Calcium Hardness Calculator

Raise: calcium hardness is raised with calcium chloride, and the dose is a mass-balance: to lift CH by Δ ppm, the product weight scales with your pool volume and inversely with the product's purity. We use the industry rule of thumb 1.25 lb of 100%-pure calcium chloride per 10,000 gallons per 10 ppm, then divide by your product grade's purity so a 77% flake user gets the right (larger) weight — an unlabeled dose assumes 100% and under-doses flake by about 30%.

chGap        = target − current                         (ppm, raise only)
dose (100%)  = 1.25 × (gallons / 10,000) × (chGap / 10)  lb
dose (grade) = dose(100%) / purity     (flake 0.77 · pellet 0.90 · anhydrous 1.00)

Lower (no chemical removes calcium — dilute):
fraction     = (current − target) / (current − fillWater)
drain %      = fraction × 100
gallons      = fraction × pool gallons
resulting CH = fillWater + (current − fillWater) × (1 − fraction)   (= target)

Lower: no chemical lowers calcium hardness — the only route down is replacing pool water with lower-CH source water. The drain fraction is (current − target) / (current − fill-water CH); with CH-free fill water (the default) that reduces to 1 − target/current. Entering your fill water's own hardness makes the answer honest for hard-water areas. Above ~40% we flag a single big drain and point you to the staged dilution plan rather than draining the whole pool at once.

Surface presets set the recommended band — plaster/gunite 250–350, vinyl 150–250, fiberglass 200–400 — and pre-fill the target to the midpoint; you can override it. Scaling above roughly 400 ppm and etching below roughly 150 ppm are general industry consensus, not a single sourced figure.

Sources: Pool Advisor (dilution worked example, 500 → 400 ppm = 20% drain); Balance My Pool (product grades flake 77% / pellet 90% and the surface presets); Test Your Own Pool (the ~1.2–1.25 lb / 10k / 10 ppm rule-of-thumb constant, product close to 100% pure).