Free tool
Salinity & specific-gravity converter
Specific gravity, ppt, a hydrometer read at the wrong temperature — the most muddled number in the hobby. Convert between them, see if you're on the 35 ppt reef target, and correct a glass-hydrometer reading for temperature — the bit most converters quietly skip.
Sources & method
- Randy Holmes-Farley — "Specific Gravity: Oh How Complicated!" (reference-temperature SG values, salinity↔SG slope, seawater density vs temperature, the temperature-correction method).
- Reef target: natural seawater ≈ 35 ppt (SG ≈ 1.0264 at 25 °C).
A free Coralkin tool — no sign-up, nothing saved. It does the conversion; your calibrated instrument has the final say. We never guess a care number.
Salinity ⇄ specific gravity quick reference
Specific gravity shown at the 25 °C / 77 °F reference (most reef refractometers & Tropic Marin). The ~35 ppt reef-ideal band is highlighted.
| Salinity (ppt) | Specific gravity (SG) | |
|---|---|---|
| 30 | 1.0226 | |
| 32 | 1.0241 | |
| 33 | 1.0249 | reef-ideal |
| 34 | 1.0256 | reef-ideal |
| 35 | 1.0264 | reef-ideal |
| 36 | 1.0272 |
Quick answers
How do I convert specific gravity to ppt (salinity)?
Anchor to natural seawater: a salinity of 35 ppt reads as specific gravity 1.0264 at the 25 °C / 77 °F reference most reef refractometers and Tropic Marin use. From there, every 0.001 change in specific gravity is about 1.3 ppt (Randy Holmes-Farley's cited slope of ~1300 ppt per 1.000 SG). So SG 1.025 is roughly 33.5 ppt at that reference. Enter your reading and this converter does it exactly.
What specific gravity should a reef tank be?
Aim for natural-seawater salinity of about 35 ppt, which is specific gravity ≈1.0264 at 25 °C / 77 °F. Salinity in ppt is the real target; specific gravity is just a temperature-dependent proxy. Corals and inverts are happiest at ~35 ppt — hold it steady rather than chasing an exact SG number.
Why does the same salinity show a different specific gravity?
Because specific gravity is the density of your sample divided by the density of pure water at a reference temperature, so it shifts with which reference your instrument uses. A salinity of 35 reads 1.0264 at 77 °F / 25 °C, 1.0266 at 68 °F / 20 °C, and 1.0269 at 60 °F / 15.6 °C (many lab glass hydrometers). Always know your instrument's reference temperature.
How do I correct a hydrometer reading for temperature?
A glass hydrometer only reads true at its calibration temperature; warmer or cooler water throws it off. The correction multiplies your reading by the ratio of seawater density at the calibration temperature to density at the sample temperature (Randy Holmes-Farley's method) — this tool does it for you. It only works if you KNOW your hydrometer's calibration temperature; a correction with the wrong reference is worse than none. Refractometers with automatic temperature compensation handle this for you.
Should I calibrate my refractometer?
Yes — calibrate a refractometer with RO/DI water (which should read 0) or with calibration fluid before you trust it. Salinity (ppt) is the number that matters, and your calibrated instrument has the final say; different salt mixes and ion balances shift specific gravity by a hair.