Density of Seawater: 1,025 kg/m³ — Salinity, Temperature, Depth, and Buoyancy

PSU = Practical Salinity Unit (approximately equivalent to grams of salt per kilogram of seawater, or ‰ parts per thousand).

Average ocean salinity is 34.5–35 PSU. The Dead Sea reaches ~340 g/kg (not PSU-scale) — far beyond normal seawater range.

Salinity is the most direct chemical factor affecting seawater density. Each 1 PSU increase in salinity raises seawater density by approximately 0.8 kg/m³.

The Dead Sea has a salinity of approximately 280–340 g/kg — roughly 10× the average ocean — giving it a density of about 1,240 kg/m³. This extreme density makes it impossible to sink, even without swimming effort.

For broader material comparisons, open the density table.

Like fresh water, seawater density decreases as temperature rises. Unlike fresh water, seawater does not have the 4°C maximum-density anomaly: dissolved salts remove that behavior, so seawater density increases steadily as it cools toward the freezing point.

Cold polar water (near −2°C) is significantly denser than warm tropical surface water (30°C) — a density difference of about 6.4 kg/m³. This density contrast drives the global thermohaline circulation (ocean conveyor belt), as cold dense water sinks at the poles and flows along the ocean floor toward the equator.

As depth increases, the weight of the water column above applies pressure to seawater, compressing water molecules more tightly and increasing density. This compression effect is almost negligible in shallow water, but becomes significant in the deep ocean (>2,000 m).

At the bottom of the Mariana Trench (10,911 m), seawater density reaches approximately 1,071 kg/m³ — about 4.5% denser than surface seawater. Despite this compression, water is still far less compressible than most solids; the volume change from surface to full ocean depth is only about 5%.

According to Archimedes' principle, the buoyant force on an object equals the weight of the fluid it displaces. Because seawater (1,025 kg/m³) is denser than fresh water (1,000 kg/m³), it exerts a greater buoyant force for the same displaced volume. An object that barely floats in fresh water will float more easily — and sit higher — in seawater. For the base concept, see what is density.

The 2.5% density difference between seawater and fresh water means that seawater provides 2.5% more buoyant force per unit volume. For comparison with floating fluids, see density of oil. For a 70 kg swimmer, the buoyant force in seawater is approximately 1.7 N greater than in fresh water — enough to make a noticeable difference in how much effort is needed to stay afloat.

The Plimsoll line (load line) on a ship's hull marks the maximum safe loading depth in different water conditions. Ships float higher in denser seawater than in fresh water or brackish water, so the Plimsoll line has multiple marks: TF (Tropical Fresh), F (Fresh), T (Tropical), S (Summer), W (Winter), and WNA (Winter North Atlantic). A ship loaded to its Summer Saltwater mark in the ocean will sit lower in the water — and may be overloaded — if it then enters a fresh water river.

The global ocean circulation — sometimes called the "ocean conveyor belt" or thermohaline circulation — is driven entirely by density differences in seawater. "Thermo" refers to temperature and "haline" refers to salinity, the two primary factors controlling seawater density. Dense water sinks; less dense water rises to replace it, creating a continuous global circulation pattern.

In the North Atlantic, warm surface water flows northward (as the Gulf Stream), releasing heat to the atmosphere and becoming cooler and saltier through evaporation. At high latitudes, this water becomes dense enough to sink to the ocean floor, forming North Atlantic Deep Water (NADW) — one of the densest water masses in the ocean at approximately 1,027.8 kg/m³.

This density-driven circulation transports enormous amounts of heat around the planet, moderating climates in Western Europe and regulating global carbon cycling. Climate scientists monitor seawater density profiles closely because freshwater input from melting ice sheets can reduce surface salinity and density, potentially disrupting the sinking mechanism that drives the conveyor belt. For the frozen-water reference behind that meltwater input, see density of ice.