Salt Mix Calculator
Estimate how much salt mix you need to bring fresh RO/DI water to your target salinity, or to nudge an existing tank up from its current reading.
Assumes a typical reef mix at ~36.5 g/L for 35 ppt; brands range roughly 35–38 g/L, so verify the result with a refractometer. Never add dry salt directly to a stocked tank — mix it in a separate container, let it dissolve fully, and match temperature before use. With livestock present, raise salinity slowly, no more than about 1 ppt per day.
How much salt is really in a batch
Manufacturer instructions converge on roughly 36.5 grams of dry mix per liter of water to reach 35 ppt (about half a cup per gallon), but the real number varies from roughly 35 to 38 g/L between brands and even between batches, because salt mix absorbs moisture from the air over time. That's why the output here is a starting estimate, not a recipe: mix, let it dissolve and circulate for several hours, then measure with a calibrated refractometer and adjust. Cup measurements are rougher still — packing density differs enough between brands that weight is always the better measure.
The safety rules matter more than the arithmetic. Never pour dry salt into a tank with animals in it: undissolved grains create pockets of extreme salinity that burn fish and invertebrate tissue on contact. Mix in a separate container, dissolve fully, and match temperature before adding. When raising the salinity of a stocked tank, go slowly — about 1 ppt per day at most — because invertebrates in particular handle a gradual climb far better than a sudden jump. Dropping salinity can go somewhat faster, but the same principle of small steps applies.