Pool & spa
Pool Calcium Hardness Calculator
Calcium chloride to raise hardness, which protects plaster and stops the water pulling calcium out of surfaces.
Result
Dose against a test result, never against a schedule. Add chemicals in stages with the pump running, wait for a full turnover, then retest before adding more. Never mix chemicals, and never add water to acid. Where a product label differs from the figure here, the label wins. Calcium hardness cannot be lowered chemically. The only way down is draining and refilling with softer water, which is why it is worth not overshooting. Calcium chloride gives off heat as it dissolves, so add it slowly.
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About this calculator
Why does hardness matter?
Water low in calcium dissolves it from wherever it can find it, which means the plaster, grout and tile. Too high and it scales on surfaces and in the heater.
What range is right?
Roughly 200 to 400 ppm for a plaster pool, lower for vinyl and fibreglass where there is no calcium surface to protect.
Can I lower it?
Only by dilution. Partially drain and refill with water lower in calcium, which is the reason to approach the target from below.
The formula
about 1.25 lb of calcium chloride per 10,000 gallons raises hardness 10 ppm
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