Reef Two Part Dosing Schedule Calculator

Enter your tank size, current water test results, and coral load to get an instant correction dose (to bring parameters up today) plus a 7-day daily maintenance schedule. All chemistry runs in your browser — nothing is sent to any server.

Tank Setup
Solution Type

Dose volumes depend on how concentrated your two-part solution is. Choose the option that matches what you use.

Current & Target Parameters

NSW defaults pre-filled. Enter your latest test results in the Current column.

Current
Target
Unit
Calcium
ppm
Alkalinity
dKH
Magnesium
ppm
Coral Density / Weekly Consumption

Select your coral load for estimated consumption, or enter measured weekly drawdown values manually.

Correction vs. Daily Maintenance

Correction = total volume needed to raise parameters to target. Maintenance = daily replenishment for ongoing coral consumption. Correct values are shown per day if splitting is required.

Correction Dose
Daily Maintenance Dose

7-Day Dosing Schedule

Correction is spread over the days required; maintenance runs every day. Add Part A (Ca) and Part B (Alk) at least 2 hours apart — never at the same time in the same spot.

Date Ca Part A Alk Part B Mg Part C Type
All doses in ml.

How it works

Two-part dosing uses two separate solutions — Part A (calcium chloride) and Part B (sodium carbonate / bicarbonate) — to independently replenish the calcium and alkalinity consumed by corals and coralline algae. A third part (magnesium chloride + sulfate blend) keeps magnesium levels stable.

Correction dose — Calcium Based on CaCl₂ solution at ~43 mg Ca/ml (standard BRS powder recipe).
dose (ml) = ΔCa × vol_L / 43.2
Correction dose — Alkalinity Based on Na₂CO₃ solution at ~1.695 meq/ml (BRS powder, ¾ lb/gal).
dose (ml) = Δ(dKH) × vol_L × 0.357 / 1.695
Correction dose — Magnesium Based on MgCl₂·6H₂O + MgSO₄·7H₂O blend at ~19.25 mg Mg/ml.
dose (ml) = ΔMg × vol_L / 19.25
Daily maintenance dose Weekly consumption ÷ 7, then same formula. Consumption scales to your tank volume.
daily = weekly_drawdown / 7 → correction formula

The "Standard DIY" mode is calibrated to BRS powder recipe concentrations — the most common hobbyist two-part approach. The "Commercial Concentrate" mode applies a 3× multiplier to match ready-made liquid products (Red Sea, Seachem, etc.) whose solutions are roughly 3× more concentrated. If your doses seem off, double-check your product's label for mg-per-mL or ppm-per-mL-per-100-gal figures.

Frequently asked questions

What is reef two-part dosing and when should I use it?
Two-part dosing is the most beginner-friendly method to replenish the calcium, alkalinity, and magnesium that corals and coralline algae consume. You add a liquid calcium solution (Part A) and a liquid alkalinity solution (Part B) daily — either by hand or with a dosing pump — to replace what the reef uses. It is ideal for tanks under ~200 gallons, mixed reefs, and anyone who wants predictable, safe chemistry control without investing in a calcium reactor. As coral demand grows, you simply increase the daily dose. Kalkwasser is an alternative but pH-sensitive and harder to dose precisely; a calcium reactor becomes cost-effective only on very large, heavy SPS systems.
How do I figure out my actual weekly Ca/Alk/Mg consumption?
The most reliable method is a direct drawdown test: test your water, stop dosing for exactly 7 days, then test again. The drop in each parameter is your weekly consumption for your current coral load. Enter those numbers in "Manual Entry" above for the most accurate schedule. The density presets (FOWLR, Low, Mixed, SPS) are typical ranges based on published reef chemistry data — they are good starting points but your tank's actual draw will vary based on total coral mass, growth phase, and lighting intensity. Re-measure every few months as your reef matures.
Can I give the correction dose all at once?
For small corrections (Ca < 10 ppm, Alk < 0.5 dKH, Mg < 25 ppm) a single dose added slowly to a high-flow area is generally safe. For larger swings the calculator automatically splits the correction across multiple days with a warning. Never try to correct large Alk deficits (2+ dKH) in a single dose — this can precipitate calcium carbonate, crashing pH and harming corals. Ca and Mg corrections are more forgiving, but always add solution to a turbulent area and monitor the tank for stress signs. Rule of thumb: slow and steady wins. Most reefers correct over 1–2 weeks rather than 1–2 days.
Why do the dose volumes look so much larger than what I've read online?
Dose volume depends entirely on how concentrated your solution is. This calculator defaults to the standard BRS powder recipe (~43 mg Ca per ml). Commercial ready-made two-part liquids are typically 3× or more concentrated, so you'd add 3× less volume. If your supplier's label says something like "1 mL raises Ca 0.3 ppm per gallon," use that to derive your own rate; the math is: dose_ml = ΔCa_ppm × vol_gal / rate_ppm_per_ml_per_gal. Switch to the "Commercial Concentrate" tab to apply a 3× dilution factor automatically.
Should I dose Ca (Part A) and Alk (Part B) at the same time?
No — never dose calcium and alkalinity solutions in the same location or at the same time. When concentrated Ca²⁺ and CO₃²⁻ / HCO₃⁻ meet directly, they precipitate calcium carbonate (a white snowstorm), which wastes both additives and can crash alkalinity. Best practice: dose Part A in the morning into a high-flow area, and Part B several hours later into the sump return chamber. A dosing pump with separate inlets and an offset timer handles this automatically. Magnesium can generally be dosed at any time independent of Ca and Alk.