Free tool
Reef dosing calculator
Your alkalinity, calcium or magnesium is low — how much do you actually add? Tell us your water volume, your test result and your target, and we'll work out the dose from the real chemistry, then split it safely over as many days as it needs. Test, don't guess.
- Always dissolve the chemical fully in RO/DI water first, then add it slowly to a high-flow spot — never dump dry powder in the tank.
- Test first, and test again before re-dosing. A dose is only as good as the test behind it.
- Don't dose calcium and alkalinity at the same time or the same spot — mixed at high concentration they precipitate back out (and can cloud the tank).
- Get magnesium into range (~1300 ppm) first — calcium and alkalinity won't hold if magnesium is low.
- Weigh it. Teaspoon "guesstimates" vary a lot by product; a cheap kitchen scale is the "test, don't guess" of dosing.
Sources & method
- Constants derived from standard molar chemistry, benchmarked to the canonical figure (~1 tsp baking soda raises ~50 US gal by ~1 dKH).
- Randy Holmes-Farley — Optimal Parameters for a Coral Reef Aquarium (targets + reef chemistry method).
- Bulk Reef Supply — Pharma dosing instructions (safe daily limits: calcium ≤50 ppm/day; magnesium ≤25 g/80 L/day; dissolve first, dose slowly, test again).
A free Coralkin tool — no sign-up, nothing saved. It does the arithmetic; your test kit and your product's label have the final say. We never guess a care number.
Quick answers
How much does a dose of baking soda raise alkalinity?
It follows the compound's exact chemistry. The canonical hobby benchmark is that about 1 level teaspoon (~6 g) of baking soda (sodium bicarbonate) raises roughly 50 US gallons (~189 L) by about 1 dKH — which is the constant this calculator uses (30 mg of NaHCO₃ per litre per dKH). Enter your real water volume, your current dKH and your target, and it works out the grams from there.
How do I use a reef dosing calculator?
Pick the parameter you're raising (alkalinity, calcium or magnesium) and the exact product, enter your real water volume (display plus sump, minus roughly 10% for rock and sand — not the tank's rated size), then your current test result and your target. The tool returns the total grams and splits it safely over as many days as it needs. Reef-standard ranges are alkalinity ~8–11 dKH, calcium ~400–450 ppm and magnesium ~1300–1400 ppm.
How fast can I safely raise alkalinity, calcium or magnesium?
Go slow — a sudden swing, especially in alkalinity, is the single most common coral-killer. Raise alkalinity by about 1 dKH per day at most (many reefers keep it to ~0.5 dKH/day). Raise calcium by no more than ~50 ppm per day and magnesium by roughly 35 ppm per day (Bulk Reef Supply). This calculator automatically splits any bigger correction across multiple days to stay under those limits.
Why does the dose change with the calcium chloride I use?
Because the dose depends on the exact compound and hydrate. Calcium chloride dihydrate (CaCl₂·2H₂O) needs about 3.67 mg per litre per ppm of calcium, while the moisture-free anhydrous form (CaCl₂) needs less — about 2.77 mg — so the same target takes a different amount. That's why the tool asks which product you have, and why you should always cross-check against your product's own label.
Does this tool lower a parameter that's too high?
No — it only raises a parameter. To lower alkalinity, calcium or magnesium the safe route is a water change, not a chemical. Also get magnesium into range (~1300 ppm) first, because calcium and alkalinity won't hold if magnesium is low, and never dose calcium and alkalinity at the same time or the same spot — mixed at high concentration they precipitate back out.