The weight of any liquid, including water, is determined by its density—the amount of mass contained within a specific volume. Density is the fundamental concept for understanding why a gallon of saltwater weighs more than a gallon of freshwater. The total weight depends on the physical state of the water and the amount of dissolved substances. Consequently, water from a tropical sea will weigh differently than water from a polar ocean or a mountain lake.
Standard Weight of Seawater Per Gallon
For standardization, a U.S. liquid gallon of average ocean water is considered to weigh approximately 8.55 pounds. This figure is based on open ocean surface water with a salinity of about 35 parts per thousand, measured near 63.5 degrees Fahrenheit. In comparison, a U.S. liquid gallon of pure freshwater, measured at its maximum density, weighs about 8.34 pounds. The difference of roughly 0.21 pounds per gallon is a direct result of the dissolved salts.
This standard weight is only an average, as global surface seawater density ranges from about 1020 to 1029 kilograms per cubic meter. The higher weight of seawater compared to freshwater is due to non-water components packed into the same volume. The density of seawater is about 1.025 times greater than freshwater.
The Role of Salt Content
The primary reason saltwater weighs more than freshwater is the presence of dissolved inorganic ions, mainly sodium chloride, known as salinity. These dissolved solids, which average 35 grams for every 1,000 grams of seawater, increase the overall mass without significantly changing the water’s volume. Adding mass to the same fixed volume increases the density, making a gallon of salty water heavier.
Salinity is the most significant variable affecting weight and is not uniform across all bodies of water. For example, in the Baltic Sea, high freshwater runoff results in salinity as low as 7.21 parts per thousand, making the water lighter. Conversely, in highly evaporative bodies like the Red Sea, salinity can rise to nearly 40 parts per thousand, resulting in a much heavier gallon of water.
How Temperature Changes the Weight
Temperature acts as a secondary influence on the weight of saltwater through thermal expansion. As seawater warms, its molecules move faster and spread farther apart, causing the water to expand and become less dense. Consequently, a gallon of warm surface water in the tropics will weigh slightly less than a gallon of colder water from the deep ocean or polar regions.
Conversely, as seawater cools, the molecules move closer together, causing the water to contract and become denser and heavier per gallon. This contraction continues until the water freezes, a point lower than freshwater due to the dissolved salts. In many parts of the open ocean, the density effect from temperature changes often outweighs the effect of salinity on the water’s density.
Why This Weight Difference Matters
The difference in density between saltwater and freshwater has practical consequences, most notably in the principle of buoyancy. Since denser seawater exerts a greater upward buoyant force, large cargo ships float noticeably higher in the ocean than they do in a freshwater river or lake. This also explains why swimming in the ocean feels easier and why people float more effortlessly in highly saline bodies of water like the Dead Sea.
On a global scale, density differences drive the massive, slow-moving currents of the deep ocean, referred to as thermohaline circulation. Cold, salty, and dense water in polar regions sinks and moves along the ocean floor. This circulation distributes heat and nutrients throughout the world’s oceans.