Calcium hardness dose chart
| Pool size | +10 ppm | +25 ppm | +50 ppm | +100 ppm |
|---|---|---|---|---|
| 10,000 gal | 1.3 lb | 3.1 lb | 6.3 lb | 12.5 lb |
| 15,000 gal | 1.9 lb | 4.7 lb | 9.4 lb | 18.8 lb |
| 20,000 gal | 2.5 lb | 6.3 lb | 12.5 lb | 25.0 lb |
| 25,000 gal | 3.1 lb | 7.8 lb | 15.6 lb | 31.3 lb |
| 30,000 gal | 3.8 lb | 9.4 lb | 18.8 lb | 37.5 lb |
Using 94 to 97% anhydrous calcium chloride? Multiply by about 0.8.
The formula
lb of calcium chloride = gallons / 10,000 x ppm rise / 10 x 1.25
Worked example: a 15,000 gallon plaster pool at 180 ppm with a 300 ppm target needs a 120 ppm rise. 15,000 / 10,000 x 120 / 10 x 1.25 = 22.5 lb of 77% calcium chloride, added as two doses of about 11 lb a day apart.
Ideal calcium hardness by pool type
| Surface | Target CH | Why |
|---|---|---|
| Plaster / gunite | 250 to 400 ppm | Low calcium etches plaster and grout. |
| Pebble / quartz | 250 to 400 ppm | Same as plaster; the finish is cement-based. |
| Vinyl | 175 to 250 ppm | Nothing to etch; very hard water can still scale tile. |
| Fiberglass | 175 to 250 ppm | High calcium can scale and stain the gelcoat. |
| Salt (any surface) | Low end of the surface range | Cells scale up faster above about 400 ppm. |
| Spa / hot tub | 150 to 250 ppm | Hot water scales sooner; keep it lower. |
More on surfaces: plaster vs pebble and fiberglass pool maintenance.
Signs your calcium hardness is off
- Too low: etching, rough or pitted plaster, plaster dust, grout washing out. Calcium hardness too low.
- Too high: cloudy water, white scale at the waterline, a crusty salt cell, rough heater elements. Calcium hardness too high.
Calcium is one of four inputs to water balance. Check the result with the LSI calculator, and balance alkalinity and pH first. Background on what calcium does: calcium hardness explained.