Salinity Converter: SG, ppt, Conductivity

All three describe how much salt is in the water. Salinity in parts per thousand (ppt) is grams of salt per kilogram of water. Specific gravity (SG) compares the water’s density with pure water, and it rises as salt is added. Conductivity is how well the water carries an electric current, which depends on its dissolved ions.

Natural seawater is about 35 ppt, which is a specific gravity of about 1.026 and a conductivity of about 53 mS/cm at 25 °C (77 °F). Most reef tanks are kept between about 1.025 and 1.026.

A floating hydrometer measures density, and water gets denser as it cools. The same water therefore reads higher when it is cold and lower when it is warm. Hydrometers are calibrated for one temperature, usually 77 °F (25 °C), and are only accurate there.

This calculator takes your reading and water temperature, works back to the actual salinity, and gives you the corrected specific gravity at 77 °F. It also shows what the same water would read at other temperatures.

A refractometer measures how much the water bends light, which rises with dissolved salt. Its SG and ppt scales are two labels for the same reading, so you can read either.

• Calibrate with a 35 ppt solution (SG 1.026, about 53 mS/cm), not only with RODI. Calibrating on pure water can leave a reading off at reef salinity, and the 35 ppt fluid sets the range you actually use.
• Let the sample match the instrument. Automatic temperature compensation (ATC) corrects for temperature, but only works properly over a limited range, typically around 10 to 30 °C, and when the sample, prism and cover have settled to the same temperature.
• Use a drop, wipe the prism between readings and read the edge in good light. Salt crystals and a smear of water both shift the line.
• Check it against a fresh 35 ppt standard every few weeks. If it has drifted, recalibrate.

Because a refractometer reads light rather than density, you can enter its reading here as specific gravity or ppt without a temperature correction. Use the temperature fields only for a hydrometer.

A conductivity meter reads ions rather than density. Seawater at 35 ppt is about 53 mS/cm at 25 °C, but conductivity climbs steeply with temperature, so most meters compensate to 25 °C automatically. Tick that option if yours does. If you read a raw value at the water’s own temperature, untick it and enter that temperature. Calibrate with a standard solution close to the range you measure.

Density comes from the UNESCO 1981 equation of state for seawater, and conductivity from the Practical Salinity Scale of 1978, the same standards oceanographers use. They are fitted for 2 to 42 ppt and 0 to 40 °C, so the results are most reliable inside that range. Here, ppt means practical salinity, which for aquarium purposes is grams of salt per kilogram of water.

Many reef charts round 35 ppt to exactly 1.026. The physical value at 77 °F is 1.0264. At 1.025 that gives 33.2 ppt here, while charts that fix 35 ppt at 1.026 show about 33.7 ppt. The Salt mix calculator uses the chart convention.