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How Much Chlorine Does Algae Treatment Need?

Reviewed 2026-04

Enter measured stabilizer (CYA) and free chlorine (FC) to estimate an algae-treatment addition. Retest before each addition and maintain the target until the completion checks pass.

See how the calculations work in our methodology

Example volumes (gallons):

Stabilizer binds chlorine, so this chart uses higher FC treatment targets as CYA rises. Enter a measured reading. The chart covers CYA 20–100 ppm; outside that range, follow the product label and seek qualified guidance.

Product: Check that your product label matches the chemical and available-chlorine strength selected above.
Confirm your inputs

Before you use a dose, compare every input with your fresh tests and measured pool volume. Confirm each example value only after you’ve checked it. When you change a value, you’ll need to confirm it again.

Compare pool shock

Enter your measured gallons and fresh tests above to work out how much you need.

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Product option

In The Swim

Calcium Hypochlorite Shock · 12 x 1 lb bags

Check price on Amazon →

In The Swim

Sodium Dichlor Chlorine Shock Granules · 5 lb

View product at In The Swim →

Frequently asked questions

How do I calculate each algae-treatment dose?

Enter your gallons, measured CYA and fresh FC reading. The dose fills the gap to the target for your CYA. Test FC again before you calculate each later addition. Use a fresh FC reading to calculate each addition back to the CYA-based treatment target, and apply it as the chlorine label directs. Circulate, then retest in about one hour initially. If chlorine demand is rapid, test more often. Recalculate every addition; never repeat an old dose. Brush daily and filter continuously. Clean at the filter manual’s pressure or condition trigger, and recharge D.E. after backwashing as that manual directs.

How do I know algae treatment has worked?

Finish treatment only when these three checks pass together: the water is clear with no visible algae, combined chlorine is at or below 0.5 ppm, and overnight FC loss is at or below 1 ppm. Test after sunset and before sunrise without adding or generating chlorine between readings. Then resume routine FC targets and meet the label’s swimming conditions. Complete every check, even if the water looks clear or you’ve been treating it for several days.

What must I test before swimming after treatment?

A dose calculation cannot clear a pool for swimming. Before anyone swims, test free chlorine (FC) and pH. Keep pH at 7.2–7.8, with clear water, the bottom visible and no visible algae. FC must meet both the minimum for measured CYA and the chlorine label’s re-entry range. CDC minimums are 1 ppm without stabilizer and 2 ppm with it; a pool with CYA can require more. Test FC and pH at least twice daily, and more often during heavy use.

Does this calculator cover mustard or black algae?

The calculator gives an ordinary-algae target. For persistent mustard or black algae, have a pool professional assess the problem before you increase treatment strength. Clean the pool surfaces and accessories using methods safe for your finish.

Do I need shock on a weekly schedule?

Base that decision on your readings and the pool’s condition. Keep routine FC at the target for your measured CYA. Visible algae, combined chlorine or unexpected chlorine demand need investigation. If you’re opening a pool with no algae, use those checks to decide whether shock is needed.

Chlorine Calculator →CYA Calculator →

How we calculate this
Your result uses the formulas explained in our methodology. Read the assumptions for your calculation before you use the result.