Salinity Converter

Salinity measures the dissolved salt content in water.

Common units:

• ppt (parts per thousand): grams of salt per kilogram of water
• PSU (Practical Salinity Units): dimensionless, numerically equal to ppt
• Specific Gravity (SG): density compared to pure water (1.000)
• Conductivity (mS/cm): electrical conductivity at 25°C

Conversions (approximate):

• ppt ≈ PSU (for practical purposes, they are interchangeable)
• SG ≈ 1 + (ppt × 0.00075) (at 25°C, so 35 ppt ≈ SG 1.026)
• Conductivity ≈ ppt × 1.514 mS/cm (anchored to standard seawater: 35 PSU = 53.0 mS/cm at 25°C)

Reference values:

These conversions are approximate. Exact values depend on temperature, pressure, and ionic composition. The linear formulas hold in the 0–40 ppt range. At brine extremes like the Dead Sea both of them break down, and in opposite directions: run 340 ppt through the specific gravity formula and it says 1.255 against a measured 1.240, while the conductivity formula says over 500 mS/cm against a measured 192, because conductivity peaks and then falls once the ions start crowding each other. That row is reference only, and the converter will happily give you the wrong answer for it.

The unit you reach for depends on what you’re doing. Oceanographers and lab work use PSU, which is derived from conductivity and is what modern instruments actually measure. Aquarists tend to think in specific gravity, because the cheap swing-arm hydrometer reads it directly, while ppt is the plain grams-of-salt-per-kilogram figure most people find intuitive. They’re close cousins: seawater sits near 35 ppt, 35 PSU, and 1.025 specific gravity all at once.

Specific gravity is the one to be careful with, because it shifts with temperature. A hydrometer calibrated for 25°C reads too low in a warmer tank, which fools people into thinking the water is fresher than it is, so they add salt and push the true salinity too high, stressing the livestock. That’s exactly why serious reef keepers switch to a refractometer, which is far less temperature-sensitive and far more repeatable. Whatever tool you use, calibrate it and read at a consistent temperature; a salinity number without a known temperature is only half a measurement.