The weight of water depends on its density, which is defined as mass per unit volume. While a standard density is used for practical calculations, the actual weight of a large volume, such as 200 gallons, can fluctuate based on environmental factors. Calculating the total weight requires understanding this standard measurement and the variables that cause it to deviate.
The Standard Weight of 200 Gallons
Two hundred US liquid gallons of fresh water weigh approximately 1,668 pounds under standard conditions. This calculation uses the accepted standard density for water in the US customary system. The precise weight is determined by multiplying the volume (200 gallons) by the standard weight per gallon (8.34 pounds), resulting in 1,668 pounds.
For international reference, this weight translates to about 756 kilograms. This significant mass highlights why understanding water’s weight is important for engineering, construction, and transport. This 1,668-pound figure represents the weight of pure, fresh water at a specific, cool temperature.
The Conversion Factor: Why Water Weighs 8.34 Pounds Per Gallon
The standard conversion factor of 8.34 pounds per US liquid gallon is derived from water’s density at a specific temperature. Density measures how tightly water molecules are packed within a set volume. In the United States, the weight of one gallon is specified at \(62^\circ\text{F}\) (\(17^\circ\text{C}\)), where it is precisely 8.3454 pounds.
The US liquid gallon is formally defined as 231 cubic inches. Thus, the 8.34 pounds per gallon figure represents the mass of 231 cubic inches of water at the standard temperature. This precise figure is an industry-accepted standard for fresh water calculations. Unlike the metric system where one liter of water weighs one kilogram, the US system requires the 8.34 conversion factor.
How Temperature and Salinity Influence Density
The standard weight assumes a specific temperature because water’s density changes with heat. Pure water reaches its maximum density just above its freezing point, at about \(39.2^\circ\text{F}\) (\(4^\circ\text{C}\)). At this temperature, a gallon of water is at its heaviest, weighing slightly more than the standard 8.34 pounds.
As temperature increases above \(39.2^\circ\text{F}\), water molecules expand, causing density to decrease and the water to become lighter per gallon. For instance, water near its boiling point weighs approximately 8.00 pounds per gallon. This change becomes significant when calculating the total weight of hundreds of gallons.
Salinity, or dissolved salts, influences water density, making salt water heavier than fresh water. Seawater has an average salinity of about 35 parts per thousand, with a density of \(1.02\) to \(1.03\) grams per cubic centimeter. Since fresh water is \(1.00\) gram per cubic centimeter, 200 gallons of seawater would weigh more than 1,668 pounds.