How Much Does Water Weigh?

Our lives revolve around water — for hydration, recreation and so much more. And understanding how factors like temperature, state and density affect the weight of water can help you when it comes to cooking.

Scientists use two measurement systems to define the weight of water. In the metric system, which is more widely used around the world and in cooking, the unit weight of water is approximately 1 gram per cubic centimeter at 25 degrees Celsius, according to the National Library of Medicine. In imperial measurements, it's about 62.4 pounds per cubic foot at 77 degrees Fahrenheit.

But the weight of water is best understood when you think about it in terms of common quantities:

• A cup of water weighs about 224 grams (half a pound), per the National Institute of Standards and Technology.
• A liter of water weighs about 1 kilogram (2.2 pounds), according to the United States Geological Survey (USGS).
• A gallon of water weighs about 3.79 kilograms (8.35 pounds), per the USGS.

The weight of water is approximate, however, because it can vary based on factors like temperature and pressure, according to the USGS. It can also fluctuate if there are substances in the water, like salt.

Water weight is important when it comes to cooking. Many recipes are based on the weight of ingredients, rather than fixed amounts like tablespoons or cups. So if you're lacking a food scale, understanding roughly how much certain quantities of water weigh can help you ace any meal.

The density of any substance will change as factors like temperature and pressure make the material expand and contract, and water is no exception, according to the USGS.

Density is a measure of how much mass a substance has, per the U.S. Nuclear Regulatory Commission (NRC). A small volume of a very dense material like lead will be heavy, whereas a large volume of a low-density material like Styrofoam weighs much less.

Density is normally stated in units of mass per volume; for example, grams per cubic centimeter. On Earth, this means that the density of water at room temperature is the same as the weights listed above, per the USGS.

Most people don't make a distinction between mass and weight, but to a scientist, these are two different measurements. Mass is the actual physical bulk of a substance, according to the NRC. Weight is the force with which gravity pulls on that mass.

For example, a cat has more mass than a mouse. But a cat on the moon might weigh less than a mouse on Earth due to different gravitational pulls. But because you're likely only dealing with water on Earth, the weight of water won't fluctuate based on gravity.

The weight and density of water can fluctuate based on its state, however. In addition to liquid, water can be a solid (ice) or gas (vapor), per the USGS. The density of ice, gas and liquid are different, which consequently changes water's weight from form to form.

Ever wonder why ice floats? That's because ice is less dense than water. When water freezes at 32 degrees F (0 degrees C), its molecules spread out to form a more rigid structure, according to the USGS.

Vapor, on the other hand, occurs when liquid water hits 212 degrees F (100 degrees C) and the molecules move around rapidly and evaporate, according to the USGS. Consequently, gas is less dense than the other states of water.

Then there's salt water to consider. Salt water is more dense — and thus weighs more — than fresh water due to the added mass of salt, according to the USGS.