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Reef Dosing Calculator (Ca/Alk/Mg)

Based on commonly published standard DIY two-part dosing ratios. Always check your specific commercial product's label, since strength varies by brand.

Your numbers
Dosing note

Keep a simple reef dosing log

Keep a simple dosing log — reef parameters drift slowly, and a log makes it much easier to spot a trend before it becomes a real problem.

Where these ratios come from

The per-dose figures used here (roughly +1.4 dKH, +40 ppm calcium, or +15 ppm magnesium per teaspoon per 5 gallons) match commonly published standard homemade two-part recipes long used in the reefing hobby, built around baking soda/washing soda for alkalinity and calcium chloride for calcium. They're a widely used reference point, not a universal constant — commercial two-part and all-in-one products are frequently formulated at different strengths, so always check your specific product's label for its actual ppm-per-dose rating and use that instead when available.

Why large swings should be corrected gradually

Corals and other reef invertebrates are sensitive to rapid parameter swings, especially in alkalinity and calcium. A large calculated dose to close a big gap is generally better split across several days of smaller doses, with retesting in between, rather than added all at once.

Why Ca, Alk and Mg are dosed together in practice

Calcium and alkalinity are chemically linked (both are consumed by coral calcification, and pushing one without the other out of balance can cause problems), and magnesium acts as a buffer that helps keep both stable. Many reef keepers track and dose all three together rather than adjusting just one in isolation.

Source: Dosing ratios reflect the standard homemade two-part recipe published by reef chemistry author Randy Holmes-Farley (Advanced Aquarist), a widely cited reference for DIY reef supplementation.

Frequently asked questions

Why does my commercial 2-part product's dosing chart give a different number than this calculator?

Commercial products vary in concentration and are often stronger or weaker per dose than the standard homemade DIY ratio used here. Always follow your specific product's own dosing chart when you have it; use this calculator as a general planning reference or for homemade dosing solutions.

Should I dose Ca/Alk/Mg before or after testing?

Test first to know your current level and how much you actually need, then dose, then retest after the dose has fully mixed (often a few hours) before making further adjustments.

What if alkalinity and calcium are both low?

Dose them on an offset schedule (e.g. alkalinity in the morning, calcium in the evening) rather than mixing concentrated solutions together directly, since combining concentrated calcium and carbonate/bicarbonate solutions can cause them to precipitate out of solution.

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