Aquarium Salt Mix Calculator
Find how much marine salt to add to reach a target salinity in a saltwater or brackish aquarium of any volume.
How it works
Salt (g) = (target − current) ppt × volume (L) · volume (L) = gallons × 3.7854
Salinity in parts per thousand is almost exactly grams of salt per litre of water, because a litre of water masses about a kilogram. So the salt you must add is simply the salinity gap you want to close multiplied by the water volume in litres. Always mix salt into the water — never the reverse — dissolve it fully in a separate container of dechlorinated water, and check with a refractometer before adding it to livestock, since the exact grams-per-litre varies a little between salt brands.
Worked example
A 50-gallon tank holds 50 × 3.7854 = 189.3 L. Going from 0 to 35 ppt is a 35 ppt gap, so you need 35 × 189.3 ≈ 6,626 g — about 6.6 kg — of marine salt mix.
Frequently asked questions
How does salinity relate to specific gravity?
Roughly: 35 ppt corresponds to a specific gravity near 1.026 at 25 °C, the reef-tank target. Each 1.0 ppt is about 0.00075 in specific gravity, but temperature affects the reading, so a refractometer calibrated at 25 °C is the reliable tool.
Can I use table salt?
No. Marine aquariums need a full synthetic sea-salt mix that supplies calcium, magnesium, carbonate and trace elements. Plain sodium chloride lacks these and will not support corals or invertebrates.
Should I add dry salt straight to the tank?
Never with livestock present — undissolved salt creates dangerous local salinity spikes and can burn animals. Pre-mix and dissolve the salt in a separate bucket of RO/dechlorinated water, let it aerate, then add.
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This calculator is for educational purposes. Double-check important results before acting on them.