Aquarium Salt Mix Ratio Calculator

Scale your salt mix product's own recommended dosing ratio (from its label) up or down to match your target water volume — every salt mix brand has a different ratio for its target salinity, so this uses your product's specific instructions rather than assuming a fixed ratio.

Product's Recommended Amount (cups)

...per this many gallons

Your Target Water Volume (gallons)

Scaling your own label ratio

There is no universal salt-to-water figure, so the tool scales whatever your bag says:

cups needed = (label cups / label gallons) × target gallons

A label reading half a cup per gallon works out at 0.5 / 1 × 30 = 15 cups for a 30 gallon batch. The same arithmetic runs the other way: if your bag quotes 2.5 cups per 5 gallons, that is the same 0.5 cups/gal, and a 10 gallon change wants 5 cups.

Mixing the batch

Start with RO/DI or distilled water at roughly tank temperature. Tap water brings copper, chlorine, phosphate and nitrate that no salt mix removes.
Salt into water, never water onto salt, added gradually with a powerhead running. Dumping the lot in leaves a crust that dissolves for hours.
Mix short and adjust up. Undissolved salt occupies volume, so hold back the last cup, let it circulate, then bring the reading up to target with small additions.
Circulate and aerate 12-24 hours before use. A fresh mix is cloudy, off-balance in pH and often a degree or two warm from the reaction.

Cups are an approximation - salt settles and compacts, so two scoops of the same brand differ by a few percent. The measurement that counts is specific gravity, read with a calibrated refractometer at the end.

Frequently asked questions

How much salt mix do I need for 30 gallons?

15 cups at a half-cup-per-gallon label rate. Check your own bag first: brands vary by 10-15% for the same salinity, and low-salinity blends differ more.

What specific gravity should I mix saltwater to?

1.026 (35 ppt) is standard for reef tanks and 1.020-1.025 is common for fish-only systems. Match the tank you are pouring into, so a water change does not shift salinity.

Can I use table salt or freshwater aquarium salt?

No. Both are essentially sodium chloride. A marine mix supplies magnesium, calcium, carbonate and trace elements in seawater proportions, which is what makes the water usable.