Pool calcium hardness calculator
Raise calcium with the dose for the bag you actually own, or find out how much water to replace to bring it down. It counts your tap water's hardness, which decides whether draining can work at all.
Example numbers are filled in so you can see an answer. Type your own over them.
Don't know your gallons? Work it out here →
Exact geometry at 7.48 gal/ft³. Kidney shapes, diving hoppers and salt/turnover figures live in the full Pool Volume Calculator.
Calcium chloride per 10 ppm
| Bag | 10,000 gal (37,854 L) | 15,000 gal (56,781 L) | 20,000 gal (75,708 L) | 30,000 gal (113,562 L) |
|---|---|---|---|---|
| Flake, ~77% | 1.2 lb (544 g) | 1.8 lb (816 g) | 2.4 lb (1.1 kg) | 3.6 lb (1.6 kg) |
| 83–87% | 1.1 lb (499 g) | 1.6 lb (726 g) | 2.2 lb (998 g) | 3.3 lb (1.5 kg) |
| Anhydrous, 94–97% | 15.8 oz (448 g) | 1.5 lb (680 g) | 2.0 lb (907 g) | 3.0 lb (1.4 kg) |
How much to drain to reach 350 ppm
| Calcium now | Fill water 0 ppm | Fill water 100 ppm | Fill water 200 ppm |
|---|---|---|---|
| 500 ppm | 30% | 38% | 50% |
| 600 ppm | 42% | 50% | 63% |
| 800 ppm | 56% | 64% | 75% |
The bag's strength matters
Calcium chloride is sold as flake (about 77%, the dihydrate form), as 83–87% pellets, and as near-anhydrous 94–97% prills. The same rise takes about 18% less of the strong stuff. A calculator that assumes one form over-doses the other, and there is no easy way back: calcium leaves only when water does. The strength is printed on the bag.
Adding it
- Dissolve it in a bucket of pool water first. It gets hot, so add the calcium to the water, not the other way round.
- Pour slowly around the edge with the pump running. Split a big dose into two or three parts a few hours apart.
- Get pH and alkalinity right first, then calcium, then check the water balance (LSI).
Dose: stoichiometric (110.98 g CaCl2 per 100.09 g CaCO3 equivalent) divided by the stated strength, which also matches the ~1 lb per 10 ppm per 10,000 gal printed on a near-anhydrous bag. Drain: dilution arithmetic including fill-water hardness.
FAQ
How much calcium chloride do I add to raise calcium hardness?
In 10,000 gallons, about 1.2 lb (544 g) of 77% flake raises calcium hardness 10 ppm, or about 15.8 oz (448 g) of a 94–97% anhydrous bag. Pick the strength printed on your bag in the calculator: the difference is about 18%, and calcium only comes back down by draining.
What should calcium hardness be?
It depends on the surface. Plaster: 300–400 ppm, or 400 and a little over if the pool freezes. Vinyl liner: 200–250 ppm, which sits inside the ranges Latham, Loop-Loc and GLI print for their liners. Fiberglass and pebble finishes: use the number your shell or finish maker prints, because their warranties are written against it and the makers disagree (fiberglass runs from Leisure Pools' 80–120 to Latham's 250–350; Pebble Tec prints 200–300). A salt pool uses the same range as a chlorine pool.
How do I lower calcium hardness?
Only by replacing water: nothing you pour in removes calcium. The calculator gives the percentage to drain, counting the calcium already in your fill water. If your tap water is at or above your target, draining cannot get there; a reverse-osmosis service can.
Why is my calcium going up?
Evaporation leaves calcium behind, hard fill water adds more, and cal-hypo chlorine adds about 0.8 ppm of calcium for every 1 ppm of chlorine. Switching to liquid chlorine stops the last one.
Is high calcium bad for a salt pool?
It raises the risk of scale on the cell plates, where the water is hottest and pH is highest. Keep the whole water balance in range, not just calcium: the LSI calculator shows whether the combination is scaling.