To raise 100 L of water by 10 mg/L of calcium you need about 3.7 g of calcium chloride dihydrate. To raise it by 10 mg/L of magnesium you need about 8.4 g of magnesium chloride hexahydrate. The calculator scales this to your tank and splits it so calcium rises no more than 50 mg/L a day and magnesium no more than 100 mg/L.
Fill in your volume and both readings to see how much to add.
How the sums work
Each salt carries a fixed share of calcium or magnesium by weight, set by its chemistry. The calculator multiplies the rise you need by your volume and that factor. For a liquid product, it uses the label dose instead.
| Salt | Raises | Grams |
|---|---|---|
| Calcium chloride dihydrate | Calcium | 3.7 g |
| Calcium chloride, anhydrous | Calcium | 2.8 g |
| Magnesium chloride hexahydrate | Magnesium | 8.4 g |
| Magnesium sulfate heptahydrate | Magnesium | 10.1 g |
How fast is safe
Bulk Reef Supply suggests raising calcium by no more than 50 mg/L a day and keeping magnesium changes under 100 mg/L a day. Dissolve the salt fully in RODI water and add it slowly to a high-flow area.
Magnesium: chloride and sulfate
Bulk Reef Supply’s magnesium mixes blend magnesium chloride with some magnesium sulfate, to keep the balance of chloride and sulfate in the tank. Its general mix is about 10 parts chloride to 1 part sulfate by weight. Epsom salt alone adds a lot of sulfate. See magnesium.
Common questions
How much calcium chloride raises calcium by 10 ppm?
About 3.7 g of the dihydrate per 100 L.
Can I raise magnesium in one go?
Bulk Reef Supply suggests under 100 mg/L a day. Larger rises are split across days.
Do I need to dose magnesium?
Only if it falls between water changes. Test first.
Sources and limits
Last checked 13 October 2026. Amounts come from the chemistry of each salt and assume your volume and readings are right. Add less rather than more, and test before each further dose.
- Bulk Reef Supply: calcium chloride dosing instructions
- Bulk Reef Supply: magnesium supplement mixing guidance
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
Calcium and magnesium dosing calculator · Free Reef Companion · All calculators