Convert between salinity in parts per thousand (ppt) and specific gravity (SG). Pick the reference temperature your instrument uses. Most reef refractometers and hobby charts use 77 °F (25 °C).
Salinity to specific gravity table
| Salinity (ppt) | SG at 77 °F (25/25) | SG at 60 °F (60/60) |
|---|---|---|
| 30 | 1.0226 | 1.0230 |
| 31 | 1.0233 | 1.0238 |
| 32 | 1.0241 | 1.0245 |
| 33 | 1.0249 | 1.0253 |
| 33.5 | 1.0252 | 1.0257 |
| 34 | 1.0256 | 1.0261 |
| 34.5 | 1.0260 | 1.0265 |
| 35 | 1.0264 | 1.0269 |
| 35.5 | 1.0268 | 1.0272 |
| 36 | 1.0271 | 1.0276 |
How it's calculated
Specific gravity here is the density of seawater divided by the density of pure water at the same temperature. Both densities come from the international one-atmosphere equation of state of seawater (EOS-80), as published in Fofonoff & Millard (1983), Algorithms for Computation of Fundamental Properties of Seawater, UNESCO Technical Papers in Marine Science 44. Salinity from SG is found by solving that equation backwards.
Calibrate refractometers with a 35 ppt reference solution, not RO/DI water, and read at the temperature the instrument is compensated for. See how to calibrate a refractometer and salinity and RO/DI basics.
FAQ
What specific gravity is 35 ppt? About 1.0264 at 77 °F (25 °C), using the international equation of state of seawater with SG measured against pure water at the same temperature. At the 60 °F reference some hydrometers use, it is about 1.0269.
What salinity should a reef tank be? Most reefers aim for natural seawater, about 35 ppt (roughly 1.026 SG at 77 °F). Many keep 33.5 to 35 ppt. Stability matters more than hitting an exact number.
Why does my hydrometer disagree with my refractometer? Specific gravity depends on temperature and on the reference temperature the instrument was made for. A refractometer calibrated with 35 ppt standard (not RO/DI water) is usually the more reliable hobby tool.