What calcium hardness does
Water that’s low in calcium pulls it from wherever it can, and in a plaster, pebble or tiled pool that means the surface and the grout. Water that’s high in calcium does the opposite and deposits white scale on tile, in salt cells and inside heaters. Calcium hardness (CH) is one of the levers that keeps the water in the balanced middle.
Plaster pools usually aim for 200 to 400 ppm. Vinyl and fiberglass surfaces don’t contain calcium, so they can run lower, around 150 to 250 ppm, though very soft water can still be hard on metal parts. The CDC’s Model Aquatic Health Code sets an upper limit of 2,500 ppm for public pools, which is far above where scale becomes a nuisance.
The formula
Calcium chloride (g) = CH rise (ppm) ÷ 100.09 × 110.98 × litres ÷ 1,000 ÷ purity
Calcium hardness is measured as calcium carbonate (100.09 g/mol). Each mole of calcium chloride (110.98 g/mol) adds one mole of calcium. Flake calcium chloride is about 77% pure (the rest is bound water); pellets are about 94%.So 1.2 lb of 77% flakes, or 16 oz of 94% pellets, raises 10,000 gallons by 10 ppm.
Worked example
15,000 gallons, CH 150 to 250 ppm
- Rise = 100 ppm
- 77% flakes = 100 ÷ 100.09 × 110.98 × 56,781 ÷ 1,000 ÷ 0.77 = 18 lb
- Or 94% pellets: 14.8 lb
Calcium is one part of the balance
Whether calcium causes problems depends on pH, alkalinity and temperature too. The Langelier Saturation Index pulls them together. Holding pH 7.5, alkalinity 90 ppm, CYA 40 and 82°F water, here is how calcium moves the index:
Saturation index as calcium hardness rises
Show the numbers
| CH (ppm) | LSI |
|---|---|
| 100 | -0.55 |
| 150 | -0.37 |
| 200 | -0.24 |
| 250 | -0.15 |
| 300 | -0.07 |
| 350 | 0 |
| 400 | 0.06 |
| 450 | 0.11 |
| 500 | 0.15 |
Check your own numbers with the LSI calculator.
How to add calcium chloride
- Fill a plastic bucket most of the way with pool water, then add the calcium chloride to the water. Never add water to the dry product.
- It gets hot as it dissolves. Stir with a wooden or plastic stick and let it dissolve fully.
- Pour it slowly around the deep end with the pump running.
- Add no more than about half the dose at once, then retest after a full turnover.
Lowering calcium
There’s no practical chemical that removes calcium from pool water. You replace some water with softer water. If your tap water itself has 150 ppm, going from 600 to 400 ppm means replacing 44% of the pool. The drain and refill calculator handles the fill-water correction. If your fill water is hard and you can’t dilute, run pH and alkalinity at the lower end of their ranges to keep scale away.
Frequently asked questions
How much calcium chloride raises hardness by 10 ppm?
1.2 lb of 77% flakes, or 16 oz of 94% pellets, per 10,000 gallons.
What should calcium hardness be?
About 200 to 400 ppm for plaster, pebble and tile pools, and around 150 to 250 ppm for vinyl and fiberglass. Follow your builder’s or manufacturer’s range if it differs.
Is calcium hardness increaser the same as calcium chloride?
Yes. Pool “hardness increaser” is calcium chloride, usually the 77% flake or the 94% pellet form.
What happens if calcium hardness is too low?
In plaster pools, low-calcium water can slowly etch the surface and grout. In any pool it makes the water more aggressive toward metal parts.
What happens if calcium hardness is too high?
White scale on tile at the water line, cloudy water when pH runs high, and crusted salt cells and heater exchangers.
Can I use calcium chloride from the hardware store?
Only if it’s pure calcium chloride. Ice-melt blends often contain other salts and additives. Check the label for the percentage of calcium chloride.
Sources
- Model Aquatic Health Code, 5th edition (2024). CDC.
- Understanding LSI: The Langelier Saturation Index. Orenda Technologies.
- Langelier Saturation Index (LSI). Mountain Empire Community College, water technology course notes.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.