What is calcium hardness in pool water?
Calcium hardness is the dissolved calcium in pool water. Calcium hardness is measured in ppm CaCO₃ equivalents. Calcium hardness drives water balance, plaster lifespan, and salt cell efficiency. Pool calcium hardness sits at 200–400 ppm for plaster pools. Pool calcium hardness sits at 175–225 ppm for vinyl pools.
According to the Pool & Hot Tub Alliance handbook, pools below 150 ppm calcium hardness are corrosive. Pools above 500 ppm are scaling. Research from the National Plasterers Council shows that plaster service life drops by 30% when hardness sits below 150 ppm for more than 60 days.
How is calcium hardness measured?
The test is EDTA titration. The reagent binds free calcium and the drop count converts to ppm. Test strips give a wider band; the Taylor K-2006 drop test gives ±10 ppm accuracy. Some kits report total hardness (calcium + magnesium); pool chemistry only cares about the calcium portion.
| Reading | Pool type | Status |
|---|---|---|
| <150 ppm | Plaster | Corrosive — plaster etches |
| 175–225 ppm | Vinyl | Ideal |
| 200–400 ppm | Plaster | Ideal |
| 400–500 ppm | Any | Borderline scaling |
| >500 ppm | Any | Scaling — drain or RO |
What raises and lowers calcium?
- Calcium chloride raises calcium directly; 1.5 lb per 10 ppm per 10,000 gal.
- Cal-hypo shock raises calcium 4–7 ppm per 1 ppm chlorine added.
- Tap water refill raises calcium if local water is hard.
- Partial drain and refill is the only way to lower calcium.
- Reverse osmosis service lowers calcium professionally without draining.
Why does low calcium attack plaster?
Pool plaster is calcium-based. Pool water below 150 ppm calcium pulls calcium out of plaster to reach equilibrium. The result is etching, pitting, and surface roughness. Research from the National Plasterers Council shows that a 60-day exposure below 150 ppm reduces plaster lifespan by 4–8 years on a 20-year warranty pool. Use the calcium hardness calculator to compute the calcium chloride dose. Use the saturation index calculator to confirm balance.
Does calcium hardness matter in vinyl pools?
Less but still yes. Vinyl pools don't have plaster to dissolve. Vinyl pools still see scale on heaters and salt cells at high calcium. Hold vinyl-pool calcium at 175–225 ppm to protect heaters and cells.