Salinity & Dosing Calculator

Salinity is the foundation parameter on a reef, and it's the one that drifts on its own as water evaporates. Enter your tank volume and your current versus target salinity — in ppt or specific gravity — and this calculator works out roughly how much salt to add or how much RO/DI water to swap in to close the gap. Below it is a plain-English calcium, alkalinity and magnesium target-range reference. The golden rule with every adjustment: go slow. Big, fast salinity swings stress livestock. This is general reef-keeping information, not veterinary advice — test your own water and follow each product's label.

Salinity adjustment

Total water volume in US gallons.

Reef target ≈ 35 ppt ≈ 1.026 SG.

What your refractometer / meter reads now.

Where you want to be. 35 ppt / 1.026 SG is a common reef target.

Ca / Alk / Mg target-range reference

Common reef target ranges for a mixed reef, drawn from established reef references. Stable matters more than perfect — pick a point in each range and hold it. Adjust gently and follow your supplement's label for dose rates; this reference does not calculate doses for a specific product.

Alkalinity (dKH)
~8–9 dKH
The fastest-moving parameter — corals consume it building skeleton. Watch the trend.
Calcium (ppm)
~420–440 ppm
Half of the skeleton-building pair with alkalinity. Moves slower than alk.
Magnesium (ppm)
~1300–1400 ppm
The quiet enabler — keeps calcium and alkalinity both in solution.
Salinity
~35 ppt / 1.026 SG
Hold it steady with an ATO so evaporation doesn't push it up.
How this is calculated
  • SG → ppt. Specific gravity is converted to ppt for the math (roughly ppt ≈ (SG − 1) × 1000 × 1.34 near reef salinity), so you can enter either unit. Reef target ≈ 35 ppt ≈ 1.026 SG.
  • Raising salinity. To raise salinity you add salt mix. The estimate is the extra salt mass needed to lift your volume from current to target ppt (about 38 g of mix makes ~1 ppt per US gallon, near reef strength). Add it slowly, pre-dissolved, over hours — never dump dry salt into the display.
  • Lowering salinity. To lower salinity you replace some saltwater with RO/DI fresh water. The estimate is the fraction of your volume to swap out for fresh water to reach the target, done gradually across small changes.
  • Go slow. Aim to move salinity no more than ~1 ppt (≈0.001 SG) per day for a stocked reef. The tool shows the total change needed; spread it over days, not minutes.
  • Ca / Alk / Mg reference. Target ranges are common reef figures from established references. Dose volumes for a specific 2-part, calcium or magnesium supplement come from that product's own label — this reference does not prescribe a brand-specific dose.

These are rules-of-thumb from established reef references, not bench measurements and not a prescription for your specific tank or livestock. Salt-mix strength varies by brand — always test your own water, follow each product's instructions, and make changes slowly. This is general reef-keeping information, not veterinary advice.

The gear that holds these parameters steady — heaters, ATO and dosing — is covered in the reef stability hub. If you're sizing a heater for your tank, the best reef tank heaters guide compares titanium heaters and controllers, and the Reef Matcher sizes your ATO, skimmer and flow.

Affiliate note: our guides earn from qualifying Amazon purchases at no cost to you. Full disclosure. Specs are verified against manufacturer details and current listings — no hands-on testing claims. Dosing and parameter figures are general reef-keeping information, not veterinary advice — follow each product's label.