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How the calculation works
Specific gravity is a ratio of densities, so its temperature reference matters. This converter uses seawater density divided by pure-water density, both at 25°C and atmospheric pressure. It reuses TankForge’s tested EOS-80 conversion rather than a fixed multiplier or a lookup rounded to a few aquarium values.
The supported interval is 0–42 ppt, with SG 1 representing freshwater in this model. The inverse solves the density relation numerically. It preserves calculation precision internally and rounds only the visible answer. Values outside the model range are reported as errors rather than extrapolated.
Check the calibration solution, temperature compensation and reference stated for your refractometer or hydrometer. A reading calibrated to another reference is not automatically interchangeable. Practical salinity in ppt is the aquarium convention here; it is not a recipe for weighing dry salt into water.
Worked example
At 35 ppt, seawater density is about 1,023.343 kg/m³ and freshwater density about 997.048 kg/m³ at 25°C. Their ratio is about 1.02637 SG. Converting that unrounded ratio back returns 35 ppt.
Common questions
Why does another chart give a different SG?
Temperature reference, instrument calibration and rounding can differ. Compare the reference conventions before interpreting the difference as a tank change.
Can this tell me how many grams of salt to add?
No. Dry blends and moisture differ. Use the exact marine-salt manufacturer’s mixing directions, then measure the fully dissolved water.