For freshwater, a single cubic foot weighs approximately 62.4 pounds. A cubic foot is a standard unit of volume defined as a cube with sides that are each one foot long. This weight-to-volume relationship, known as density, is a fundamental measurement used in applications ranging from engineering to household planning.
The Standard Measurement
The precise figure of 62.4 pounds per cubic foot (62.4 lbs/ft³) is based on a specific physical property of pure water. This measurement is taken at the point of water’s maximum density, which occurs at a temperature of about 39.2°F (4°C). At this temperature, the water molecules are packed most closely together, resulting in the greatest mass for a given volume. Density itself is defined as mass per unit volume. The 62.4 lbs/ft³ figure is the Imperial system conversion of this maximum metric density value, serving as the standard reference point for water weight calculations.
Factors Affecting Water Weight
The weight of a cubic foot of water is not constant and can change due to two main environmental factors: temperature and salinity. The effect of temperature is unique for water, as its density decreases when it is either heated above 39.2°F or cooled below it. For example, water at room temperature (about 68°F or 20°C) weighs slightly less, closer to 62.3 lbs/ft³.
When water freezes into ice at 32°F (0°C), it expands considerably, forming a crystalline structure that is less compact than its liquid state. This expansion causes a significant drop in density, resulting in a cubic foot of ice weighing only about 57.2 pounds. The presence of dissolved salts also changes the density of the water. Saltwater is denser because the dissolved compounds add mass to the same volume, meaning a cubic foot of average seawater weighs more than freshwater, typically around 64.0 to 64.3 pounds.
Practical Applications
Knowing the specific weight of a cubic foot of water is necessary for structural and capacity planning scenarios. For homeowners, this figure is crucial when calculating the total load of a large object like an aquarium or a hot tub placed inside a home. A 100-gallon aquarium contains about 13.4 cubic feet of water, translating to a water weight of over 830 pounds, not including the tank or decorations.
This calculation allows a person to determine if the existing floor joists or foundation can safely support the weight before installation. In civil engineering, the figure is used for large-scale projects, such as designing water storage tanks and reservoirs. Engineers rely on the precise density to calculate the hydrostatic pressure exerted by water, which is fundamental to ensuring structural integrity.