Types of pool chlorine compared
All pool chlorine products release the same sanitizer, hypochlorous acid, but they leave different things behind in the water.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (10 sections)
For every 1 ppm of chlorine, trichlor adds 0.6 ppm of stabilizer (CYA) and dichlor adds 0.9 ppm. Cal-hypo adds about 0.7 ppm of calcium, and liquid chlorine and bleach add only a little salt. Pick the form by what your water can still absorb, not by the label on the bucket.
Comparison table: what each form adds per 1 ppm
Amounts are for raising free chlorine (FC) by 1 ppm in 10,000 gallons, calculated from each product's available chlorine. "Side effect" is what stays in the water after the chlorine has done its job.
| Form | Typical strength | Amount for +1 ppm | Also adds per 1 ppm FC | Effect on pH |
|---|---|---|---|---|
| Liquid chlorine (sodium hypochlorite) | 10% or 12.5% (trade %) | 12.8 fl oz (10%), 10.2 fl oz (12.5%) | Salt only | Small rise when added, roughly neutral once used up |
| Household bleach (sodium hypochlorite) | 6–8.25% (by weight) | 20.6 fl oz (6%), 14.5 fl oz (8.25%) | Salt only | Same as liquid chlorine |
| Cal-hypo (calcium hypochlorite) | 65–73% | 2.05 oz (65%) | +0.7 ppm calcium hardness | Raises pH |
| Dichlor (sodium dichloroisocyanurate) | 56% or 62% | 2.4 oz (56%), 2.15 oz (62%) | +0.9 ppm CYA | Close to neutral |
| Trichlor (trichloroisocyanuric acid) | 90% | 1.48 oz | +0.6 ppm CYA; −0.7 ppm total alkalinity (TA) as it is used | Lowers pH |
| Non-chlorine shock (MPS) | Label dose | Does not raise FC | Sulfate | Lowers pH slightly |
Sources: CYA per ppm from PHTA; cal-hypo calcium and trichlor alkalinity figures from reaction stoichiometry (our calculation); ounces checked against Taylor's K-1005 tables. Liquid chlorine uses trade percent, and bleach uses weight percent (explained below).
Per pound, the solids differ as well. One pound in 10,000 gallons raises FC by about 10.8 ppm with trichlor 90%, 7.8 ppm with cal-hypo 65%, and 6.7 ppm with dichlor 56%. A gallon of 12.5% liquid chlorine raises the same pool by 12.5 ppm.
Why the side effects matter becomes clear over a season. A pool that uses 2 ppm of chlorine a day for 100 days consumes about 200 ppm of FC in total. On trichlor alone, that adds roughly 120 ppm of CYA. On dichlor alone, it adds 180 ppm. On cal-hypo alone, it adds about 141 ppm of calcium hardness. None of those leave the water except by draining. That single fact explains most "my chlorine stopped working in August" stories.
Use the calculator to see the dose of any form for your pool.
Liquid chlorine and bleach
Both are sodium hypochlorite in water. Pool-store liquid chlorine is usually 10% or 12.5% and labeled in trade percent (grams of available chlorine per liter ÷ 10). Household bleach is 6% to 8.25% sodium hypochlorite by weight, which works out to about 6.2 to 8.8 in trade terms. Use plain, unscented bleach with sodium hypochlorite as the only active ingredient.
Good for: daily chlorinating in any pool, shocking when the product's label gives pool shock directions, and pools where stabilizer or calcium is already high. It adds nothing you will have to drain out later except a little salt.
Watch for: it weakens in storage, and faster in heat. PHTA figures show 12.5% losing half its strength in about 180 days at 77 °F and in about 48 days at 95 °F. It is heavy to carry and must be dosed often. Doses: liquid chlorine calculator and bleach calculator.
Trichlor tablets
Trichlor is the familiar 1-inch or 3-inch tablet (also sold as sticks and granules), about 90% available chlorine. It dissolves slowly in a floater or feeder, so it keeps chlorinating while you are away.
Is trichlor stabilized? Yes. Cyanuric acid is part of the molecule, which is why every 1 ppm of chlorine from trichlor leaves about 0.6 ppm of CYA behind (PHTA). It is also acidic: as the chlorine is used, alkalinity drops by roughly 0.7 ppm per 1 ppm FC, and pH trends down.
Good for: building stabilizer early in the season, and for owners who can't dose daily. Watch for: CYA buildup. Once CYA passes about 50 ppm, the chlorine level you need rises with it. The chlorine tablets calculator shows the week your pool will cross that line.
Dichlor
Dichlor is a fast-dissolving granule, 56% (dihydrate) or 62% (anhydrous) available chlorine per PHTA. It is close to pH-neutral, which makes it popular in small pools and hot tubs and as a "shock" in pools with low stabilizer.
