Specific gravity, ppt, and conductivity, with the temperature correction that most conversion tables leave out.
| Salinity | S.G. 25/25 | mS/cm at 25 °C | Notes |
|---|---|---|---|
| 30 ppt | 1.0226 | 46.2 | Low, fish only range |
| 33 ppt | 1.0249 | 50.4 | Bottom of the reef range |
| 35 ppt | 1.0264 | 53.1 | Natural seawater |
| 36 ppt | 1.0271 | 54.4 | Top of the reef range |
Reef tanks are normally held between 33 and 35 ppt, which is 1.0248 to 1.0264 specific gravity at 25 °C. Stability matters more than the exact figure. Pick one and hold it.
density from the UNESCO EOS-80 equation of state, salinity from PSS-78 → 35.036 ppt
Specific gravity is a ratio of two densities, so it means nothing until you say at what temperature each was measured. Water expands as it warms. The same salt water reads a different specific gravity at 22 °C than at 27 °C, and the reference water matters just as much as the sample.
This calculator uses the Millero and Poisson 1981 one atmosphere equation of state, the density equation behind UNESCO EOS-80. At 35 ppt and 25 °C it gives 1023.343 kg/m³ against 997.047 for pure water, so specific gravity is 1.02637. That is where the familiar 1.0264 comes from, and it only holds at 25 °C referenced to 25 °C.
The same water on a swing arm hydrometer calibrated at 60 °F reads about 1.0244, because the reference water is colder and therefore denser. Neither number is wrong. They answer different questions.
Conductivity comes from PSS-78, the practical salinity scale. Standard seawater at 35 and 15 °C has a conductivity of 42.914 mS/cm by definition, and the same water at 25 °C reads about 53.1 mS/cm. Digital salinity meters measure conductivity and convert, which is why they need a temperature probe to be trusted.
Refractometers have a separate problem. Most cheap ones are built for brine and graduated on a sodium chloride scale, and seawater bends light slightly differently from an equally salty NaCl solution. A brine refractometer reads roughly one to one and a half ppt high on real seawater. Buy one sold as a seawater or reef refractometer, and calibrate it with 35 ppt standard solution rather than RO water.
Automatic temperature compensation on a refractometer corrects the prism, not the fluid. It buys you accuracy across roughly 10 to 30 °C and does not remove the need to calibrate.
Salinity units: ppt, PSU, and practical salinity are used interchangeably in this hobby and the differences are far below test error. Treat them as the same number.