To raise calcium hardness from 150 ppm to 250 ppm in a 15,000 gallon pool, add 18 lb of 77% calcium chloride, or 14.8 lb of 94%. To lower it, you cannot add a chemical: you replace water. Going from 600 ppm to 300 ppm means replacing 50% of the pool with fill water near 0 ppm, which is 7,500 gallons, or 60% (9,000 gallons) if your tap water has 100 ppm of calcium.
How the dose is calculated
Calcium hardness is measured as calcium carbonate. Pure calcium chloride is 110.98 g per mole, so each ppm in 1 cubic metre takes 1.109 g of 100% product, and more of the weaker grades.
Calcium chloride (lb) = gallons ÷ 10,000 × (target − current) ÷ 10 × 1.20 lb (77%)
That is the NSPF table figure: 1.2 lb of 77% raises hardness 10 ppm in 10,000 gal (0.9 lb of pure product). Yakima County lists 1.25 lb and Swim University 1 to 1.25 lb. For 15,000 gal and a 100 ppm rise: 1.5 × 10 × 1.20 = 18 lb. One pound of 77% adds about 5.5 ppm to 15,000 gal.
Target calcium hardness by surface
| Surface | We use | Where it comes from |
|---|---|---|
| Plaster, concrete, pebble | 200 to 400 ppm | PHTA ideal, in the Hayward, Pentair and CircuPool manuals; Swim University says 200 to 275 |
| Vinyl liner | 150 to 250 ppm | Latham; Orenda says vinyl makers quote it, and argues 150 is too low in most of the US |
| Fiberglass | 150 to 200 ppm | Orenda: makers often say under 200; Thursday Pools and River Pools say under 120; Swim University 175 to 225 |
These disagree, and they should. Plaster is calcium based, so soft water pulls calcium out of the surface and grout. Vinyl and fiberglass do not dissolve, but very low calcium still lets water etch grout, tile and heater parts, while high calcium with a high pH scales on them. For fiberglass we follow the warranty: if your maker says under 120 ppm, type 120 as the target, and do not use calcium hypochlorite or calcium increasers. The tool fills the middle of the range and lets you change it. The engine's floor is 150 ppm and the CDC's ceiling is 2,500 ppm.
Raising calcium
Add calcium chloride pre-dissolved in a bucket, because it releases heat. Pour it around the deep end with the pump running, keep it running for at least 8 hours and retest the next day. Adding more than about 100 ppm at once can cloud the water, so split large rises. Calcium hypochlorite shock also adds calcium, about 0.8 ppm per ppm of chlorine (PHTA), so a season of shocking may raise it enough on its own. Check saturation in the LSI calculator afterwards: for the same pH and alkalinity, calcium of 200 ppm reads about 0.18 lower than 300 ppm.
Lowering calcium
Drain and refill. Hard tap water refills add calcium back, so enter your fill-water reading; if it is at or above your target, draining cannot work and the calculator says so. A hose filter for hardness helps. Otherwise a stain and scale product keeps calcium dissolved while rain and top-ups dilute it. Never empty a plaster pool. Recheck alkalinity and pH when the pool is full, since a fresh fill changes both.
Salt pools and calcium
Salt cells scale when calcium and pH are high, and CircuPool's manual says not to use calcium chloride as the salt. Keep hardness in your surface's range, and add salt with the pool salt calculator. For a full plan from every test result use the pool chemical calculator, and find gallons in the pool volume calculator. New plaster? Resurfacing costs are in the pool resurfacing cost calculator. Everything is listed in pool chemistry.