers mean anything without a test you can trust, and the one that decides your chlorine target — CYA — is the number a pool-store machine most often gets wrong. How to test your water covers the drop-based kits that read all five, and which one is worth the money.

Want to catch a number drifting before it bites? Log every test in the free, on-device Pool Tracker — it judges each reading against your pool’s target, hands you the exact dose, and shows the trend.