Calcium Hardness (CH)
Calcium hardness (CH) measures how much dissolved calcium is in your pool water. Too little and the water turns corrosive — pulling calcium from plaster, grout, and metal equipment to satisfy itself. Too much and excess calcium precipitates as white scale on surfaces, heaters, and salt cells. It’s the biggest single lever for adjusting LSI.
Target ranges
| Pool type | Target CH | Notes |
|---|---|---|
| Plaster / gunite | 200–400 ppm | Aim for ~300 ppm as a midpoint |
| Vinyl liner | 150–250 ppm | No plaster to protect, so lower end is fine |
| Fiberglass | 200–350 ppm | More forgiving surface, but still needs balance |
| Below 150 ppm | Corrosive | Water actively dissolves surfaces regardless of pH |
| Above 500 ppm | Scale risk | Scale likely without tight pH/alkalinity control |
What CH does
Water has a natural drive toward mineral equilibrium. When calcium concentration is low, the water is “hungry” — it will dissolve calcium from whatever surface it contacts. In a plaster pool that means etching the surface, leaving it rough and chalky. In any pool it means attacking grout, corroding copper fittings, and degrading heater components.
When calcium is too high, the water is oversaturated — it holds more dissolved calcium than is stable, and that excess deposits as calcium carbonate scale. Scale coats tile lines, clogs salt cell plates, and reduces heater efficiency. Unlike most chemistry problems, high calcium has no chemical fix: the only way to reduce it is to remove water.
Calcium hardness and LSI
Calcium hardness is one of the five inputs to the Langelier Saturation Index (LSI) — the single number that tells you whether your water is balanced, corrosive, or scale-forming. Of all five inputs, CH has the largest practical range and produces the biggest LSI swings.
CH is the biggest LSI lever
Raising CH from 150 to 300 ppm shifts LSI by about +0.30 — the same impact as raising pH by 0.3 units, but without affecting alkalinity stability. When LSI is persistently negative and pH is already in range, calcium hardness is usually the most direct fix.
This also means calcium accumulation matters even before it causes visible scale. In hot or arid climates, evaporation concentrates everything in your water — including calcium. CH can climb 20–30 ppm over a summer without adding anything, gradually pushing LSI positive.
How to raise calcium hardness
Add calcium chloride (CaCl⊂2;), sold as “calcium hardness increaser” or pool-grade calcium chloride. As a rough guide, 1.25 lbs per 10,000 gallons raises CH by about 10 ppm. Dissolve in a bucket of water first and add slowly around the perimeter with the pump running. Calcium chloride generates significant heat when dissolved — always add chemical to water, not water to chemical.
CH raises permanently. It doesn’t evaporate, react out, or get consumed by chlorine. Once you hit your target you generally only need to add more after a partial drain or significant dilution from rain.
How to lower calcium hardness
No chemical lowers calcium hardness. The only options are:
- Partial drain and refill — drain 25–50% of the pool and replace with fresh water. If your fill water is soft (low CH), this is effective and inexpensive. Retest after refilling and repeat if needed.
- Reverse osmosis service — a specialist can filter calcium out without draining. Available in limited regions and more expensive, but preserves the water entirely.
This is why prevention matters. Once CH climbs above 500 ppm, bringing it down is disruptive. Keeping an eye on CH through the season — especially in evaporation-heavy climates — avoids the problem entirely.
Want to...
Track calcium hardness over time
PoolChem Tracker logs your CH alongside pH, alkalinity, and temperature — and calculates your LSI automatically every time you test, so you can see when calcium drift is affecting your water balance before it becomes a problem.
