FC/CYA Target Ratio Calculator

Under the Trouble Free Pool (TFP) FC/CYA method, your free-chlorine targets scale with your stabilizer (CYA), not fixed numbers — a "normal" 3 ppm is dangerously low at CYA 80 and wastefully high at CYA 20. Every number below is a multiple of your CYA.

Minimum FC  = CYA × 7.5%   (regular)   or   CYA × 5%   (salt / SWG)
Target FC   = CYA × 10%  →  CYA × 13%   (a band, not a point)
SLAM / shock = CYA × 40%   (a sustained level held over time, NOT one dose)

FC/CYA ratio % = current FC ÷ CYA × 100

The salt distinction matters: SWG pools use a 5% minimum (not 7.5%) because the salt cell replenishes chlorine continuously — getting that wrong is the most common error in lookalike calculators.

Status verdict (when you enter a current FC): below minimum, low (above minimum but below the target floor), in target range, above target, or in the SLAM range. The "FC to add" figure is a target gap, not a chemical dose — turn it into a product amount in the chlorine-dosing tools.

The reference chart is pinned to the published PoolChem Tracker grid, so the SLAM column at CYA 80/90/100 reads 31/35/39 (the chart cells) rather than a bare ×0.40. The headline at your exact CYA uses the multipliers so any value works. This is the hobbyist TFP method every consumer tool uses — not a regulatory standard. Commercial pools follow MAHC / state health-department minimums that differ.

Sources: PoolChem Tracker — FC/CYA Chart; PoolDial — FC/CYA Ratio Calculator; Trouble Free Pool — SLAM blog + CYA wiki.