Pool Calcium Hardness Calculator (Calcium Chloride to Raise, Water to Drain to Lower)
How much calcium chloride raises your pool's calcium hardness, or how much water to replace to lower it, based on your pool size and fill-water hardness.
Formula
Raise: calcium chloride (lb) = (target − current) ÷ 10 × gallons ÷ 10,000 × 1.2 for 77% (0.9 for 100%). Lower (dilution): share to drain = (current − target) ÷ (current − fill water); gallons = share × pool volume.
Worked example
Inputs: Pool volume = 15000 gal; Current calcium hardness (test result) = 150 ppm; Target calcium hardness (from your pool builder or test kit guide) = 250 ppm; Calcium chloride strength (on the label) = 77; Calcium hardness of your fill water = 50 ppm.
| Change ppm | 100 |
|---|---|
| Calcium chloride lb | 18 |
| Drain pct | — |
| Gallons to drain | — |
Dissolve it in water before adding it to the pool, add large amounts in thirds, then circulate and retest. Follow the product label.
FAQ
How much calcium chloride raises hardness 10 ppm?
1.2 lb of 77% calcium chloride, or 0.9 lb of 100%, per 10,000 gallons. For a 15,000 gallon pool going from 150 to 250 ppm: 100 ÷ 10 × 1.5 × 1.2 = 18 lb of 77%.
How do I lower calcium hardness?
By replacing some of the water with softer water; no chemical removes it. With 100 ppm fill water, going from 600 to 400 ppm means replacing (600 − 400) ÷ (600 − 100) = 40% of the water.
What if my tap water is hard?
If your fill water is at or above the target, draining cannot get you there. Test your tap water first; the calculator warns you when that is the case.
In what order should I adjust chemicals?
The Indiana guide's order is free chlorine, total alkalinity, pH, cyanuric acid (outdoor pools), then hardness.
How do I add it?
Dissolve it in water first, add large amounts in thirds over a couple of hours with the pump running, keep swimmers out, then retest. Follow the label.
Sources
Related
- Pool Alkalinity Calculator (Baking Soda to Raise, Muriatic Acid to Lower)
- Pool Volume Calculator: Rectangular (Gallons)
More Pool calculators
- Pool Volume Calculator: Round / Above-Ground (Gallons)
- Pool Volume Calculator: Oval (Gallons)
- Pool Salt Calculator (Pounds of Salt for a Saltwater Pool)
- Pool Liquid Chlorine Calculator (Ounces to Raise Free Chlorine)
- Pool Stabilizer (Cyanuric Acid) Calculator: How Much to Add or Drain
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Last reviewed 2026-10-20.