FC/CYA chart
Also called: free chlorine / CYA chart, FC/CYA relationship.
The FC/CYA chart is the table that gives the free chlorine (FC) a pool needs at each cyanuric acid (CYA) level: a minimum of about 7.5% of CYA, a target band above that minimum, and a SLAM level of about 40% of CYA.
Why it matters
Free chlorine is not a fixed number, because active chlorine tracks the ratio of free chlorine to CYA. Richard Falk, known as chem geek on Trouble Free Pool, worked out that about 7.5% of CYA holds active chlorine near the level that reliably prevents algae. The target is not a clean percentage, which is why you read it off the chart. A quick way to hold the floor in your head: keep at least 1 ppm of free chlorine for every ~13 ppm of CYA. Borates do not lower that floor.
In numbers
At CYA 30 the target band is 4–6 ppm; at CYA 100 it is 11–13 ppm. At CYA 50 the floor is about 3.75 ppm.