Low-calcium water is hungry water. If there isn't enough calcium dissolved in it, the water goes looking for some — and it pulls it out of your plaster, grout, and metal. Here's the ideal range and exactly how much calcium chloride it takes to get there.
Why low calcium hardness is corrosive
Calcium hardness (CH) is one of the inputs to the water's Langelier Saturation Index (LSI) — the single number that predicts whether water will etch or scale. Too little calcium drives LSI negative, and negative water is aggressive: it etches plaster, dissolves the cement in grout lines, and corrodes metal in the heater and equipment. Even vinyl and fiberglass pools want some calcium to protect their metal parts. Check your LSI →
The ideal range
For a plaster or gunite pool, aim for 200–400 ppm (many pros target ~300–350 for a comfortable LSI cushion). Vinyl and fiberglass pools have no plaster to protect, so 150–250 ppm is plenty — mostly to keep the water from attacking the heater and fittings.
The dose: how much calcium chloride
The rule the calculator uses: about 1.25 lb of calcium chloride per 10,000 gallons raises CH roughly 10 ppm. So lifting a 20,000-gallon plaster pool from 150 to 350 ppm (a 200-ppm bump) takes about 50 lb. Enter your surface type and volume for an exact figure. Calcium calculator →
How to add it — carefully
- Pre-dissolve it. Add the calcium chloride to a bucket of water (never water to the chemical), a bit at a time. The reaction is exothermic — the bucket will get hot, which is normal.
- Pour the solution in slowly near a running return, with the pump on, so it disperses instead of settling.
- Add in stages. Do about three-quarters of the dose, let it circulate a few hours, then re-test before topping up.
- Keep it off the plaster. Don't dump dry calcium chloride straight onto the pool floor — undissolved piles can stain or etch.
You can't easily lower it
There's no chemical that removes calcium — the only way down is dilution (partially drain and refill with softer water). That's exactly why you add it in stages and aim for the lower half of the range: overshooting means draining. If your fill water is already very hard, you may not need to add any. See the right order to balance →.