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How Much Does Water Weigh? (Metric + Imperial)

We use water all throughout the day without giving it much thought.  Drinking, cleaning, and showering are just a few of the ways in which we make use of it. Because we use water so much, knowing what it weighs is important.  And, knowing how to convert its weight between the imperial (avoirdupois) and metric systems are equally important.

water molecule model, Science or medical background, 3d illustration.

It was the chemist Henry Cavendish (1731 – 1810), who discovered the composition of water.

The Weight of a Water Molecule

The weight of a single molecule of water is determined by the combined atomic masses of its individual atoms.  A water molecule is made up of 3 atoms: two hydrogen and one oxygen. We measure atomic mass in grams per mole (g/mol). The masses of hydrogen and oxygen atoms are determined by the number of protons and neutrons in their nuclei. 

A hydrogen atom has 1 proton and 0 neutrons.  An oxygen atom has 8 protons and 8 neutrons.  Therefore, two hydrogen atoms plus one oxygen atom equals 18 g/mol. So, the atomic mass of a water molecule is 18 g/mol. From here, we can use the atomic mass to figure out the weight of a single water molecule.


  • 1 g/mol = .022 pounds per mole (lbs/mol)

Therefore, the weight of a single water molecule is 0.396 lbs/mol.


  • 1 g/mol = .001 kilos per mole (kg/mol)

Therefore, the weight of a single water molecule is 0.018 kg/mol.

Calculate Weight From the Volume of Water

Calculating the weight of water based on its volume measure is one of the most common calculation methods.  Under the imperial system, fluid ounces (fl oz) is the most common unit of measure for water volume.  A second, less common measure of volume is cubic inches (in3).

Under the metric system, milliliters (ml) is the most common unit of measure for water volume. Therefore, a bottle of water in the US will be listed in terms of fluid ounces whereas the same bottle would be listed in milliliters throughout the rest of the world. Here’s how to calculate water weight in both systems


  • 1 fluid ounce of water = 0.065 pounds (lbs)
  • 1 cubic inch of water = 0.036 lbs

Weight of an average US-sized water bottle (16.9 fl oz) = 1.102 lbs. If you want to calculate the weight of water in pounds based on its volume in fluid ounces, multiple the volume by 0.065 pounds.


  • 1 milliliter of water = 0.001 kilos (kg)

Weight of an average European-sized water bottle (500 ml) = 0.5 kg. If you want to calculate the weight of water in kilos based on its volume in milliliters, multiply the volume by 500.

Calculate the Weight of a Gallon of Water

The gallon (gal) is a common unit of measurement for water in the imperial system.  It is another measurement of volume. However, a US liquid gallon is far larger than a US fluid ounce.  Knowing what a gallon of water weighs is important for measuring out large quantities of water. 

Whereas knowing what a fluid ounce of water weighs is important for measuring out small quantities of water. The metric equivalent for measuring large quantities of water is liters (L).  Use liters for measuring out large quantities of water and milliliters for small quantities.


  • 1 US gallon of water = 8.35 lbs

If you want to calculate the weight of water in pounds based on its volume in gallons, multiply the volume by 8.35.


  • 1 liter of water = 1 kg

Very simply put, a single liter of water equals a single kilo in weight.  Therefore, liters and kilos have a one-to-one ratio.  

Calculate Weight From the Density of Water

Another less common method of weighing water is through its density.  Density is typically measured in grams per milliliter (g/ml).  Other times, density is measured in kilograms per cubic meter (kg/m3). The density of water is equal to 1 g/ml.  1 g/ml of water is equal to 8.35 gallons per liter (gal/l). While density is not the most common way to weigh water, it plays a huge role in the weight of water amidst temperature changes.

Thermometer measures the temperature of the water

Water temperature and density have an inverse relationship.

How Weight Changes With Temperature

As the temperature of water changes, its weight will also change.  Cold or room temperature water weighs more than the same water if it was heated to be warm water.  For example, a gallon of cold water will lose some of its weight if we were to boil it. This occurs because, when water is heated up, its density decreases. 

Density decreases because some molecules of the water transform into gas as it heats up.  As a result, the atmospheric pressure of the air will increase and the water will lose weight. Water will weigh its most when it is at its freezing point.  When it is frozen, it is at its maximum density.  Therefore, if you take warm water and freeze it, its weight will increase.

Sea Water vs Fresh Water

Have you ever been in the ocean or a saltwater lake and felt like it seemed different from the freshwater lakes you’ve been in?  What you were feeling was a difference in the weight of saltwater and freshwater. One cubic foot of saltwater weighs 64.1 lbs (29.1 kg).  One cubic foot of freshwater weighs 62.4 lbs (28.3 kg).

Saltwater weighs slightly more than freshwater because it has dissolved salt in it.  Similarly, purified or pure water weighs less than non-purified water because water that has not been purified still carries the weight of its natural minerals.  Pure water has its minerals removed so it weighs less. 

Conclusion: Why Water Weight is Important

Understanding how much water weighs and how to calculate water weight is important.  It will save you time when cooking or calculating how much water you should drink in a day.  Additionally, if you travel a lot, being able to switch between the metric and imperial systems of measurement is important. 

If you’re in a hurry and need to quickly weigh out your water based on its volume or density, use this water weight calculator as a converter. At one point or another, understanding how to weigh water will come in handy because water is an inevitable part of everyone’s life.

Bonus tip: Check out these top picks for water softeners that alter the weight and density of water by adding salt to it.

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Joshua Perkins

Joshua spent ten years as a water systems technician in California before settling down with his wife and two young children in Nashville in 2018. Through all of his experience, he learned the benefits and shortfalls of so many different types of water filtration systems, from pitchers to whole-house installations. He started Water Filter Authority in 2019 to empower other families to make the right decision for their long-term health and wellness.