Reef Dosing Calculator
Enter a reading for alkalinity, calcium, or magnesium and see where it sits against the reef range, how far it is from your target, and — the part that actually protects your corals — how many days a correction should take. Add your supplement's chart to turn that into millilitres.
Magnesium anchors calcium and alkalinity — if your magnesium is low, correct it first or alkalinity and calcium will never hold. Never add dry supplement straight to the tank: dissolve it in RO/DI water first, then dose slowly into high flow. This is an estimate to confirm with a test kit, not a recipe — stability matters more than hitting an exact number.
Why the pace matters more than the number
Corals tolerate a slightly “off” number far better than a fast-moving one. A reef sitting steadily at 7.5 dKH is healthier than one yo-yoing between 7 and 9 as you chase a target. That's why this calculator leads with the safe correction time, not just the dose: the widely used ceilings are about 1 dKH of alkalinity, 20 ppm of calcium, and 50 ppm of magnesium per day. When your reading is a long way off, the fix is patience — split the correction across several days and re-test as you go.
The big three are chemically linked, so the order of operations matters. Magnesium keeps calcium and alkalinity in solution; if magnesium is low, you can pour in calcium and buffer all week and the numbers still won't hold. Correct magnesium first, then bring alkalinity and calcium to target together. Typical reef aims are roughly 8–9.5 dKH, 400–440 ppm calcium, and 1300–1380 ppm magnesium, but the exact figures matter less than holding them steady.
Two safety rules override the arithmetic. Never add a dry supplement straight to the display — undissolved grains create pockets of extreme pH and chemistry that burn coral tissue on contact; dissolve it in RO/DI water first and dose slowly into high flow. And to lowera parameter, use water changes or ease off your dosing rather than a “reducer” product. When in doubt, dose less and test more. If you're still building your parameter habit, the reef parameters guide explains what each number does and how often to check it.