How much calcium chloride to raise hardness?
Doses below are for the common "77%" flake (calcium chloride dihydrate). The anhydrous form is more concentrated — about a quarter less product for the same raise; the calculator handles both:
| Raise CH by | 10,000 gal | 15,000 gal | 20,000 gal |
|---|---|---|---|
| 25 ppm | 3.1 lb | 4.6 lb | 6.1 lb |
| 50 ppm | 6.1 lb | 9.2 lb | 12.3 lb |
| 100 ppm | 12.3 lb | 18.4 lb | 24.5 lb |
Values from the calculator above. Raise CH in steps of ~50 ppm, retesting between doses — overshooting has no chemical undo.
Dissolving calcium chloride releases serious heat: it's the active ingredient in ice-melt and hand warmers. Never pre-dissolve it in a bucket the way you would baking soda — a concentrated bucket can reach near-boiling and spatter. Broadcast the dry granules across the deep end with the pump running, brush anything that settles, and it'll be dissolved within the hour, warming the pool an unmeasurable amount.
What should calcium hardness be?
| Surface | CH range | Why |
|---|---|---|
| Plaster / pebble / tile | 250–450 ppm | The surface is calcium-based — undersaturated water dissolves it (etching), oversaturated water deposits on it (scale) |
| Fiberglass | 220–320 ppm | The gelcoat doesn't feed the water calcium, but balanced water protects it and the equipment |
| Vinyl liner | Low CH is fine | The liner doesn't care — keep some hardness (~150+) mainly for the heater's copper exchanger |
The number that actually decides whether your CH is a problem isn't CH alone — it's the saturation index, which weighs CH together with pH, alkalinity and temperature. A plaster pool at CH 500 with low pH and TA can still be etching; one at CH 250 with high pH can still scale. Check the index before treating any CH reading as an emergency.
How to lower calcium hardness
Nothing removes calcium from solution at pool scale — "calcium reducers" only sequester it (it's still there, just not scaling for a while, with weekly maintenance doses forever). Lowering CH means replacing water, and here's the wrinkle this calculator models that most skip: your fill water has calcium too. Replacing half the water doesn't halve your CH — it moves you halfway toward your fill water's CH. Enter your fill water's reading above (test your tap the same way you test the pool) and the drain percentage accounts for it — including telling you when your target simply isn't reachable by dilution.
In genuinely hard-water regions — much of Arizona, Texas, and anywhere on a well — fill water can run 300+ ppm, which makes dilution a treadmill. The realistic options there: accept a higher CH and manage the saturation index downward with pH and TA, or hire a mobile reverse osmosis service, which filters the pool's own water in place without a drain. RO isn't cheap, but against a full drain-and-refill (water cost, chemical restart, and drain risks to plaster and liners in high water tables) it's often closer than you'd think.
If your CH climbs every season, the usual suspects are evaporation (water leaves, calcium stays — every top-up adds its own) and cal-hypo shock: every 10 ppm of FC from cal-hypo adds about 7 ppm of CH permanently. On hard fill water, that's a good argument for liquid chlorine as your shock of choice — it adds nothing that accumulates.