CYA Chlorine Chart: Chart by Pool Size

Your minimum free chlorine is your CYA level times 5% if a salt cell makes your chlorine, and times 7.5% if you add it by hand. Shock is CYA times 40%. At CYA 80 that is 4.0 ppm minimum for a salt pool, 6.0 ppm for a manual one, and 32 ppm to shock. The whole table is below.

This is the numbers page. The product question behind it, which tablets exist without stabilizer in them and whether you need any, is on chlorine tablets without CYA. Here we are only doing arithmetic.

The chart

Every number in that table is from Pool Chem Tracker’s FC/CYA chart, which reproduces the Trouble Free Pool methodology as a table. It is not Trouble Free Pool itself, and that distinction is worth keeping straight, because TFP’s own explainer makes the argument without publishing the numbers. It establishes that the free chlorine you must hold depends on your CYA level and rises as CYA rises, and that letting FC fall too low for your CYA gives you algae. The table is what that argument looks like as figures.

Our page on optimal chlorine levels in a saltwater pool declines to give a ratio for exactly that reason. This page is where the ratio lives now.

The three formulas

You do not need to keep the table open. Three multiplications reproduce almost all of it.

• Minimum FC, salt generator: CYA x 5%. 80 x 0.05 = 4.0.
• Minimum FC, manual chlorine: CYA x 7.5%. 80 x 0.075 = 6.0.
• Shock: CYA x 40%. 80 x 0.40 = 32.

We checked all 27 cells against those three formulas. Twenty six match exactly. One does not: at CYA 20 the published shock level is 10 ppm, and 20 x 0.40 is 8. Round up to the published figure there. Everywhere else the multiplication and the chart agree to the decimal.

The target column is not a formula, it is a working band that sits above the minimum with enough headroom that a hot week does not push you under it before you next test.

Why the salt column is lower

The two minimum columns differ by a third, and the reason given is production, not chemistry. A cell makes chlorine continuously while the pump runs, so it replaces consumption as consumption happens. A pool chlorinated by hand gets a slug in the evening and then coasts, so it has to start higher to still be above the floor at 4pm the next day.

That is the same mechanism that makes a salt pool want more stabilizer in the first place, which is laid out on best range of CYA for a salt pool. The catch is that the chart cuts both ways. A cell that has stopped producing, or a pump on a short schedule, turns your pool into a manually chlorinated pool without telling you, and the column you should be reading moves from 4.0 to 6.0 at the same CYA.

The problem with your manufacturer’s chart

Here is the part worth the page.

Hayward’s AquaRite manual publishes a water balance chart. It asks for free chlorine of 1.0 to 3.0 ppm. On the same chart it asks for stabilizer of 60 to 80 ppm, with 80 listed as the target.

Put those two instructions together and do the multiplication.

At Hayward’s own stated target of CYA 80, a pool run exactly to Hayward’s own chlorine range is below the FC/CYA chart’s minimum across the whole range, and at the bottom of the range it is at a quarter of it. Not by a rounding error. By a full ppm at the best case and 3 ppm at the worst.

Both of those are real published positions from sources people rely on. We are not going to tell you one of them is simply wrong, because we cannot demonstrate it, and because the warranty on your equipment follows the manual and not a chart on the internet.

What we will say is that they cannot both be describing the same pool, and you are the tiebreaker.

The practical test is algae

TFP’s claim is testable and it is testable in your own backyard. If free chlorine is too low for your CYA, you get algae. So:

Run Hayward’s numbers and watch the walls. If you hold 2.5 to 3.0 ppm at CYA 80 all season, the water stays clear, and the walls are not slippery, then the manufacturer’s chart works for your pool and there is nothing to fix. Plenty of pools run this way.

If you get algae anyway, you have your answer. Slippery walls, a green tint at the deep end, water that clouds after a hot weekend. That is not a mystery requiring a new product. It is the chart telling you the chlorine you hold is not enough for the stabilizer you hold. The likely-cause order is on algae in a salt water pool and salt water pool green.

There are two moves from there and you should think of them as a pair. Hold more free chlorine, or carry less CYA. Both close the same gap. Lowering CYA is the expensive one because stabilizer only leaves with water, which is why Orenda calls draining and diluting the most economical and practical method and the only realistic alternative is reverse osmosis. Read reduce cyanuric acid in a swimming pool before you start a hose.

What changes with pool size, and what does not

The title of this page says chart by pool size, so let us be direct about that, because it is the single most common misreading of a chart like this one.

None of the ppm figures change with pool size. Parts per million is a concentration. A 12,000 gallon pool at CYA 80 needs the same 4.0 ppm minimum free chlorine as a 40,000 gallon pool at CYA 80. There is no small-pool column and no big-pool column, and any chart offering you one is measuring something else. The chart above is the whole chart.

What changes with pool size is how much product it takes to get there, and that scales in a straight line with gallons.

• Stabilizer. HTH’s figure is 1 lb per 10,000 gallons to raise CYA by about 10 ppm. So 20 ppm in a 20,000 gallon pool is 4 lb. Add it slowly to the skimmer with the pump running, leave the pump running, and do not backwash for 48 hours. It dissolves slowly, so wait 24 to 48 hours before you retest or you will read low and overdose.
• Shock. In The Swim puts a full shock at 3 lbs of cal hypo per 10,000 gallons, or 5 gallons of liquid bleach per 10,000 gallons. They also say to add 50% more above CYA 20 and to double it above CYA 40, which is the same relationship the shock column expresses.
• Free chlorine day to day. In a salt pool you are not dosing this at all. You are setting an output percentage and a run time, and a bigger pool needs more of both to hold the same ppm.

All three of those need your real gallons, not the number on the listing. Work it out on pool volume or with the pool volume calculator, and if you are sizing a salt dose at the same time, the pool salt calculator handles that one.

Before you trust any of it, test CYA properly

Every number on this page is a multiple of one measurement, and that measurement is the least precise one in your test kit. It is a turbidity test judged by eye. A reading of 70 that is really 90 moves your minimum from 3.5 to 4.5 and you will never notice the difference on an FC test.

That does not make the chart useless, it makes the input worth care. How to read the tube, and why the pool store’s number disagrees with yours, is on the CYA pool test kit. What is in the bag you are pouring in is on chlorine stabilizer for pool.

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