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.
The one thing to remember: salinity (in ppt) is the real target — specific gravity is just a proxy that shifts with temperature and with your instrument's reference. So always know your instrument's reference temperature, and calibrate a refractometer with RO/DI (0) or calibration fluid before you trust it.
An honest tool, not a promise. Every value is anchored to Randy Holmes-Farley's published figures (S=35 = 1.0264 at 77 °F; ~1300 ppt per SG unit; seawater density vs temperature), and quoted to about ±0.0002 SG / ±0.5 ppt — finer than any hobby hydrometer. Different salt mixes and ion balances shift this a hair; your calibrated refractometer has the final say.
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.