The factors come from straight molar chemistry — calcium hardness is measured "as calcium carbonate," and each product carries a different amount of usable calcium per pound. Check your bag: "calcium chloride dihydrate" or 77% means the first factor; "anhydrous" or 94%+ the second.
A worked example
A 15,000-gallon plaster pool at 180 ppm targeting 300: a 120 ppm rise needs 1.23 × 1.5 × 12 ≈ 22 lbs of standard flake — three 10-lb buckets with some left over. Add a third at a time across a day, retesting between rounds.
Why plaster pools care so much
Water wants calcium; if the water is soft, it takes calcium from the most available source — a plaster, tile-grout, or pebble surface — slowly etching it. Vinyl and fiberglass have nothing to give up, which is why their range is relaxed. If you also run a heater, moderate calcium protects its heat exchanger regardless of surface type.
Frequently asked questions
What should calcium hardness be in a pool?
250–450 ppm for plaster, tile, and pebble pools, where the water will dissolve calcium out of the surface if it can't get it elsewhere. Vinyl and fiberglass pools are far more forgiving — 150–250 ppm is fine there, and low calcium mostly matters for foaming and heater longevity.
How much calcium chloride do I add?
About 1.2 lbs of standard flake calcium chloride (77%, the common 'calcium increaser') raises calcium hardness by 10 ppm in 10,000 gallons. The stronger anhydrous form (94%+) needs about 0.9 lbs for the same rise.
How do I lower calcium hardness?
Only by replacing water with lower-calcium water — nothing removes it chemically at pool scale. That's why it pays to add in stages: overshooting calcium means a partial drain, just like CYA.
Is it true calcium chloride gets hot when added to water?
Yes — dissolving it is strongly exothermic. Add it by broadcasting small amounts directly over the deep end with the pump running, never pre-dissolved in a bucket in concentration, and never mixed with any other chemical.