A reef dosing calculator helps you determine the precise amount of calcium, alkalinity, and magnesium supplements needed to bring your reef tank from current to target levels. Unlike brand-specific tools, this calculator is brand-agnostic — enter your product concentration and tank volume to get exact daily doses for any two-part, kalkwasser, or dry supplement.
Tank & Dosing Setup
Calcium (Ca²⁺) — target: 380–450 ppm
Alkalinity (dKH) — target: 8–12 dKH
Magnesium (Mg²⁺) — target: 1200–1350 ppm
Ideal Reef Parameter Ranges
| Parameter | LPS / Softie | Mixed Reef | SPS Dominant |
|---|---|---|---|
| Calcium (ppm) | 380–420 | 400–430 | 420–450 |
| Alkalinity (dKH) | 8–10 | 9–11 | 10–12 |
| Magnesium (ppm) | 1200–1300 | 1250–1320 | 1280–1350 |
| Salinity (SG) | 1.023–1.025 | 1.025–1.026 | 1.025–1.026 |
| Temperature (°F) | 76–80 | 77–79 | 76–78 |
How to Use the Reef Dosing Calculator
Maintaining stable calcium, alkalinity, and magnesium is the cornerstone of a healthy reef tank. Coral skeletons are made from calcium carbonate, and corals consume these elements as they grow. A reef dosing calculator takes the guesswork out of supplementation by converting your measured deficit into a safe, precisely sized daily dose — regardless of which brand of supplement you use.
Step 1: Test Your Water Parameters
Before entering values, test your tank with a reliable test kit or send a sample for ICP analysis. For calcium, a salifert or Hanna checker is accurate. For alkalinity, a Hanna DKH checker or titration kit. For magnesium, a salifert kit. Enter your current tested values — not estimates — to get accurate dose calculations.
Step 2: Set Your Target Levels
Set targets appropriate for your reef type. Softie and LPS tanks do well with calcium around 400–420 ppm, alkalinity 8–10 dKH, and magnesium 1200–1280 ppm. SPS-dominant tanks typically target the higher end: calcium 420–450 ppm, alkalinity 10–12 dKH, magnesium 1280–1350 ppm. Avoid chasing exact numbers — stability matters more than hitting a precise target.
Step 3: Select Your Product Type
The calculator supports the most common reef dosing formats. Two-part liquid systems are the most popular for simplicity. Dry salts like calcium chloride, sodium bicarbonate, and magnesium chloride are more economical for larger tanks. Kalkwasser (calcium hydroxide) simultaneously raises calcium and alkalinity while precipitating phosphate — ideal for top-off dosing with an ATO.
Step 4: Review the Dose and Schedule
The calculator outputs the total dose needed and spreads it over multiple days if the deficit is large. For alkalinity, never raise more than 1–2 dKH per day. For calcium, limit changes to 20–30 ppm per day. Magnesium changes up to 50 ppm per day are generally safe. The schedule table shows exactly what to dose on each day to safely reach your targets.
Step 5: Maintain with Regular Testing
After reaching target levels, switch to maintenance dosing — testing every 7–14 days to measure consumption rates and adjust your daily dose accordingly. Coral consumption increases as your tank matures, so expect to gradually increase doses over time. Many reefers automate dosing with a peristaltic pump or calcium reactor once they have established their tank's consumption rate.
Why Magnesium Matters
Magnesium is often overlooked but critical. Low magnesium (below 1200 ppm) causes calcium and alkalinity to precipitate out of solution, making them impossible to hold at target levels. If you find your calcium and alkalinity consistently dropping despite dosing, check your magnesium first. Raise it to 1250–1350 ppm and your other parameters will become much easier to maintain.
FAQ
Is this reef dosing calculator free?
Yes, this reef dosing calculator is completely free with no signup or account required. All calculations run locally in your browser — your tank parameters are never sent to any server.
Is my reef tank data private?
Absolutely. Everything runs in your browser using JavaScript. Your tank volume and chemistry parameters never leave your device. No cookies, no tracking, no server-side storage.
What are the ideal calcium, alkalinity, and magnesium levels for a reef tank?
Most reef keepers target calcium between 380–450 ppm, alkalinity between 8–12 dKH (2.9–4.3 meq/L), and magnesium between 1200–1350 ppm. SPS-dominant tanks often aim for the higher end of these ranges, while LPS and softy tanks are more forgiving.
How does two-part dosing work?
Two-part dosing uses equal volumes of a calcium/magnesium solution (Part A) and an alkalinity solution (Part B) to raise both parameters simultaneously. The calculator shows you the daily dose in mL based on your tank volume and the deficit between current and target levels.
Why should I raise alkalinity slowly?
Rapid alkalinity swings stress corals and can cause tissue death, bleaching, or the dreaded alkalinity crash. Never raise alkalinity more than 1–2 dKH per day. The calculator flags doses that would cause a large single-day swing so you can spread them over multiple days.
What's the difference between dKH and meq/L for alkalinity?
Both are units of alkalinity. dKH (degrees of carbonate hardness) is more common in the reefing hobby. 1 dKH equals 0.357 meq/L. A target of 8–12 dKH is the same as 2.86–4.29 meq/L. This calculator uses dKH as the primary unit.
How often should I test my reef water parameters?
Test calcium, alkalinity, and magnesium at least weekly, especially when dialing in your dosing regimen. Once you have stable consumption rates and a consistent dose schedule, testing every 2 weeks is adequate for established tanks. Always test after adding new corals or after any equipment issue.
Why does magnesium matter for a reef tank?
Magnesium acts as a natural inhibitor that prevents calcium and carbonate from precipitating out of solution. Low magnesium causes calcium and alkalinity to fall rapidly and makes it difficult to hold target levels. A proper magnesium level (1250–1350 ppm) stabilizes the entire calcium-alkalinity chemistry.