Cal Hypo Calculator: How Much to Add

Cal hypo (calcium hypochlorite) is a dry chlorine sold as granules and tablets. It adds no stabilizer (cyanuric acid, CYA), which makes it useful when CYA is already high. Pick the strength that matches the available chlorine line on your label. Enter your gallons, your free chlorine (FC) now, and your goal. The calculator gives ounces and grams. It covers everyday doses; for a shock, use the amount on your product's label.

How much cal hypo per ppm

The dose depends on three things: your gallons, how many ppm you want to add, and the product's available chlorine. These figures come from the calculator's engine for 10,000 gallons. For another size, scale in proportion or use the calculator above.

Use a kitchen or postal scale. Granules vary in size, so a cup of one product doesn't weigh the same as a cup of another. 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 your label's directions and the calculator differ, the label wins.

Keep everyday doses within your target range. With the default stabilizer of 30 ppm, the calculator shows a target of 2.5–4.5 ppm and a minimum of 2.5 ppm. It warns you if your goal is above 4 ppm, the swim limit on many chlorine labels. The target for your own CYA is on pool chlorine levels.

65%, 68%, 73%, 78%: read the available chlorine line

Cal hypo labels list two numbers, and they are easy to mix up:

• Calcium hypochlorite %, the active ingredient, listed in the ingredient statement.
• Available chlorine %, which is what raises your FC reading and what the dose depends on.

The second number is a few points lower. EPA-accepted labels we checked show calcium hypochlorite 68% with at least 65% available chlorine, and calcium hypochlorite 78% with at least 75% available chlorine. For products sold as "73%", the calculator assumes about 70% available chlorine. So:

• If your label says calcium hypochlorite 68% and available chlorine 65%, choose 65% in the calculator.
• If your label says 73% calcium hypochlorite, choose 73%; the calculator uses about 70%. If the label gives the exact available chlorine, type it in the "Strength on your label" field under More options.
• If your label says 78% with 75% available chlorine, choose 78%.
• If your label lists a different available chlorine number, type it in the "Strength on your label" field. The general chlorine dose calculator covers liquid chlorine, bleach, and other forms.

Some shock bags blend cal hypo with other ingredients and have much lower available chlorine. One EPA label we found lists 45%. Always read the number on your own bag.

"65% vs 73%" is about strength, not quality. A stronger product needs fewer ounces for the same rise in FC; the calcium added per ppm of chlorine is the same.

What cal hypo does to calcium and pH

Calcium. Every 1 ppm of chlorine from cal hypo adds about 0.7 ppm of calcium hardness (CH). This follows from the chemistry, so it holds for every strength. In a pool with low CH, that is a bonus. In a pool with hard fill water or a salt cell, it adds up: 2 ppm of chlorine a day for 100 days adds about 141 ppm of CH. Calcium leaves only with water you drain. Test CH monthly if cal hypo is your main chlorine, and see calcium hardness for targets.

pH. Dissolved cal hypo is strongly alkaline (product fact sheets list a pH of roughly 10–12 for a solution; we could not confirm one figure for all brands), so pH often rises after you add it. Check pH the next day and bring it back into range with the pH calculator. High pH plus high calcium makes cloudy water and scale more likely. You can check that risk with the LSI calculator.

Stabilizer. None. Cal hypo doesn't raise CYA, so it can't cause the "chlorine lock" that comes from too much stabilizer. See chlorine lock.

How to add it: follow your label's directions

Cal hypo labels don't all say the same thing about dissolving, so read yours before you start. Here is what labels we checked include:

• Pre-dissolve in a bucket. Some labels say to dissolve the product first in a clean plastic bucket of pool water, stirring with a clean plastic or wooden stick, then pour the liquid around the pool. They warn that direct contact with colored plaster, vinyl, or fiberglass can damage the surface.
• Product into water, never water into product. The labels we checked state this plainly.
• Broadcast or skimmer methods. Other labels allow spreading granules over deep water or adding through the skimmer with the pump running, with notes to avoid bleaching vinyl. Use these methods only if your label lists them.
• Leftover granules. Labels that allow broadcasting say to brush settled granules so they dissolve instead of sitting on the floor.

Use a clean bucket that is only for cal hypo, never one that has held acid, other chlorine products, or anything else.

Cal hypo in a salt water pool

You can raise chlorine in a salt pool with any chlorine product labeled for pool use, including cal hypo. The concern is calcium. Hayward's AquaRite manual, for example, lists calcium at 200–400 ppm, warns that water balanced toward scaling leaves scale, and tells owners to inspect the cell for scale. Because cal hypo adds calcium with every dose, many salt pool owners use liquid chlorine for top-ups instead. If you use cal hypo, test CH and pH more often and inspect the cell. For running a salt pool, see salt water pool care.

Shock doses come from the label

Many shock bags are cal hypo. This calculator covers everyday doses only. For a shock, use the amount on your product's label, scaled to your gallons; labels usually give an amount per 10,000 gallons. The label also tells you when the pool can be used again. Don't swim until FC is back at or below the swim limit on your label. Shock steps and timing are on how to shock a pool, and all chlorine forms are compared on types of pool chlorine.

FAQ

Is 73% cal hypo stronger than 65%?

Yes. A 73% product has about 70% available chlorine, so you need fewer ounces for the same rise in FC. In 10,000 gallons, about 1.91 oz raises FC 1 ppm, compared with 2.05 oz of a 65% product.

How much calcium does cal hypo add?

About 0.7 ppm of calcium hardness for every 1 ppm of chlorine, whatever the strength. Over a season, that can add more than 100 ppm, so test calcium monthly.

Do you have to pre-dissolve cal hypo?

It depends on the label. Some labels require dissolving it in a bucket of water first to protect the pool surface; others allow broadcasting or skimmer addition. Follow yours, and always add product to water, never water to product.

Can you use cal hypo in a salt water pool?

Yes, if its label allows pool use. But it adds calcium, and high calcium raises the risk of scale on the salt cell. Many salt pool owners use liquid chlorine instead. If you use cal hypo, test calcium hardness often.

Sources

• U.S. EPA, HTH 300 gram tablets master label (calcium hypochlorite 68%, minimum available chlorine 65%): EPA label (PDF)
• U.S. EPA, HTH 75 superchlorinator shock master label (calcium hypochlorite 78%, minimum available chlorine 75%; broadcast and skimmer methods): EPA label (PDF)
• U.S. EPA, cal hypo pool label (pre-dissolve in a plastic bucket; surface damage from direct contact; product into water): EPA label (PDF)
• U.S. EPA, HTH pool shock master label (blended product, minimum available chlorine 45%): EPA label (PDF)
• OSHA, Hazard information bulletin: calcium hypochlorite (decomposition when not kept dry; fire or explosion when stored with organic or flammable materials): osha.gov
• Hayward, AquaRite manual (calcium 200–400 ppm, scaling): PDF
• Doses and calcium added per ppm: our engine (calcium from reaction stoichiometry); see methodology.