Reef alkalinity, calcium & magnesium dosing calculator

How it works

Every supplement is a known amount of the ion you want per gram (or per millilitre). The calculator multiplies the rise you want by your system’s volume and by that amount:

rise   = target − current        (meq/L for alkalinity; mg/L = ppm for calcium and magnesium)
amount = rise × volume (L) × product factor
days   = rounded-up (rise ÷ maximum safe rise per day)

• Alkalinity is measured in meq/L, and 1 meq/L = 2.8 dKH = 50.04 ppm as calcium carbonate. Baking soda supplies 84.0 mg of bicarbonate salt per meq; soda ash supplies 53.0 mg.
• Calcium and magnesium are found from the element’s share of the salt: calcium chloride dihydrate is 27.3% calcium, anhydrous calcium chloride 36.1%, magnesium chloride hexahydrate 12.0% magnesium and Epsom salt 9.9%.
• Brand products use each maker’s own rate: Red Sea’s 1 mL per 100 litres, and the gram-per-gallon formulas from Brightwell and Seachem.
• The gauge shows the level after dosing against the range Holmes-Farley suggests: alkalinity 7–11 dKH, calcium 380–450 ppm, magnesium 1250–1350 ppm. Amber and red bands are the warning zones either side.
• Daily limits are the most cautious the makers publish: 20 ppm calcium, 1.4 dKH alkalinity and 10 ppm magnesium a day.

Worked example

A 75 gallon system at 7 dKH alkalinity, target 8 dKH, using baking soda:

• Rise: 1 dKH ÷ 2.8 = 0.357 meq/L
• Volume: 75 gal = 283.9 L
• Baking soda: 0.357 × 283.9 × 0.0840 = 8.5 g (0.30 oz), within the 1.4 dKH daily limit, so one dose.

The same system needing calcium raised from 400 to 420 ppm with calcium chloride dihydrate: 20 × 283.9 × 0.00367 = 20.8 g. Raising magnesium by 100 ppm with magnesium chloride hexahydrate takes about 237 g, spread over ten days at the 10 ppm daily limit.

Dose safely

• Go slowly and test. Split anything larger than the daily limit over several days, and test before every dose.
• Dissolve and add to flow. Mix powders in a cup of tank or RO water and pour it slowly into the sump or a high-flow spot, never straight onto corals.
• Check salinity first. Red Sea notes that a 1 ppt rise in salinity from evaporation raises calcium by about 13 ppm and magnesium by about 40 ppm, so top off with fresh water before you decide a parameter is low.
• Keep them in balance. Corals use alkalinity and calcium together, about 1 meq/L of alkalinity for every 20 ppm of calcium. Dosing one without the other makes the gap grow.

Tips from the fish room

• Use your real volume. Subtract rock and sand from the tank size and add the sump. Most systems hold less water than the label says.
• Watch the trend, not one test. Two tests a week show how fast you use each element, which is the number you actually need to keep levels steady.
• Pure chemicals are cheapest, brands are easiest. Baking soda and calcium chloride cost very little per dose. Branded liquids cost more but are pre-mixed and easy to meter.
• Epsom salt adds sulfate. It raises magnesium, but the sulfate stays behind; Holmes-Farley’s recipes balance it with magnesium chloride.

FAQ

Sources

1. An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System, Randy Holmes-Farley, Reefkeeping. 297 g of baking soda in a gallon is about 950 meq/L; 250 g of calcium chloride dihydrate in a gallon is about 18,500 ppm calcium; suggested ranges of calcium 380–450 ppm and alkalinity 2.5–4 meq/L (7–11 dKH).
2. Do-It-Yourself Magnesium Supplements for the Reef Aquarium, Randy Holmes-Farley, Reefkeeping. Natural seawater magnesium is about 1285 ppm (1250–1350 is fine); a 100 gallon tank holds about a pound of it.
3. Reef Care Program Foundation, Testing and Supplementing user manual, Red Sea. Maximum daily rise of calcium 20 ppm, alkalinity 0.5 meq/L (1.4 dKH), magnesium 10 ppm; dosing charts are based on 100 liters / 25 gallons.
4. Reef Foundation A, B and C product pages, Bulk Reef Supply (Red Sea labels). 1 mL raises the calcium of 25 gallons by 2 ppm (A), the alkalinity by 0.036 meq/L or 0.1 dKH (B), and the magnesium by 1 ppm (C).
5. Alkalin8.3-P dosing directions, Brightwell Aquatics (via Bulk Reef Supply). Gallons × dKH × 0.089 = grams; gallons × meq/L × 0.25 = grams.
6. Reef Advantage Calcium and Reef Advantage Magnesium dosing instructions, Seachem. Calcium grams = gallons × mg/L × 0.0104, at most 24 mg/L a day; magnesium grams = gallons × mg/L × 0.05, at most 25 mg/L a day.

Formula and sources last checked September 30, 2026. How we test formulas.

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Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.