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.

Calcium never sits on its own. It is one of the three terms the saturation index calculator weighs against pH and alkalinity, which is the check that decides whether a high reading is actually scaling your heater or merely high on paper, and the whole balanced set is laid out on the pool chemistry target chart. When the answer is dilution, the staged plan for a big drop lives in the drain and dilution planner, which splits one oversized drain into cycles. Hot tubs run the same mineral on a far smaller volume and a much shorter fuse, so dose those on the spa and hot tub dosing calculator rather than scaling a pool figure down by eye.

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).