Calcium Hardness
Calcium hardness measures the concentration of dissolved calcium ions in pool water, expressed in ppm.
Definition
Calcium hardness measures the concentration of dissolved calcium ions in pool water, expressed in ppm.
Typical Values: Plaster pools: 200–400 ppm; Vinyl/fibreglass: 150–250 ppm; Hot tubs: 150–250 ppm
In Plain Language
Calcium hardness determines whether water is aggressive (under-saturated, will dissolve calcium from surfaces) or scaling (over-saturated, will deposit calcium carbonate). Along with pH, temperature, and alkalinity, it determines the Langelier Saturation Index. Plaster pools require higher hardness (200–400 ppm) to protect surfaces. Vinyl and fibreglass tolerate lower levels (150–250 ppm).
Why It Matters
Low hardness corrodes plaster surfaces and equipment. High hardness deposits calcium scale on heaters, pipes, and tile.
Typical Values
Plaster pools: 200–400 ppm; Vinyl/fibreglass: 150–250 ppm; Hot tubs: 150–250 ppm
Last reviewed: 2026-06-01