Salinity converter
Convert salinity between PSU, specific gravity and conductivity — the three ways an aquarium or a research ship measures the same water.
That's about the same as the open ocean. Every liter holds about 35 g of dissolved salt.
What it means
How salty is that?
Specific gravity depends on temperature. Aquarium hydrometers are usually calibrated at 25 °C, and a reading taken cold will be wrong.
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Your salinity in every unit
Learn
Three things worth knowing
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35 PSU is 1.026 specific gravity
Every PSU adds about 0.00075 to the specific gravity. A reef tank held at 1.025 is running at 33 PSU.
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Measured by conductivity
Evaporating seawater and weighing the salt is slow and imprecise. Since 1978 salinity has been defined by conductivity against a standard potassium chloride solution.
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PSU has no unit
It is a ratio of conductivities, so strictly it is dimensionless — which is why you will see 35 PSU, 35 ppt and just plain 35 for the same water.
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Popular salinity conversions
Background
About salinity
Ocean water is about 35 PSU: thirty-five grams of salt in every kilogram. PSU is measured by conductivity and calibrated so the numbers match the old weight figures almost exactly.
Aquarists use specific gravity instead, where 35 PSU reads 1.026 on a hydrometer.