CYA to Free Chlorine Ratio: Working the 45:1 Math

The MAHC 45:1 line is CYA divided by DPD free chlorine: above 45 in a pool using stabilizers is an imminent health hazard (MAHC 6.6.3.1, item 4, where your state adopted it). To stay under it, free chlorine must be at least CYA / 45, so 2.0 ppm at 90 ppm CYA and 2.67 ppm at 120 ppm.

Why do the 2.0 ppm floor, the 90 ppm cap and the 45:1 line fit together?

The Annex explains that the 45:1 closure ratio was chosen to match the 2018 MAHC limits of 2 ppm minimum free chlorine when CYA is present and a maximum of 90 ppm CYA. Check it: 90 / 2.0 = 45. The three numbers describe one corner of the operating box.

Two consequences follow:

• With CYA at or below 90 ppm and free chlorine at or above the 2.0 ppm floor (5.7.3.1.1.2.2), the ratio cannot exceed 45:1. The floor is the binding rule.
• The ratio becomes the active rule when CYA is over 90 ppm. Free chlorine of 2.5 ppm passes the ratio at CYA 100 (40:1) and fails it at CYA 120 (48:1), though both pools are already over the cap.

The same free chlorine reading therefore means different things in two pools. Without CYA the floor is 1.0 ppm (5.7.3.1.1.2.1), and CDC's operating page lists the same 1.0 and 2.0 ppm minimums in its splash pad guidance.

Which three thresholds trigger which action?

Threshold	Code section	What it triggers
CYA above 90 ppm	5.7.3.1.3.2	Out of compliance with the cap. Dilute by drain and refill.
CYA:DPD-FC above 45:1	6.6.3.1 item 4	Imminent health hazard. The owner, operator or person in charge must close (6.6.3.2), and the health department can order correction or closure.
CYA 300 ppm or higher	5.7.3.1.3.3 and 6.6.3.1 item 23	Closed until remediated. Item 23 lists CYA above 300 ppm with bathers present as an imminent hazard.

The Annex also notes that the cap was cut from 100 to 90 ppm because 100 ppm is the highest value the turbidimetric CYA test reads without diluting the sample. If your test returns "100 or more", dilute and retest before you can compute a ratio.

Why does the ratio matter more than the free chlorine number?

CYA binds chlorine and lowers the HOCl concentration, and HOCl is what kills. The Annex says the HOCl concentration is fairly consistent at a constant CYA:DPD-FC ratio (Table 5.7.3.1.1.2(c), shown at 77 F, with HOCl in ppm as Cl2):

CYA:FC ratio	HOCl, ppm
5:1	0.10
10:1	0.042 to 0.044
20:1	0.020
40:1	0.0095 to 0.0098
70:1	0.0053 to 0.0055
100:1	0.0037 to 0.0038

At a fixed 2.0 ppm free chlorine (Annex Table 5.7.3.1.1.2(d)), HOCl is 0.027 ppm at 30 ppm CYA and 0.0087 ppm at 90 ppm. That is 0.027 / 0.0087 = 3.1 times less kill power at the same reading. With no CYA at 1.0 ppm free chlorine, the same table gives 0.49 ppm.

DPD-FC includes cyanurate-bound chlorine as well as HOCl and OCl- (Annex glossary), so the ratio uses what your kit reports. The kit does not show how much of it is working.

How do you run it as a daily number?

Ratio = CYA / free chlorine. Required free chlorine = CYA / 45. Carry the last CYA test forward and compare it with each day's lowest free chlorine reading:

CYA, ppm	CYA / 45, ppm FC	FC that applies (higher of floor and CYA / 45)	Ratio at that FC	HOCl at that FC, ppm
30	0.67	2.0	15:1	0.027
50	1.11	2.0	25:1	0.016
70	1.56	2.0	35:1	0.011
90	2.00	2.0	45:1	0.0087
120 (over the cap)	2.67	2.67	45:1	not in Annex table

If your SOP wants a margin, set an internal action ratio below 45:1. That is your own choice and not a code number; the Annex tables show the cost of every step up in ratio.

How do trichlor feeders walk a pool to the line?

