Saltwater Aquarium Salt Mix Calculator

Saltwater aquariums require precise salinity levels to keep marine fish, corals, and invertebrates healthy.
Salinity is controlled by dissolving commercial marine salt mix into purified water, meaning RO/DI (reverse osmosis and deionization). The amount of salt needed depends on the water volume, the target salinity, and the brand of mix, since each brand has a slightly different density.

Salinity Measurement

Salinity is measured in three interchangeable ways:

FOWLR = Fish Only With Live Rock (fish are more tolerant of salinity variation than corals).

Salt Mix Formula

Salt (cups) = Water Volume (gallons) × Salt Rate (cups/gallon)

These rates are approximate. Different salt mix brands vary by 5–10%. Always measure with a refractometer after mixing.

Mixing Process

1. Fill container with RO/DI water at target temperature (75–78°F / 24–26°C)
2. Add salt mix gradually while stirring with a powerhead or pump
3. Mix for 15–30 minutes until fully dissolved and clear
4. Measure salinity with a calibrated refractometer
5. Adjust: add more salt to raise SG, or more RO/DI water to lower it
6. Aerate for 8–24 hours before adding to the tank (allows pH to stabilize and dissolved gases to equalize)

Worked Example: 75-Gallon Reef Tank, SG 1.025

Water volume: 75 gallons (284 liters). Salt rate: 0.54 cups/gallon. Salt needed: 75 × 0.54 = 40.5 cups, which is roughly 22 lbs or 9.9 kg, since marine salt runs about 245 g per cup.

For a 10% water change on this tank: 7.5 gallons × 0.54 = 4.05 cups of salt mix.

Water Change Salt Calculations

Regular water changes are the foundation of reef tank maintenance. Typical schedule:

Temperature Affects Salinity Reading

Specific gravity readings change with temperature. A refractometer calibrated at 77°F (25°C) will read incorrectly if the water is significantly warmer or cooler. Always measure at the calibration temperature or use a temperature-compensating refractometer (most modern ones do this automatically).

Correcting salinity, which is a different problem

Choosing “Salinity Correction” changes what the calculator does, and it has to, because the answer is not the same shape. Mixing a new batch of water is a question of how much salt goes into an empty container. Correcting a tank that has drifted low is a question of how much salt to add to water that already has salt in it, and the two differ by an enormous factor: a 75 gallon reef that has slipped from 1.025 to 1.023 needs about 3.5 cups, not the 40 cups a fresh mix of that volume would take.

So in correction mode the calculator asks for your current reading and works from the gap. It also holds you to the safe rate: never raise salinity by more than 0.002 SG in a day. A correction of more than that is a multi-day job, and the calculator says how many days.

The relationship it uses across this range is close enough to linear to work with directly: each 0.001 of specific gravity is worth about 1.5 g of salt per liter, which you can read straight off the rate table above.

Why RO/DI Water

Never mix salt with tap water. Tap water contains chlorine, chloramine, phosphates, silicates, and heavy metals that promote algae growth and harm corals. RO/DI systems remove 95–99% of dissolved solids. If you cannot get an RO/DI unit, purchase RO water from your local fish store.