PSU stands for Practical Salinity Unit, a measure of how salty a body of water is. It’s based on the electrical conductivity of a water sample rather than directly weighing the dissolved salts. Average ocean surface salinity is about 35 PSU, meaning roughly 35 grams of dissolved salt per kilogram of seawater.
How PSU Is Defined
PSU comes from the Practical Salinity Scale of 1978 (PSS-78), an international standard developed to give oceanographers a consistent way to report salinity worldwide. Instead of measuring the actual mass of salt dissolved in water, PSS-78 measures how well a water sample conducts electricity relative to a reference solution.
That reference is a standard potassium chloride solution. When a seawater sample conducts electricity at the same rate as this reference solution, both measured at 15 °C and normal atmospheric pressure, the salinity is defined as exactly 35. The result is technically dimensionless: it’s a ratio of two conductivity measurements, so there are no true “units” attached. Oceanographers still write “PSU” or “PSS” after the number as a practical label, even though purists consider this technically incorrect.
Why Conductivity Instead of Weighing Salt
Before PSS-78, salinity was reported in parts per thousand (ppt), representing kilograms of salt per kilogram of water. The problem was consistency. Different oceans have slightly different mixes of dissolved ions, and the equations used to convert chemical measurements into ppt were calibrated with samples from various regions, making results hard to compare. Salinometer work also suffered from inconsistent reference standards.
The switch to a conductivity-based system solved this by tying everything to a single, reproducible standard. A conductivity ratio can be measured quickly and precisely on a ship or in a lab without evaporating water and weighing residue. The tradeoff is that PSU captures the conductivity signature of one assumed ion mix (North Atlantic seawater) rather than the true mass of dissolved material. In most of the ocean the numbers are nearly identical to the old ppt values, but in unusual water bodies like the Baltic Sea, where the ion composition differs, PSS-78 can be off by small but measurable amounts on the order of 0.02.
How Salinity Is Measured in Practice
The workhorse instrument for salinity measurement is the CTD sensor, which stands for Conductivity, Temperature, and Depth. A shipboard CTD consists of a set of small probes mounted on a metal frame called a rosette. The rosette is lowered on a cable from the ship to the seafloor, streaming data back in real time through a conducting cable to a computer on deck. Scientists watch the salinity profile build as the instrument descends, revealing layers of saltier or fresher water.
Smaller, low-power versions of these sensors ride on autonomous platforms like underwater gliders, profiling floats, and moored instruments that move up and down a cable. All of them work the same way: they measure how well the surrounding water conducts electricity, record the temperature and pressure at that moment, then calculate practical salinity from those three inputs. Temperature and pressure matter because both change how easily water conducts electricity independent of how much salt is present.
Typical Salinity Ranges
Open ocean surface salinity generally falls between 32 and 37 PSU, with a global average around 35. The subtropics tend toward the higher end because intense evaporation concentrates salts, while regions near the equator or at high latitudes can be fresher due to heavy rainfall or melting ice.
Estuaries and coastal waters show much wider swings. A tidal estuary might range from nearly 0 PSU at its freshwater headwaters to over 30 PSU near its mouth. Brackish seas like the Baltic sit well below open-ocean values. At the other extreme, hypersaline environments push far beyond 35. The Dead Sea, for example, averages about 300 PSU, and its evaporation ponds reach 350 to 384 PSU, roughly ten times saltier than the ocean.
Why PSU Matters for Marine Life
Salinity directly controls the movement of water across cell membranes. Fish, shellfish, and invertebrates all maintain internal salt concentrations within a narrow range, so a sudden change in surrounding salinity forces their bodies to work harder to keep that balance. This is why many marine species can’t survive in fresh water, and vice versa.
Research on intertidal invertebrates along the Pacific and Atlantic coasts illustrates the range of tolerance. In experiments that gradually lowered salinity from 25 PSU down to zero, different species hit their lethal thresholds at very different points. Periwinkle snails from the genus Littorina survived to salinities below 2 PSU, while predatory dog whelks died at around 5 PSU. Acorn barnacles fell somewhere in between, with high individual variability. These thresholds help ecologists predict which species will persist as climate change alters rainfall patterns and freshwater runoff into coastal waters.
PSU vs. Absolute Salinity
Since 2010, the international oceanographic community has been shifting toward a newer framework called TEOS-10 (Thermodynamic Equation of Seawater 2010). Its key innovation is Absolute Salinity, expressed in grams per kilogram (g/kg), which is a true SI unit of concentration representing the actual mass of dissolved material in seawater.
The reason for the change is that practical salinity, being based purely on conductivity, misses dissolved substances that don’t conduct electricity well, like silica. It also assumes the same ion ratios everywhere. Absolute Salinity includes correction procedures that account for regional differences in seawater composition. In the deep Pacific, for instance, PSS-78 salinity can introduce density errors of up to 0.02 kg/m³, enough to matter for climate modeling and deep-water circulation studies.
In practice, instruments still measure conductivity and compute practical salinity first. Absolute Salinity is then calculated from that value plus a regional correction. So PSU hasn’t disappeared from daily oceanographic work. You’ll still see it in published data, aquarium guides, fisheries reports, and environmental monitoring, and for most purposes it remains a reliable and intuitive way to describe how salty water is.