DIY Calcium Chloride (Randy Recipe 1) raises calcium by 0.3698 ppm per mL per 100 L of tank water (manufacturer data: source). Enter your tank volume, current and target calcium below for an exact dose, split automatically if it exceeds the safe daily limit.
Open the full calculatorA 75 gallon tank (284 L) raising calcium by 20 ppm needs (20 / 0.3698) x (284 / 100) = 153.6 mL of DIY Calcium Chloride (Randy Recipe 1).
This is the error that quietly ruins an otherwise correct calculation. A tank sold as 75 gallons does not hold 75 gallons once rock, sand and displacement are counted, and a sump adds only some of it back. Real volume usually lands 10 to 20 percent below the number on the box. Dose against the label figure and you overshoot every time, always in the same direction, which reads as "DIY Calcium Chloride (Randy Recipe 1) is stronger than the calculator says" when the calculator was fine and the volume was wrong.
The dose above is only as good as the current reading you put into it. Titration kits carry genuine error, and a single result that disagrees sharply with where the tank has been sitting is more often a fumbled test than a real swing. Retest before acting on a surprise, ideally with fresh reagent, because a bad number and a real problem look identical on paper and only one of them needs DIY Calcium Chloride (Randy Recipe 1).
Do not chase it back down with anything. Calcium that has gone high is diluted out by ordinary water changes over the following weeks, not corrected chemically. The damage in reef chemistry comes from the speed of the change far more than from the number itself, so a slow return is the correct return. If calcium will not hold no matter how much you add, check magnesium before adding more: low magnesium stops calcium staying in solution.