Calcium hardness calculator
Calcium hardness (CH) protects plaster and tile grout from being slowly dissolved by the water. Enter your readings to see how much calcium chloride to add, or how much water to replace if hardness is too high.
What level to aim for
The right level depends on the pool's surface:
| Pool surface | Common target range | Why |
|---|---|---|
| Plaster, pebble, quartz | 250–450 ppm | Soft water slowly dissolves calcium out of the cement, roughening and etching the surface. |
| Vinyl liner, fiberglass, painted | 150–250 ppm | No cement to protect, but very soft water can still be aggressive to metal parts and heaters. |
Calcium hardness is one part of the water's overall balance, together with pH, alkalinity and temperature. The LSI calculator combines them to show whether the water tends to dissolve or deposit calcium. Very hard water, above about 500 ppm, makes scale more likely, especially on heaters and salt cells.
The formula
Calcium hardness is reported as calcium carbonate (100.09 g per mole). Each mole of calcium needs one mole of calcium chloride (110.98 g). Flake calcium chloride is a hydrate that's about 77% calcium chloride by weight, while anhydrous pellets are about 94% or more. Check the purity on your product's label.
| Raise 10,000 gallons by 10 ppm | Amount |
|---|---|
| Flake calcium chloride (77%) | 1.2 lb |
| Anhydrous calcium chloride (94%) | 15.8 oz |
Worked example
A 15,000-gallon plaster pool reads 150 ppm, and you want 250 ppm. 100 mg per liter × 56,781 liters ≈ 5,678 g of hardness as calcium carbonate, which is 5,678 ÷ 100.09 ≈ 56.7 moles of calcium. That takes 56.7 × 110.98 ≈ 6,296 g of pure calcium chloride. As 77% flake, that's 6,296 ÷ 0.77 ≈ 8,177 g, about 18 lb.
How to add calcium chloride
- Fill a clean plastic bucket about three-quarters with pool water.
- Slowly add the calcium chloride to the water, a few pounds at a time, and stir. It gives off a lot of heat as it dissolves: never add water to the dry chemical, and use a bucket that can take the heat.
- Pour it around the pool with the pump running. Split large doses over a day or two.
- Retest after 24 hours.
Lowering calcium hardness
There's no practical home chemical that removes calcium. The usual fix is replacing some of the water with softer fill water. The share to replace is (current − target) ÷ (current − fill water hardness). For example, going from 600 to 400 ppm with 100 ppm fill water means replacing (600 − 400) ÷ (600 − 100) = 40% of the water. Enter your fill water's hardness above, or use the drain and refill calculator. Some pool stores offer reverse-osmosis water treatment as an alternative.
Frequently asked questions
What's the difference between calcium hardness and total hardness?
Total hardness includes magnesium as well as calcium. Pool tests measure calcium hardness, because calcium is what protects plaster and forms scale. Use a test that reports calcium hardness specifically.
Does cal-hypo shock raise calcium hardness?
Yes, by about 0.7 ppm for every 1 ppm of chlorine it adds. Pools shocked often with cal-hypo can see hardness creep up over a season. The chlorine calculator shows the increase.
Why is there white scale on my tile line?
Scale forms when water is oversaturated with calcium: high pH, high alkalinity, high hardness or warm water, or a mix of these. Evaporation at the waterline concentrates it. Check the LSI and bring pH down first.