About this tool
The Salinity & Specific Gravity Converter is a free, in-browser tool that translates any one of the three ways marine aquarists measure salt content — salinity in ppt, specific gravity, or conductivity in mS/cm — into the other two. It uses the standard 25 °C/77 °F reference where 35 ppt = SG 1.0264 = 53.0 mS/cm. Enter one value plus the sample temperature and it outputs the rest at the 25 °C reference. Everything is computed locally in your browser, so nothing you enter is uploaded.
The practical relations are salinity (ppt) = (SG_25 − 1) ÷ 0.000754, its inverse SG_25 = 1 + ppt × 0.000754, and conductivity (mS/cm) ≈ ppt × 1.514 across the seawater range of about 25–40 ppt. Glass hydrometers are calibrated at a fixed temperature (usually 25 °C), and seawater density falls by roughly 0.00025 SG per °C as it warms, so a hydrometer reading taken warm reads low. The tool corrects specific-gravity input to the 25 °C reference with SG_25 = SG_measured + 0.00025 × (T_sample − T_calibration) before converting. It also flags target bands: fish-only tanks run about 30–34 ppt and reef tanks about 33–35 ppt.
As a worked example, a swing-arm hydrometer reads SG 1.026 in a sample at 27 °C, calibrated at 25 °C. The corrected value is 1.026 + 0.00025 × (27 − 25) = 1.0265, which is (1.0265 − 1) ÷ 0.000754 ≈ 35.1 ppt and about 53.2 mS/cm — solidly in the reef range. Note that refractometers and conductivity meters with automatic temperature compensation already report the 25 °C value, so for those instruments leave the sample temperature at the calibration temperature.