FC/CYA Chart: How Much Chlorine Your Pool Needs

The quick answer: your free chlorine (FC) target depends on your cyanuric acid (CYA) level.

• Keep FC at no less than about 7.5% of CYA.
• Use 40% of CYA as your shock (SLAM) level.
• With CYA 40, that means a minimum of 3 ppm, a daily target of 6 ppm and a SLAM level of 16 ppm.

The chart and calculator below give the numbers for your water.

The chart

These are the same numbers the calculator above and the Pool Calc app use, computed by the same engine: minimum 7.5% of CYA (never below 1 ppm), target 3 ppm above the minimum, top of the daily range 2 ppm above the target (never above 10 ppm), and SLAM 40% of CYA (never below 10 ppm).

All values in ppm free chlorine, manual chlorine (liquid chlorine, bleach, tablets). Salt water pools use a 5% minimum and a 7.5% target; the calculator switches when you pick a salt system.

How to read it:

• Minimum FC: never let FC fall below this, even right before your next dose. Below it, algae can start to grow.
• Daily target: a cushion above the minimum. Dose up to it (or a little above, up to the top of the range) in the evening, and you'll still be above the minimum after a sunny day. Adjust it to your pool's own daily chlorine use.
• SLAM FC: for clearing algae, cloudy water or a new-season opening, held until three clear conditions are met. This is not for routine use, and nobody swims during a SLAM. See how much shock to add.

Home test kits read to 0.5 or 1 ppm, so aim for the nearest reading at or above the number. Indoor or shaded pools with no CYA follow different practice; ask before copying an outdoor-pool target.

When can you swim? Swim when free chlorine is back within your normal range and at or below the re-entry level on your chlorine label (many labels say 4 ppm). Never swim during a SLAM or with free chlorine above 10 ppm. Your product label always comes first.

Why CYA changes how much chlorine you need

When you add chlorine, it forms hypochlorous acid (HOCl), the form that actually kills germs and algae. Sunlight destroys HOCl within hours, so outdoor pools add cyanuric acid.

CYA binds most of the chlorine into a protected reserve. That reserve doesn't break down in sunlight, and it releases HOCl as HOCl gets used up.

The catch is that with more CYA, a smaller share of your FC is active HOCl at any moment. Your test kit reports total free chlorine (bound plus active) as one number. So 3 ppm FC with 30 CYA is a strong pool, while 3 ppm FC with 90 CYA is a weak one, even though the test reads the same.

Keeping FC in proportion to CYA keeps active HOCl at a consistent, effective level. That's all the chart does. It's also why advice like "keep chlorine at 1–3 ppm" works for an indoor pool with no CYA and fails outdoors.

pH matters less than you've been told (when CYA is present)

Old charts show HOCl collapsing as pH rises. That's true in water with no CYA. With CYA present, the reserve buffers the effect, and pH changes the active chlorine level much less. Keep pH in range (7.2–7.8) for comfort and to protect surfaces and equipment. But if your pool is losing the algae battle, the FC/CYA ratio is far more likely to be the reason than pH 7.7.

Choosing your CYA level

• Manually chlorinated pools (liquid chlorine or bleach): around 30–50 ppm is a common sweet spot. That's enough sun protection that FC targets stay manageable.
• Salt water (SWG) pools: usually 70–80 ppm. The cell makes chlorine all day, and higher CYA protects that steady trickle. Follow the cell manufacturer's range too.
• Pools using trichlor tablets: CYA keeps climbing (about 0.6 ppm CYA for every 1 ppm FC added). Check CYA every month. Once it's high, your FC targets climb with it.

Too low a CYA level means chlorine disappears every sunny afternoon. Too high a level forces impractically high FC. If CYA is above about 90–100 ppm, read cyanuric acid too high.

Testing: get the CYA number right

Every target on this chart depends on an accurate CYA reading:

• Use a turbidity (cloudy-water) CYA test in a drop kit, rather than strips.
• Read it in shade with your back to the sun.
• Make sure the sample is near room temperature. Cold water reads high.
• Above roughly 90 ppm, most tests stop being reliable. Dilute the sample 1:1 with tap water that has no CYA, then double the result.
• Adding stabilizer? It dissolves slowly. Wait about a week before trusting the new CYA reading.

Using the chart day to day

1. Test FC and CYA (CYA monthly; FC daily or every other day).
2. Find your CYA row. Note the minimum and the target.
3. Dose to the top of the target in the evening. The liquid chlorine calculator gives the amount.
4. Watch your log. If FC drops more than about 1 ppm overnight, something is consuming chlorine (often the start of algae). Act early with a SLAM rather than waiting for green.

A worked example

Your pool tests at CYA 60 and FC 4 at 7 pm:

• Minimum for CYA 60 is 4.5 ppm. You're under the minimum, so dose tonight.
• The target is 7.5 ppm, which leaves room for tomorrow's sun.
• In a 15,000 gallon pool, going from 4 to 7.5 ppm takes 67 fl oz (0.52 gal) of 10% liquid chlorine. The calculator above works this out for your own pool and product.
• Test again tomorrow evening. If FC is still above the minimum, your dose was right. If it fell below, raise your evening target a little.

FC/CYA in the app

You don't need to memorize the chart. In Pool Calc:

• every chlorine dose targets the right FC for your latest CYA reading;
• the app switches to the SLAM level when you start a SLAM;
• it adjusts automatically for salt water pools;
• custom targets you set stay set.

One pool? Every feature, free. No account, no ads, and iCloud backup is included.

Safety

• Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
• Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
• Add chemicals to water, never water to chemicals.
• Store chemicals sealed, cool and dry, apart from each other.
• Read and follow the product label. It always comes first.