How it works
salinity (ppt) = (SG₂₅ − 1) × 1325.8, with SG₂₅ corrected for temperature
Salinity is the total dissolved salt in the water, and specific gravity is how dense the water is compared with pure water. The two are tied together by the physical properties of seawater, standardised internationally as the Practical Salinity Scale of 1978 and its successor TEOS-10. Natural seawater has a salinity of 35 parts per thousand and a specific gravity of about 1.0264 at 25 degrees Celsius, and salinity rises almost linearly with specific gravity in the aquarium range, so the calculator uses salinity equals specific gravity minus one, times about 1,325.8. Temperature matters because water expands as it warms, so a warm sample is less dense and a hydrometer or refractometer reads low. Most instruments are calibrated at 25 degrees Celsius, which is 77 Fahrenheit, so the calculator first corrects your reading to that reference using water thermal expansion of roughly 0.00025 specific gravity units per degree Celsius before converting to salinity. It then compares the corrected figure with the target band for your husbandry mode: reef tanks want 1.024 to 1.026, fish-only-with-live-rock tolerate 1.019 to 1.025, brackish setups sit at 1.005 to 1.012, and hyposalinity for treating marine ich runs around 1.008 to 1.010. If you are below the target, it works out the marine salt to mix in from the salinity gap times your water mass; if you are above it, it works out the fresh reverse-osmosis water to add to dilute down. Always change salinity slowly, over hours to days, because sudden swings stress invertebrates and fish far more than a steady value slightly off target.
Sources
- UNESCO Technical Papers in Marine Science No. 44 (1983) Fofonoff & Millard, algorithms for the Practical Salinity Scale 1978 (PSS-78) and the equation of state of seawater — the salinity↔density basis.
- TEOS-10 — Thermodynamic Equation of Seawater 2010 The current international standard for seawater salinity and density, defining reference seawater at 35 g/kg.
- USGS Water Science School — Water Density Water density falls as temperature rises, which is why a specific-gravity reading must be corrected to the instrument calibration temperature.
FAQ
What is the ideal salinity for a reef tank?
Reef tanks with corals and invertebrates do best at natural seawater levels, about 35 parts per thousand, which is a specific gravity of roughly 1.024 to 1.026 at 25 degrees Celsius. Fish-only tanks can run a little lower, around 1.019 to 1.025. Keeping salinity stable is more important than hitting an exact number, so pick a value in range and hold it steady with regular top-offs.
How do I convert specific gravity to salinity?
In the aquarium range, salinity in parts per thousand is close to the specific gravity minus one, multiplied by about 1,326, once the reading is corrected to 25 degrees Celsius. So a corrected specific gravity of 1.025 is roughly 33 parts per thousand, and 1.0264 is the natural seawater value of 35. This calculator does the temperature correction and the conversion for you.
Why does temperature affect my hydrometer reading?
Water expands as it warms, so warm water is less dense and a hydrometer floats lower, reading a lower specific gravity than the true value at the calibration temperature. Refractometers are affected too unless they are automatic-temperature-compensated. Most instruments are calibrated at 25 degrees Celsius, so measuring warmer water without correcting under-reads salinity — which is exactly the correction this calculator applies.
How much salt do I add to raise salinity?
Roughly, each part per thousand you need to raise salinity takes about one gram of marine salt per litre of water, because salinity is grams of salt per kilogram of solution. The calculator multiplies the salinity gap by your water volume to give the exact amount. Add salt gradually, pre-dissolved in some tank water, and re-test after it mixes rather than dumping it in at once.
What is hyposalinity and when is it used?
Hyposalinity is deliberately lowering the salinity, to around 1.008 to 1.010 specific gravity, in a bare quarantine tank to help treat the marine parasite Cryptocaryon irritans, or marine ich. It is a treatment protocol for fish-only quarantine, never for a reef tank, because invertebrates and corals cannot tolerate low salinity. Lower and raise it slowly, and follow a documented protocol from a veterinary or extension source.
Are hydrometers or refractometers more accurate?
Refractometers are generally more accurate and repeatable than swing-arm hydrometers, which are cheap but prone to bubbles and drift. Whichever you use, calibrate it: refractometers against a known reference solution, and treat hydrometer readings with some caution. Rinse the instrument in fresh water after each use. Because both drift over time, re-check calibration periodically rather than trusting an old reading.
Salinity is estimated from specific gravity using a standard-seawater relationship and a first-order temperature correction; hobby instruments vary, so calibrate against a reference solution. Change salinity slowly. Hyposalinity is a quarantine protocol, not reef guidance. General aquarium guidance, not veterinary advice.