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.

What is the Reef Calcium Dosing Calculator?

Enter target and current Ca, Alk, and Mg levels plus tank volume to compute the dose of two-part additive or kalkwasser needed to raise each parameter. For saltwater reef aquarium keepers. It runs free in your browser on Gera Tools, with nothing uploaded.

Reef Calcium Dosing Calculator

Name: Reef Calcium Dosing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

The relationship between the three parameters

Calcium, alkalinity, and magnesium form an interlocked chemistry system, and ignoring their interactions is the most common dosing mistake:

Calcium and alkalinity are consumed together by coral growth. Every time a coral deposits calcium carbonate skeleton, it draws down both calcium ions and carbonate alkalinity from the water simultaneously. If you chase one without the other, you create an imbalance that becomes harder to correct.
Magnesium regulates precipitation. Magnesium ions occupy the same lattice sites as calcium in spontaneous calcium carbonate precipitation, essentially slowing or blocking that precipitation. When magnesium falls below about 1,250 ppm, calcium and alkalinity start precipitating out on their own — in reactors, on heaters, in pumps — making it impossible to hold them at target values no matter how much additive you dose. Always correct magnesium first.
Target natural seawater. The reference point most hobbyists use is approximately 420 ppm calcium, 8–9 dKH alkalinity, and 1,350–1,450 ppm magnesium. SPS-dominant tanks often push alkalinity slightly higher (9–10 dKH) for faster growth, but stability matters more than chasing a specific number.

Illustrative example

For example, a 200-litre (about 53 US gallon) tank testing at:

Calcium: 390 ppm, target 420 ppm — deficit of 30 ppm
Alkalinity: 7.0 dKH, target 8.5 dKH — deficit of 1.5 dKH
Magnesium: 1,280 ppm, target 1,380 ppm — deficit of 100 ppm

At default additive strengths the calculator would return doses of roughly:

Calcium part: about 9 ml for the full 30 ppm rise (split across several days)
Alkalinity part: about 7 ml, but flagged — raising 1.5 dKH in one day is the daily limit; split across 1–2 days
Magnesium: about 10 ml (dose this first, then retest calcium and alk)

Always verify the dose against your specific bottle’s label — additive strengths vary between brands, and the tool lets you override the default strength values.

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. Test regularly — ideally two or three times per week when dialling in a new system — so your daily doses track real consumption rather than accumulating toward an overshoot.