How much calcium additive raises calcium by a given amount?

A standard concentrated calcium chloride part raises calcium by about 100 ppm per 18 ml per US gallon. This tool scales that ratio to your volume and the rise you want, so you get a precise millilitre figure rather than guessing.

Why should I not raise alkalinity by more than 1 dKH per day?

Corals adapt slowly to chemistry swings. Raising alkalinity faster than about 1 dKH per day can stress or bleach sensitive SPS corals. The tool flags doses that exceed this safe daily limit so you can split them over several days.

Do calcium and alkalinity affect each other?

Yes. Calcium and carbonate alkalinity precipitate together as calcium carbonate, so adding a lot of one consumes the other. Keep both near natural seawater values (around 420 ppm Ca and 8 dKH) and dose them in balance rather than chasing one alone.

What does magnesium have to do with calcium dosing?

Magnesium keeps calcium and alkalinity in solution by interfering with calcium carbonate precipitation. If magnesium is low (below about 1250 ppm) calcium and alkalinity become hard to hold, so correct magnesium first.

Is this dosing safe for any additive brand?

The defaults match common concentrated two-part products, but brands vary. Always check your bottle's stated dose-to-rise ratio and override the per-dose strength fields if it differs, then re-run the calculation.

Reef Calcium Dosing Calculator

Keeping a reef tank stable means holding calcium, alkalinity, and magnesium near natural seawater values. This calculator converts the gap between your measured and target levels into a precise additive dose, scaled to your real water volume, and warns you when a correction should be spread across several days.

How it works

For each parameter the dose is proportional to the rise required and the system volume, using a known strength ratio for the additive:

dose (ml) = (target − current) / strengthRise × (volume / refVolume) × refDose

The defaults assume a concentrated calcium chloride part that raises calcium by about 100 ppm per 18 ml per US gallon, a carbonate part that raises alkalinity by about 1 dKH per 4 ml per US gallon, and a magnesium part that raises Mg by about 100 ppm per 12 ml per US gallon. Volumes are converted to US gallons internally so the ratios stay consistent regardless of the unit you enter.

Tips and safe limits

Raise alkalinity by no more than about 1 dKH per day and calcium by no more than about 50 ppm per day to avoid shocking corals. The tool highlights any dose that exceeds these limits. Correct magnesium before calcium and alkalinity, because low magnesium makes the other two impossible to hold. Target natural seawater values of roughly 420 ppm calcium, 8 dKH alkalinity, and 1350 ppm magnesium, and test regularly so your daily doses track consumption rather than overshooting.