How much calcium chloride to add to raise your pool’s calcium hardness into range — so the water isn’t hungry enough to etch plaster.
Most pools aim for 200–400 ppm (plaster pools 250–350). Calcium only comes down by draining and refilling, so sneak up on your target.
Add this much calcium chloride flake
37 lb (≈ 588 oz)
to raise calcium hardness from 150 to 300 ppm in 20,000 gal
raise CH by (300 − 150) ppm in 20,000 gal → 37 lb (≈ 588 oz) of calcium chloride flake
Each 10 ppm of calcium hardness per 10,000 gal needs ≈ 1.25 lb of dihydrate flake (≈ 0.9 lb anhydrous). Linear in ppm and volume.
Pre-dissolve calcium chloride in a bucket of pool water (add the calcium to the water) — it gets hot and can cloud the water or harm a vinyl liner if dumped in dry.
Pour the dissolved solution slowly around the pool with the pump running. Calcium can’t be removed by chemicals — only dilution lowers it — so add in stages and don’t overshoot.
That’s a big jump — add it over two or three days, running the pump and re-testing between, to avoid clouding the water.
Calcium hardness is how much dissolved calcium your water carries. Water that’s too soft is hungry — it pulls calcium out of plaster, grout, and concrete to satisfy itself, etching and pitting the surfaces over time. Water that’s too hard does the opposite, dropping calcium out as scale and cloudiness. The fix for low hardness is calcium chloride, sold as flake (dihydrate) or pure anhydrous pellets.
Calcium hardness is also one leg of your water balance (the Langelier index). Raising it sits alongside pH and alkalinity in keeping water neither corrosive nor scaling — which is why plaster pools in particular want it held in the 250–350 ppm band. Remember it only comes back down by draining and diluting, so it’s better to nudge up to target than to blow past it.
A 20,000-gallon plaster pool at 150 ppm, targeting 300 ppm:
With calcium chloride flake (~1.25 lb per 10 ppm per 10,000 gallons), a 150 ppm rise across 20,000 gallons works out to about 37 lb — or roughly 28 lb of anhydrous calcium chloride for the same result. Pre-dissolve it in buckets and add it over a couple of days so the water doesn’t cloud, re-testing as you go.
About 1.25 lb of calcium chloride flake (dihydrate) per 10,000 gallons raises calcium hardness by roughly 10 ppm; the pure anhydrous form takes about 0.9 lb for the same rise. So a 20,000-gallon pool going from 150 to 300 ppm (a 150 ppm bump) needs roughly 37 lb of flake. Enter your levels above for the exact amount.
Most pools run best with calcium hardness between 200 and 400 ppm — plaster and pebble pools usually want 250–350 ppm so the water isn’t hungry for calcium and won’t etch the surface. Vinyl and fiberglass pools can sit at the lower end. Too low and the water leaches calcium from plaster and grout; too high and you get scale and cloudiness.
No chemical removes calcium — you lower hardness by dilution: drain part of the pool and refill with water that’s lower in calcium (or use a hose filter / soft fill water). If your fill water is itself very hard, partial drains only help so much, and a reverse-osmosis mobile service is the other option. Prevent the climb by keeping pH and alkalinity in check so calcium stays dissolved.
Flake is calcium chloride dihydrate (about 77% calcium chloride by weight) — the common white “calcium hardness increaser.” Anhydrous calcium chloride is the pure 100% form (often sold as ice-melt pellets), so you need less of it for the same result. Both raise hardness identically per unit of calcium; just pick the right product in the calculator so the amount matches what you’re scooping.
It can if you dump it in dry — calcium chloride releases a lot of heat as it dissolves and can cloud the water or even damage a vinyl liner. Pre-dissolve it in a bucket of pool water (add the calcium to the water, not the reverse), then pour the solution slowly around the pool with the pump running. The cloudiness from a proper dose usually clears within a day.
Yes. “Calcium hardness increaser,” “hardness up,” and “calcium plus” are calcium chloride — usually the dihydrate flake. Plain calcium chloride sold for ice melt or dust control is the same chemical and often much cheaper, as long as it’s pure calcium chloride with no additives or anti-caking agents.
Once it’s pre-dissolved and circulated — generally about 30 minutes to an hour with the pump running — it’s fine to swim. If the water clouded from the addition, wait until it clears and your levels test in range before getting in.
It’s linear in the calcium you’re adding. The tool converts your desired ppm increase and pool volume into the moles of calcium needed, then into pounds of the product you selected (dihydrate flake or anhydrous), since the two have different weights per unit of calcium.
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