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Calcium Dosing Calculator

Calcium is the slower-moving partner in two-part dosing — most reef tanks consume calcium and alkalinity in roughly equimolar amounts, but the ppm number is bigger so swings look smaller in test results. Aim for 380–450 ppm and dose at the same daily rate you dose alkalinity. This calculator uses Randy Recipe #1 (500 g CaCl₂·2H₂O per gal, ~37,000 ppm Ca stock), the BRS Pharma equivalent, Red Sea Foundation A, and Triton's dry correction.

Calcium dosing

Required change
30.0ppm Ca
Liquid dose
153.46mL

500 g calcium chloride dihydrate dissolved to 1 US gal. Stock ≈ 37,000 ppm Ca. Use 20 % less by mass if using anhydrous CaCl₂.

Source: reefs.com

How this is calculated

Randy Recipe #1 Ca stock = 500 g CaCl₂·2H₂O → 1 gal solution
                       ≈ 37,000 ppm Ca = 37 mg Ca per mL

mL = (tank_L × ΔCa_ppm) / 37
   = tank_gal × ΔCa_ppm × 3.785 / 37
   ≈ tank_gal × ΔCa_ppm × 0.1023

BRS Pharma's published reference (100 gal × 20 ppm = ~102 mL stock) lines up with the math: about 0.051 mL/gal/ppm. Red Sea Foundation A is weaker: 1 mL per 100 L raises Ca by 2 ppm. Triton's dry correction is 3 g anhydrous CaCl₂ per 100 L per 10 ppm.

Anhydrous calcium chloride needs about 20 % less by mass than the dihydrate (CaCl₂·2H₂O) because water of hydration is dead weight. If your source ships anhydrous, drop the stock recipe from 500 g to 400 g per gal — concentration is the same.

FAQ

Why is the dose smaller than for alkalinity?
Calcium stock is ~37,000 ppm — twenty times more concentrated than alk stock measured in ppm. You're moving a 50-gal by 20 ppm Ca with ~100 mL; the same tank to raise 1 dKH alk needs ~70 mL. Calcium just looks like less liquid because ppm is a bigger number scale.
Should I dose calcium separately or use two-part?
Two-part. Calcium chloride alone leaves you with sodium and chloride imbalance over time. Pairing calcium dosing with sodium bicarbonate/carbonate (the alk side) means the ions balance: 2 NaHCO₃ + CaCl₂ → CaCO₃ (consumed by coral) + 2 NaCl (the salt you started with). Water changes purge whatever drift accumulates.
What if my Ca is high but alk is low?
Don't drop calcium — raise alk and let coral consumption pull Ca down naturally. Calcium drops slowly. Removing it actively (by water changes or letting it precipitate) is wasteful and uncontrolled.
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Sources & references

  1. 01
    Randy Holmes-Farley — Two-part recipe (calcium side)
    https://reefs.com/magazine/aquarium-chemistry-a-homemade-two-part-calcium-and-alkalinity-additive-system/
  2. 02
    Bulk Reef Supply — BRS Pharma Calcium Chloride mixing
    https://www.bulkreefsupply.com/content/post/brs-pharma-calcium-chloride-mixing-and-dosing-instructions
  3. 03
    Red Sea — Foundation Complete Manual
    https://redseafish.com/wp-content/uploads/2020/11/24653_Manual-Foundation-Complete-GB-_v21a-WEB.pdf
  4. 04
    Triton — CoreChem (CaCl₂ correction)
    https://www.triton.de/en/products/corechem

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