How Much Does Water Weigh?

Quick answer: A US gallon of water weighs about 8.34 pounds (3.785 kilograms) at room temperature. A liter of water weighs 1 kilogram, and a milliliter weighs 1 gram. Temperature nudges these numbers slightly, but for everyday purposes those are the figures to remember.

How much does a gallon of water weigh?

One US gallon of water weighs 8.345 pounds at room temperature, around 70°F (21°C). Most people round that to 8.34 pounds, or simply "a little over 8 pounds." In metric terms, a US gallon is 3.785 liters and weighs about 3.785 kilograms.

Two details worth knowing:

• The US gallon and the imperial gallon are different. A UK (imperial) gallon is larger, at 4.546 liters, so it weighs about 10 pounds. If a British source tells you a gallon of water weighs 10 pounds, that is why.
• The answer is "about" because of temperature. Water expands slightly as it warms, so hot water weighs marginally less per gallon than cold water. More on that below.

Water weight conversion table

Here are the amounts people actually ask about, all for plain water near room temperature and rounded for everyday use.

The simple rule: 1 liter weighs 1 kilogram

The metric system makes water weight almost effortless, because the units were originally built around water. The density of water is approximately 1 gram per cubic centimeter (1 g/cm³), which is the same as 1 kilogram per liter. That gives you three rules that cover nearly everything:

• 1 milliliter of water weighs 1 gram
• 1 liter of water weighs 1 kilogram (about 2.2 pounds)
• Weight = volume × density, so just multiply

A 5-liter container of water weighs 5 kilograms, or about 11 pounds. A 250 mL glass of water weighs 250 grams. No lookup table needed once you know the rule.

How much does 5 gallons of water weigh?

Five US gallons of water is 18.93 liters, which weighs about 18.9 kilograms, or 41.7 pounds. That is why a full 5-gallon jug for a water cooler is genuinely awkward to carry, and why filling large containers where you need them, rather than hauling them, is usually the smarter move.

Does temperature change the weight of water?

Slightly, yes. Water expands as it warms and contracts as it cools, which changes its density. Water is densest at 4°C (39.2°F), where 1 liter weighs exactly 1 kilogram. From there:

• At 20°C (68°F), 1 liter weighs about 0.998 kg
• At 100°C (212°F), near boiling, 1 liter weighs about 0.958 kg

So a gallon of hot water weighs a bit less than a gallon of cold water, but the difference is small enough that for cooking, cleaning, and carrying, you can ignore it.

What about pressure and depth?

Underwater, pressure rises quickly with depth: roughly 1 additional atmosphere for every 10 meters. Water is nearly incompressible, though, so its density barely changes. Pressure matters a great deal to divers and engineers, but it does not meaningfully change what the water in your glass weighs.

Mass vs. weight: what is the difference?

People use "mass" and "weight" interchangeably, but scientists do not. As the National Research Council defines it, mass is the amount of matter in an object, while weight is the force gravity exerts on that mass. A gallon of water on the Moon would have exactly the same mass as on Earth, but it would weigh about one sixth as much, because lunar gravity is weaker. Here on Earth, gravity is effectively the same everywhere, so treating the two as interchangeable works fine for everyday purposes.

Bottom line

Remember two numbers and you can work out almost any water weight in your head: 8.34 pounds per US gallon, and 1 kilogram per liter. Everything else is multiplication.

For a real-world feel for these numbers: a full 10-cup Pure XP pitcher holds a little over 5 pounds of water, which is why a freshly filled pitcher always feels more substantial than people expect. If you are going to lift it, it may as well be worth drinking: the Pure XP pitcher is independently tested to reduce lead, PFAS, chlorine, and over 99.9% of microplastics. And if you are counting ounces for a hike or the gym, a filled filtered bottle adds about a pound per 16 ounces of water, filter included.

Related reading

• How much weight will I lose on a 3-day water fast?
• Benefits of baking soda in water
• Plastic water bottles and microplastics

Sources

• National Research Council: definitions of mass and weight

This article is for educational purposes and is not medical advice.