It adds even more stabilizer than trichlor: 0.9 ppm CYA per 1 ppm FC. A shock dose that raises FC by 10 ppm adds about 9 ppm of CYA at once. That's fine when you're building CYA from zero after a fresh fill. It's a problem when you're already at 50.
Cal-hypo
Calcium hypochlorite is sold as granules and tablets, usually 65% to 73% available chlorine. It contains no stabilizer, so it doesn't push your chlorine target up. But each 1 ppm of FC adds about 0.7 ppm of calcium hardness, and it raises pH.
Good for: shocking pools with high CYA, and plaster pools that are short on calcium anyway (see calcium hardness). Watch for: calcium building up in hard-water areas, cloudiness if you broadcast it into water with high pH, and bleached spots if undissolved granules sit on a vinyl liner. Pre-dissolve it in a bucket of pool water if your label allows.
Shock vs chlorine: same chemical, different dose
"Shock" describes a dose, not a separate chemical. Shocking means raising free chlorine well above your normal level for long enough to clear algae or combined chlorine. Any of the forms above can do it. The bag labeled "shock" is usually cal-hypo or dichlor.
Shock when combined chlorine (CC = total chlorine − FC) is 0.5 ppm or more, when the water turns cloudy or green, when it smells strongly of "chlorine", after heavy use, and at opening and closing. How much to add, and when you can swim again, come from the product label. Use a product labeled for shocking pools. Bleach qualifies only if its label has pool directions. Most labels give an amount per 10,000 gallons, so scale it to your pool. For example, a label that calls for 1 lb of 65% cal-hypo per 10,000 gallons means 2 lb for 20,000 gallons. That 1 lb raises FC by about 7.8 ppm in 10,000 gallons. Don't swim until FC is back at or below the label's swim limit, often 4 ppm. If algae or cloudiness hasn't cleared after a few days of following the label, call a pool professional. Details: how to shock a pool.
Non-chlorine shock (MPS)
Potassium monopersulfate (MPS) is an oxidizer, not a sanitizer. It breaks down sweat and other organics, but it does not raise free chlorine and does not kill algae. PHTA and AQUA tech notes point out that leftover MPS reads as combined chlorine on DPD tests, so test before adding it or the next morning. Dose it by the label, scaled to your volume. It is useful in hot tubs and as a quick oxidizer after a party. It is not a fix for green water.
Which to use when
- CYA below 30 ppm: trichlor or dichlor is fine for a while. They build stabilizer as they chlorinate. Stop when you reach your target.
- CYA 30–50 ppm: switch daily chlorine to liquid chlorine or bleach. Keep tablets only for trips.
- CYA above 60 ppm: no more stabilized chlorine. Liquid or cal-hypo only, and plan a partial drain (how to lower CYA).
- Calcium already high or hard fill water: avoid cal-hypo; use liquid chlorine.
- Salt water pool: the cell makes chlorine from salt. Use liquid chlorine to top up or shock.
- Price: compare price per ppm, not per bucket. The chemical cost calculator turns your local prices into cost per ppm per 10,000 gallons.
For all target levels in one place, see the pool chemistry chart, and for doses by pool size, how much chlorine to add.
Safety
- Never mix chlorine products with each other or with anything else. Cal-hypo mixed with trichlor or dichlor, or with organic material, can react violently and start a fire.
- Never mix chlorine with acid: it releases toxic chlorine gas. Add each chemical separately to the pool with the pump running.
- Add chemical to water, never water to chemical. Use a clean, dedicated bucket for each product.
- Store products apart, dry, shaded, and out of reach of children. Wear gloves and eye protection.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
- If someone swallows a product or gets it in their eyes, call Poison Control at 1-800-222-1222.
More on the pool chemical safety page.
FAQ
How much does trichlor raise CYA?
About 0.6 ppm of CYA for every 1 ppm of free chlorine it adds (PHTA). A pool using 2 ppm of chlorine a day gains about 8.4 ppm of CYA a week on tablets alone, or roughly 36 ppm a month.
Is shock the same as chlorine?
Chlorine shock is ordinary chlorine at a higher dose. Most shock bags are cal-hypo or dichlor. Liquid chlorine works just as well. The exception is non-chlorine shock (MPS), which oxidizes but does not sanitize or raise free chlorine.
Is trichlor stabilized?
Yes. Trichlor contains cyanuric acid, so it stabilizes the water as it chlorinates. That's useful until CYA reaches your target (30–50 ppm for most chlorine pools) and a problem after that, because only draining lowers CYA.
Sources
- PHTA fact sheet, Dichlor (available chlorine, CYA per ppm): phta.org
- PHTA fact sheet, Sodium hypochlorite (trade percent, decomposition): phta.org
- PHTA / AQUA Tech Notes, Potassium monopersulfate: PDF
- Taylor Technologies, K-1005 treatment tables: PDF
- CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
- CDC, Pool chemical injuries 2008–2017: archive.cdc.gov