Pool Calcium Hardness Calculator
Only used when lowering CH.
Calcium chloride to add
18 lb
Add 18 lb of calcium chloride (77%) to raise calcium hardness in 15,000 gallons from 150 to 250 ppm.
- CH increase
- 100 ppm
- Rate
- 1.2 lb per 10,000 gal per 10 ppm
Calcium chloride gets very hot as it dissolves. Pre-dissolve it in a bucket of pool water (add the chemical to the water) and pour it in slowly.
- Never mix pool chemicals with each other, wet or dry.
- Always add chemicals to water, never water to chemicals.
- Follow the manufacturer's label. It overrides any calculator, including this one.
How it's calculated
Calcium hardness is reported in ppm "as calcium carbonate" (CaCO₃, 100.09 g/mol). Calcium chloride (CaCl₂, 110.98 g/mol) delivers one calcium ion per molecule, so each ppm of CH needs 1.109 mg of pure CaCl₂ per liter.
- Purity: flakes are usually 77–80% calcium chloride (the rest is water of hydration); anhydrous pellets are 94–97%. Calcium chloride is sold as dihydrate flakes (about 77–80% CaCl2) or anhydrous pellets (about 94–97%).
- Lowering is a dilution: calcium does not leave the water except with water that leaves the pool.
Worked example
A 15,000 gallon plaster pool at 150 ppm CH, target 250 ppm, using 77% flakes:
- Increase: 100 ppm × 56,781 L = 5,678,118 mg of CaCO₃ equivalent.
- × 1.109 = 6,296,098 mg of pure calcium chloride.
- ÷ 0.77 = 8,176,750 mg = 18 lb of flakes (8.2 kg).
Lowering instead: a 20,000 gallon pool at 600 ppm with tap water at 100 ppm needs (600 − 400) ÷ (600 − 100) =40% replaced, or 8,000 gallons, to reach 400 ppm.
Adding calcium chloride safely
Dissolving calcium chloride releases a lot of heat. Fill a plastic bucket with pool water, add the flakes slowly to the water (never the other way around), stir, and pour it around the deep end with the pump running. Add big doses in parts so the water does not cloud.
Calcium chloride per 10 ppm of CH
| Pool size | 77% flakes | 94% anhydrous |
|---|---|---|
| 10,000 gal | 1.2 lb | 0.98 lb |
| 15,000 gal | 1.8 lb | 1.48 lb |
| 20,000 gal | 2.4 lb | 1.97 lb |
| 25,000 gal | 3 lb | 2.46 lb |
| 30,000 gal | 3.61 lb | 2.95 lb |
Frequently asked questions
How much calcium chloride do I need to raise calcium hardness?
About 1.2 lb of 77% calcium chloride flakes raises CH by 10 ppm in 10,000 gallons (0.98 lb of 94% anhydrous). Raising a 20,000 gallon pool from 150 to 250 ppm takes about 24 lb of flakes.
What is the ideal calcium hardness for a pool?
Generally 150–400 ppm. Plaster and pebble pools do best at 250–350 ppm plaster, because water low in calcium pulls it out of the surface. Vinyl and fiberglass pools are fine lower, around 150–250 ppm vinyl/fiberglass.
How do I lower calcium hardness in a pool?
There is no practical chemical to remove calcium. Replace part of the water with fill water that has less calcium; the calculator works out how much. Until then, keeping pH and alkalinity at the low end of their ranges limits scaling.
What happens if calcium hardness is too low?
Soft water is aggressive. In plaster pools it etches the surface and dissolves grout; in any pool it can corrode heaters and metal fittings. Use the LSI calculator to see how corrosive the water is overall.
Can I use calcium chloride from the hardware store?
Only if it is plain calcium chloride with no additives. Some de-icers are mixtures with magnesium chloride or other salts. Check the label for the calcium chloride percentage and enter it as the purity.
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