At 25 °C, a specific gravity of 1.0264 is 35 ppt, and 1.026 is about 34.5 ppt. Refractometers and most digital meters report on that 25 °C basis, so leave both temperatures at 25. For a hydrometer, enter the water temperature and the temperature it’s calibrated for.
Using a refractometer or a digital meter? Leave both temperatures at 25 °C: they report specific gravity on that basis. For a hydrometer, enter the water’s temperature and the temperature printed on the hydrometer.
Log 34.5 ppt in my Companion →Opens your free Reef Companion with the result filled in. No account needed.
Why temperature matters
Specific gravity compares how dense your water is with pure water. Warm water is less dense than cold water, so the same seawater gives a lower specific gravity when it’s warmer. A salinity in ppt doesn’t change with temperature, which is why it’s the better unit to track.
That’s why this converter asks for two temperatures: the sample’s temperature and the reference temperature the instrument compares against.
Which temperatures to enter
Pick the line that matches your instrument.
| Instrument | Water temperature | Reference temperature |
|---|---|---|
| Refractometer with ATC | 25 °C | 25 °C |
| Digital salinity meter showing SG | 25 °C | 25 °C |
| Hydrometer marked 15.6 °C / 60 °F | Your water’s temperature | 15.6 °C |
| Hydrometer marked 25 °C / 77 °F | Your water’s temperature | 25 °C |
If your hydrometer has no calibration temperature printed on it, check the maker’s instructions.
A hydrometer example
A hydrometer calibrated at 15.6 °C reads 1.023 in a tank at 26 °C. Treating 1.023 as a 25 °C reading would suggest 30.5 ppt. With the right temperatures, it’s about 33.6 ppt. The difference is more than 3 ppt, enough to change what you do next.
Specific gravity and ppt at 25 °C
For quick reference, here are common readings on the basis refractometers use.
| Specific gravity | Salinity (ppt) |
|---|---|
| 1.022 | 29.2 |
| 1.023 | 30.5 |
| 1.024 | 31.9 |
| 1.025 | 33.2 |
| 1.026 | 34.5 |
| 1.0264 | 35.0 |
| 1.027 | 35.8 |
Get a reading you can trust
Calibrate a refractometer with a seawater standard, ideally monthly. Bulk Reef Supply explains that many hobby refractometers are designed for salt-in-water brine, and a seawater standard corrects the small difference. Read more in our salinity guide.
Common questions
What is 1.025 specific gravity in ppt?
At 25 °C, about 33.2 ppt.
What specific gravity is 35 ppt?
About 1.0264 at 25 °C, usually rounded to 1.026.
Does my refractometer need a temperature correction?
Not if it has ATC (automatic temperature compensation), which most do. Leave both temperatures at 25 °C.
Sources and limits
Last checked 12 October 2026. Calculated with the UNESCO 1981 equation of state for seawater (EOS-80), valid for normal seawater. Results are only as good as your instrument and its calibration.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: how to use a refractometer