Trichlor and dichlor deliver chlorine and leave CYA behind. Using molecular weights from PubChem (trichlor 232.41, CYA 129.07, dichlor 219.94, Cl2 70.90), trichlor leaves 129.07 / (3 x 70.90) = 0.61 ppm CYA per ppm of chlorine delivered, and dichlor leaves 129.07 / (2 x 70.90) = 0.91. These are stoichiometric figures; check your product label for its stated available chlorine.

Example: an outdoor pool opens at 20 ppm CYA. The erosion feeder supplies 1.5 ppm of chlorine a day, and make-up water is ignored.

1. CYA gain: 1.5 ppm x 0.61 = 0.91 ppm per day.
2. Days to reach 90 ppm: (90 - 20) / 0.91 = 77 days.
3. 77 days is 11 weeks. At that point the floor, cap and ratio all meet, with no room left, and the next trichlor tablet pushes the pool over the cap.

Water you replace with CYA-free make-up water slows this. Each percent of volume replaced takes the same percent off the CYA reading.

How much water do you drain to bring CYA down?

Fraction to replace = 1 - (target CYA / current CYA), assuming CYA-free refill water and full mixing.

Current CYA, ppm	Target, ppm	Calculation	Fraction to replace	MAHC Table 6.5.3.2.1
30	15	1 - 15/30	50%	50%
50	15	1 - 15/50	70%	70%
70	15	1 - 15/70	79%	79%
90	15	1 - 15/90	83%	83%

That table belongs to the MAHC's diarrheal-stool hyperchlorination rule (6.5.3.2.1), which requires CYA at 15 ppm or less before raising free chlorine to 20, 30 or 40 ppm. It makes a good cross-check for the formula. For a normal reduction, example: a 90,000 gallon pool tests 120 ppm CYA and you want 60. Fraction = 1 - 60/120 = 50%, so 0.50 x 90,000 = 45,000 gallons. Two 25 percent exchanges do not equal one 50 percent exchange: 0.75 x 0.75 = 0.56, so you keep 56 percent of the CYA. For the fecal incident side, see hyperchlorination CT math and the CYA drain table.

What testing cadence does the MAHC set?

• CYA tested monthly in venues using it (5.7.5.7) and 24 hours after any CYA is added (5.7.5.7.1).
• Weekly or more often when stabilized chlorine is the primary disinfectant (5.7.5.7.2).
• No CYA or stabilized chlorine in spas and therapy pools for new construction, substantial alterations and disinfection equipment replacements (5.7.3.1.3.1), and no later than 4 years after adoption for existing ones (5.7.3.1.3.1.1).
• The Annex says CDC does not recommend CYA for indoor venues, and it cites no peer-reviewed advantage there.

See the commercial pool chlorine range and how often to test commercial pool water for the surrounding numbers.

What does this look like across 10, 30 or 60 pools?

Add a CYA column to the monthly sheet, then add three more: lowest free chlorine in the last 7 days, CYA / 45, and the margin. A negative margin means the ratio is over 45:1, which is an imminent health hazard: close that pool and raise free chlorine before reopening (6.6.3.2). Flag every pool on a trichlor erosion feeder for weekly ratio review, in line with 5.7.5.7.2. Set a stabilized or unstabilized product policy by pool type: unstabilized for spas, therapy pools and indoor pools, stabilized only where sun justifies it and someone owns the trend. Poolean builds daily or monthly logs you can download as a CSV, and its out-of-range alerts warn you before chlorine is expected to drop below your minimum, so a thin margin shows up before the next round. Book a 30-minute call to see how the logs work. Use the water test log template for the log columns.

The short version

• Ratio = CYA / free chlorine. Above 45:1 is an imminent health hazard in the MAHC. Required free chlorine = CYA / 45.
• At 90 ppm CYA and 2.0 ppm free chlorine, the cap, the floor and the ratio line all meet.
• The ratio only bites on its own above 90 ppm CYA; at or below 90, the 2.0 ppm floor decides.
• Replace 1 - (target / current) of the water to cut CYA, and check it against MAHC Table 6.5.3.2.1.
• Trichlor adds about 0.61 ppm CYA per ppm of chlorine, so test erosion-feed pools weekly.