Calcium is the parameter people panic about and the one that least deserves it. It drifts down slowly, it tolerates being corrected over a week, and a reading fifty short of where you want it is not an emergency. What it does not tolerate is being fixed at the same moment as alkalinity.
Worked from your tank's real water volume rather than the number on the box. The calculator itself needs JavaScript; everything it assumes is written out below.
Calcium and alkalinity are consumed together, so if one is low the other usually is too. Correct them together in small steps rather than one in a rush, and never in the same spot or the same minute.
Calcium chloride carries chloride with it, and a large correction dosed all at once shifts the tank's ionic balance as well as its calcium. Another argument for the slow version.
A calcium reading that will not rise while alkalinity also sags is usually a magnesium problem wearing two disguises.
400 to 450 ppm, which tracks natural seawater at roughly 412 ppm. The calculator accepts targets from 380 to 480 ppm and refuses anything outside them. Calcium is consumed far more slowly than alkalinity in most tanks, which is why it is usually the parameter that quietly drifts rather than the one that crashes.
50 ppm a day is the cap the calculator uses, and a bigger correction is split across days. Calcium forgives more than alkalinity does, but a large dose of calcium chloride into a tank with high alkalinity can precipitate calcium carbonate in the water column, which is the cloudy water and the alkalinity drop people report after dosing too much at once.
The dihydrate, which is what most reef-grade calcium chloride is, takes 0.37 g per 100 litres per ppm; the anhydrous form takes 0.28 g. The difference is the two waters of crystallisation in the dihydrate, which weigh but add no calcium, so check which form the tub says it is before you trust a number off another calculator.
The rules that apply to every dose rather than to this one, including never dosing calcium and alkalinity together, are on the <a href="/tools">calculators hub</a>.