Cal Hypo Explained: The Good, The Bad & The Calcium

Calcium hypochlorite, known as cal hypo, is a staple sanitizer for many pool professionals because it delivers strong oxidation in a compact, shelf-stable form. Its percentage strength varies by source: pros often buy 73 to 78 percent, while big-box stores carry around 65 percent. Strength by weight matters when you compare products. A practical field rule equates roughly one pound of 73 percent cal hypo to one gallon of 12.5 percent liquid chlorine, even though the true math says about 22 ounces is closer. That near-parity makes cal hypo useful for routine shocking and midweek boosts, especially when you want portability and fewer heavy jugs. Still, it’s not just about free chlorine output; you need to think through side effects, regional water conditions, and safety.

Safety sits at the center of using cal hypo well. Never mix it with trichlor or dichlor, which contain cyanuric acid; the reaction can be violently exothermic and explosive in feeders, skimmers, or buckets. Avoid putting cal hypo tablets into trichlor feeders, even brand new ones, because their design and residue risks are incompatible. Application method matters by surface too. Vinyl liners can bleach almost instantly if granules touch, and some colored fiberglass finishes can spot. On plaster and pebble, broadcasting is common, but be aware of metals. Cal hypo is a strong oxidizer that can darken copper to a scary black stain on contact; while temporary, those stains spook homeowners and create callbacks you do not want.

Every sanitizer leaves something behind. Liquid chlorine adds salt, trichlor and dichlor add cyanuric acid, and cal hypo adds calcium. That’s good or bad depending on your zip code. In regions like Southern California, where calcium hardness often runs 400 to 500 ppm, use cal hypo sparingly to limit scaling pressure. In parts of Florida or Texas with 150 to 250 ppm hardness, cal hypo’s calcium gain is manageable, and partial drains can correct drift. Expect roughly 7 ppm calcium added for every 10 ppm of free chlorine in 10,000 gallons. Also note cal hypo’s high pH near 11 to 12. Light doses raise pool pH slightly and temporarily; heavy shocking may need acid balancing. Planning dosing within a full water balance strategy keeps LSI in check and surfaces protected.

Because cal hypo is unstabilized, sunlight will strip it fast without cyanuric acid. If you run cal hypo as your primary oxidizer, set CYA around 50 to 70 ppm to slow UV burn-off while keeping chlorine efficient. Treat it like liquid chlorine in schedule and expectations: add more often during heat, bather load spikes, and strong sun. Cal hypo tablets exist, but they are not trichlor equivalents. Field experience shows you may need five or six cal hypo tabs to match the output of three trichlor tabs in a 25,000-gallon pool, and they demand higher flow. In hot markets like Arizona or Las Vegas, skimmer-placed cal hypo tabs may dissolve in two days when water hits the high 80s or 90s, undermining weekly plans. Dedicated cal hypo feeders with strong flow improve consistency, but many residential systems still rely on skimmer flow, so watch temperature and dissolution rate closely.

Where cal hypo shines is targeted problem-solving. It’s excellent for shocking without adding CYA, and the granular form can knock out stubborn algae embedded in pebble finishes or stair corners that resist brushing. Carefully placing broken cal hypo tablets on step edges can clear green tufts overnight on plaster and pebble, though avoid vinyl and be cautious with colored fiberglass. For service routes, one-pound bags are convenient to haul and leave at accounts that need frequent boosts, with 25 to 50 pound buckets staged by equipment for homeowner top-ups under guidance. Done right, cal hypo offers strong, fast oxidation, flexible logistics, and reliable algae control, provided you respect its interactions, protect it with the right CYA level, and apply it with surface safety and metals in mind.