Calculate calcium chloride amounts to protect your pool plaster. Prevent aggressive water from pitting surfaces by maintaining ideal hardness levels. Keeping your calcium in the correct range is essential for preventing permanent damage to concrete and tile grout.
Start with an accurate pool volume (use the Pool Volume tool if unsure) and a fresh Calcium Hardness (CH) reading taken from elbow-deep water, away from returns. CH is reported in parts per million (ppm) as calcium carbonate. Enter your current CH and your target. PHTA guidance places the ideal band at roughly 200-400 ppm for plaster and concrete pools, and 100-300 ppm for vinyl or fiberglass, which lack calcium surfaces to protect. Also note your water temperature and Total Alkalinity, since hardness, alkalinity, pH and temperature jointly drive scaling via the Langelier Saturation Index. The calculator raises CH by dosing calcium chloride (CaCl2), sold as a flake or granule. Because calcium does not evaporate or break down, you only ever add it to climb toward target; the tool never tells you to add when you are already at or above your goal. Soft fill water from rain-fed or softened sources is the usual reason CH runs low.
Method: calcium chloride dihydrate (77%) raises CH about 1 ppm per 0.013 lb per 1,000 gallons; in metric terms roughly 1.5 grams per 1,000 liters per 1 ppm. The tool computes pounds = volume(gal) / 1000 x ppm_increase x 0.013. Worked example: a 20,000 gal (75,700 L) plaster pool reads 150 ppm and you want 250 ppm, a 100 ppm rise. 20,000 / 1000 x 100 x 0.013 = 26 lb of 77% calcium chloride (about 11.8 kg). If your product is anhydrous 94% calcium chloride, it is more concentrated, so the tool scales the dose down by the purity ratio (roughly 0.0107 lb/1,000 gal/ppm). Add in stages: dose to about 70 ppm below target, circulate, retest, then finish. Note that calcium chloride is exothermic and releases heat as it dissolves.
Accuracy matters because hardness is permanent in practice: the only way down is draining and refilling with softer water, so overshooting wastes water and money, while undershooting leaves water aggressive. Low CH makes water 'hungry,' etching plaster, pitting surfaces and dissolving grout; high CH (above 400-500 ppm) cterm clouds water and forms scale on tile, heaters and salt cells. Pre-dissolve calcium chloride in a clean bucket of pool water, adding chemical TO water (never water to chemical), because the reaction generates significant heat and can spatter. Wear gloves and eye protection, pour slowly near a return with the pump running, and brush to disperse. Wait at least 30 minutes of circulation, ideally several hours, before retesting. Recheck pH and alkalinity afterward, then run the LSI calculator to confirm the water is balanced rather than merely 'in range.'
Calcium Hardness (CH) is the amount of dissolved calcium in the water, measured in ppm. The ideal range is 200-400 ppm (250-350 ppm for plaster pools). Too low and the water dissolves calcium out of plaster and grout; too high and it forms scale.
To raise Calcium Hardness, add calcium chloride. Dissolve it in a bucket of water first (it gives off heat as it dissolves), then pour it slowly around the pool with the pump running and retest after a few hours. Enter your volume and target above to get the exact amount.
Yes. Extremely high calcium (above 400-500 ppm) can lead to cloudy water and scale formation on your pool walls and inside your plumbing.
Calcium doesn't evaporate or break down. The only way to lower it effectively is to drain a portion of the pool and refill it with fresh, lower-calcium water.
The water becomes 'hungry' and will actively dissolve calcium from your pool's plaster and grout, causing pitting and structural weakness over time.