Calculator
Reef Dosing Calculator
Raise alkalinity, calcium, or magnesium to your target — with the exact dose for your tank, and an automatic split across days when the jump is too big to do safely at once.
Paid links: if you buy through links on this page we may earn a commission, at no extra cost to you — it never affects our ratings. How we make money.
⚠️ Dose to a test, not to a calculator. Always confirm your levels with a reliable test kit first, add supplements to a high-flow area, and never exceed the daily limits shown. Chase one parameter at a time. When in doubt, dose less and re-test.
How reef dosing works
Corals consume alkalinity, calcium, and magnesium as they grow, and your job is to replace what's used and correct anything that's drifted. The dose is simple arithmetic — how far you're raising the level, times your water volume, times the strength of your additive — but the safety is in the pace. Big, fast swings (especially in alkalinity) stress and can burn coral tissue.
Raise slowly — especially alkalinity
The community-accepted ceiling is roughly 1.4 dKH of alkalinity per day, about 50 ppm of calcium, and 100 ppm of magnesium. If your correction is bigger than that, this calculator automatically splits the total dose across enough days to stay under the limit. Rushing it is the fastest way to lose corals.
DIY two-part vs. bottled additives
Raw chemicals — soda ash or baking soda for alkalinity, calcium chloride for calcium, a magnesium mix for magnesium — are the cheapest way to dose a growing reef and are exactly what most bottled "two-part" products are. Bottled versions cost more but are premixed and convenient. Either works; pick the additive above and we'll do the math for the one you have.
Not the only way to raise Alk and Ca
This calculator covers two-part and DIY raw-chemical dosing — the most common method, and the easiest to automate with a standalone pump. It's not the only one: kalkwasser replaces Alk and Ca in a single dose using a completely different chemistry, and nudges pH up instead of leaving it alone. Many reefers run both — kalkwasser through the auto top-off for baseline demand, two-part to cover the rest.
Test first, every time
A dosing calculator is only as good as the number you feed it. Test with a kit you trust, enter your real current value, and re-test after dosing to confirm you landed where you expected — then set a daily maintenance dose to hold it steady.
Ready to automate it?
Once you know your numbers, a dosing pump adds them on schedule without you standing over the tank daily. See our dosing pump comparison — including which well-known doser can't run at all without a controller you probably don't already own. Fighting nitrate or phosphate instead? That's an unrelated dosing method — see our carbon dosing guide.
Free — one email
The Reef Gear Cheat Sheet.
The exact skimmer / pump / light picks by tank size, plus our full build-cost worksheet — as one email, no spam.
One email with the sheet. If we ever start a newsletter, you can opt out first. Privacy