Salinity Converter: Specific Gravity ↔ ppt
Hydrometers and many refractometers read specific gravity (SG), while salinity targets are usually quoted in parts per thousand (ppt or ‰). This converter translates between the two using the standard 25 °C relationship, and lists target ranges for brackish, marine and salt-dip applications.
Convert salinity
| Application | SG (25 °C) | ppt |
|---|---|---|
| Freshwater | 1.000 | 0 |
| Low-end brackish (mollies, bumblebee gobies) | 1.004–1.008 | 5–12 |
| High-end brackish (monos, scats, puffers) | 1.010–1.018 | 14–24 |
| Natural seawater / reef | 1.023–1.026 | 31–35 |
How the conversion works
Specific gravity is the density of your water relative to pure water. Dissolved salt raises density almost linearly across the aquarium range, so a simple linear fit converts between the two scales. This tool is anchored to the oceanographic reference point that natural seawater at 35 ppt has a specific gravity of about 1.0264 when measured at 25 °C, which gives:
Across 0–40 ppt this linear approximation tracks the full UNESCO equation-of-state tables to within about half a ppt — tighter than the reading error of a typical swing-arm hydrometer.
Temperature is the hidden variable
Density changes with temperature, so every SG instrument has a calibration temperature — and instruments differ. Many aquarium hydrometers are calibrated at 25 °C (77 °F); some older ones use 15.6 °C (60 °F). Reading a 60 °F-calibrated hydrometer in a 25 °C reef tank overstates the correction by roughly 0.001–0.002 SG — a meaningful error at reef salinity. Check the fine print on your instrument, and if you use a refractometer, calibrate it at room temperature with RO/DI water (or better, a 35 ppt reference fluid).
Practical accuracy advice
- Swing-arm hydrometers are convenience-grade: bubbles on the arm and salt creep commonly shift readings by 0.001–0.002. Rinse in fresh water after each use and tap bubbles off the arm.
- Refractometers are the hobby standard for marine tanks — but only as good as their calibration. Recalibrate monthly.
- Consistency beats absolute accuracy. Livestock tolerate 33 ppt vs 35 ppt far better than they tolerate salinity that swings daily. Top up evaporation with fresh water (salt doesn’t evaporate), ideally with an auto top-off.
- Mix salt outside the tank, dissolve fully, heat and aerate before adding — never add dry salt mix directly to a stocked tank.
Frequently asked questions
Are ppt and PSU the same thing?
For hobby purposes, yes — the Practical Salinity Unit scale was designed so seawater values are numerically almost identical to g/kg (ppt). Scientific literature uses PSU; salt mix instructions use ppt; you can treat them interchangeably at aquarium precision.
What salinity for a fish-only marine tank vs a reef?
Reefs are typically run at natural seawater strength, 34–35 ppt (SG ~1.025–1.026). Fish-only systems are often kept slightly lower, around 30–33 ppt. Corals and invertebrates are far less forgiving of low or unstable salinity than fish are.
Can I use this for salt dips or livebearer brackish setups?
Yes — the conversion holds at low salinities. But treatment dosing (e.g. salt baths for parasites) should follow a trusted disease-treatment reference or an aquatic veterinarian’s guidance, not just a target SG. Concentration and exposure time both matter, and some species (scaleless catfish, many plants and inverts) tolerate very little salt.
Why does my reading change after the water cools?
Because density rises as water cools. A sample that reads SG 1.024 at 25 °C will read higher once it cools on the bench. Always read at a consistent, known temperature — ideally tank temperature immediately after sampling.