What calcium hardness should I aim for?

Most plaster pools run best at 200–400 ppm; vinyl and fiberglass pools tolerate lower. The right target also depends on your alkalinity and pH through the saturation index — check your pool's guidance and test with a kit.

Why does dihydrate flake take more than anhydrous?

Because roughly a quarter of the dihydrate's weight is water of crystallization, not calcium. Anhydrous flake is more concentrated, so you use less of it — but it gives off more heat when dissolving, so add it especially slowly.

Can I lower calcium hardness with a chemical?

Not really. Once calcium is in the water the practical fix is partial draining and refilling with softer water, or using a sequestrant to manage scale. That is why it pays to dose calcium carefully and retest rather than overshooting.

Calcium Hardness Calculator

Get the calcium chloride needed to raise calcium hardness to your target — anhydrous or dihydrate flake, from your pool volume and current CH.

Inputs

Pool volume — gallons

gal

Current CH

ppm

Target CH

ppm

Product — Anhydrous CaCl₂ — 36% calcium by weight

Results update as you type — no submit

Calcium chloride to add

13.88 lb

≈ 6.3 kg

CH change100 ppm

ProductCalcium chloride (anhydrous ~100%)

Formula

raise CH by 100 ppm in 15,000 gal
calcium chloride = 13.88 lb ≈ 6.3 kg

Add chemicals to water, never water to chemicals. Broadcast over the deep end with the pump running, then retest before adding more.

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.

MeasureShedmeasureshed.com/calculator/calcium-hardness-calculator

Calcium hardness — dose sheetUNITS: US GAL · ppm

Inputs

Pool volume15,000 gal

Current CH150 ppm

Target CH250 ppm

ProductCalcium chloride (anhydrous ~100%)

Results

calcium chloride to add13.88 lb

Other units6.3 kg

CH change100 CH ppm

Safe dosing: add product to water (never the reverse) · broadcast with the pump running · wait and retest before re-dosing · keep chemicals apart and dry.

Shopping list

☐ 13.88 lb (6.3 kg) — Calcium chloride (anhydrous ~100%)

Formula

raise CH by 100 ppm in 15,000 gal

calcium chloride = 13.88 lb (6.3 kg)

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.SHEET 1 OF 1

How it works

Calcium hardness (CH) is how much dissolved calcium your water holds. Too little and the water gets aggressive, etching plaster and corroding metal; too much and it scales. When CH is low, you raise it with calcium chloride. The calculator sizes the dose from your pool volume and the gap to your target.

The dose depends on which calcium chloride you buy. Calcium hardness is read as parts per million of calcium carbonate, and calcium chloride supplies the calcium by weight: anhydrous flake (`CaCl₂`) is `36%` calcium, while the dihydrate flake (`CaCl₂·2H₂O`) is only `27%` because part of its weight is water. So the same result takes about `0.9 lb` of anhydrous or `1.25 lb` of dihydrate per `10,000 gal` per `10 ppm`.

Calcium chloride releases heat as it dissolves, so add it slowly to a bucket of water — never water to the product — and pour the solution around the pool with the pump running. Test with a kit before and after. Do not overshoot: you cannot easily remove calcium except by draining and refilling.

Worked example

Volume `15,000 gal` · calcium hardness `150 → 250 ppm` (`+100 ppm`) · anhydrous `CaCl₂`
calcium needed = (20.0 ÷ 50.0) × 100 ppm = `40 mg/L` of calcium
calcium chloride = 40 ÷ 0.36 × 15,000 × 3.785 ÷ 1,000 ÷ 1,000 = `6.3 kg` = `13.9 lb`

Questions about calcium hardness

About 0.9 lb of anhydrous calcium chloride (or 1.25 lb of dihydrate flake) per 10,000 gal raises calcium hardness by 10 ppm. A 15,000 gal pool going from 150 to 250 ppm needs roughly 14 lb of anhydrous product.

SOURCES — CaCl₂ 111 g/mol (36% Ca) · CaCl₂·2H₂O 147 g/mol (27% Ca) · CaCO₃ 50 g/eq

Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.