Calcium hardness (CH) measures the amount of dissolved calcium in pool water. It plays a key role in the Langelier Saturation Index and directly affects the condition of pool surfaces and equipment.
Key Facts
- Target calcium hardness: 200–400 ppm for plaster and concrete pools; 150–250 ppm for vinyl and fibreglass.
- Low hardness makes water aggressive — it dissolves calcium from plaster and corrodes metal equipment.
- High hardness promotes calcium carbonate scaling on heater elements, pipes, and tile lines.
- Calcium hardness can only be lowered by partially draining and refilling with fresh water.
What Hardness Does
Water has a natural tendency to be in equilibrium with calcium carbonate. Water that contains less calcium than its equilibrium level is under-saturated — it will dissolve calcium from wherever it can find it, including pool plaster, grout, and metal equipment. This is called aggressive or corrosive water. Water that contains more calcium than its equilibrium level is over-saturated — it will deposit that excess calcium as scale. Calcium hardness, along with pH, alkalinity, and temperature, determines where on this aggressive-to-scaling spectrum your water sits.
Effects of Low and High Hardness
Low calcium hardness (below 150 ppm in a plaster pool) causes etching and pitting of the pool surface, visible as rough patches. Metal equipment — ladder rails, heat exchangers, filter housings — corrodes faster. The Langelier Saturation Index drops sharply at low hardness, indicating aggressive water. High calcium hardness (above 500 ppm) leads to visible scaling on tile, fixtures, and the waterline. Heater elements are especially vulnerable to calcium scale, which acts as insulation and causes elements to overheat and fail.
Adjusting Calcium Hardness
To raise calcium hardness: add calcium chloride (sold as "hardness increaser"). It dissolves quickly and raises hardness without significantly affecting pH. Typical dose for raising hardness by 10 ppm in a 10,000-gallon pool is approximately 1.25 lbs. Add in small increments. To lower calcium hardness: there is no chemical treatment. The only option is to partially drain the pool and refill with water that has lower hardness. In areas with very hard tap water, this becomes a longer-term management challenge requiring periodic dilution.
Examples
A plaster pool shows rough patches on the bottom near the main drain, visible as dull, pitted areas about 6 months after resurfacing. Water test shows calcium hardness at 80 ppm — well below the 200 ppm minimum for plaster. The low hardness water is etching the new plaster surface. Adding calcium chloride over three days to raise hardness to 250 ppm, and adjusting pH to 7.4, brings the LSI into range and stops further etching. The existing pitting is permanent but will not worsen with correct chemistry.
Common Mistakes to Avoid
- Confusing calcium hardness with total hardness — total hardness includes magnesium; pool chemistry uses only calcium hardness.
- Adding large calcium chloride doses at once — the dissolution is exothermic (produces heat) and can damage vinyl liners if undissolved granules settle on them.
- Not monitoring hardness in refill water — in some regions, tap water is naturally very hard (400+ ppm) and can cause scaling problems from day one.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference