# 1 Gallon of Water = Weight? (Answered & Explained)

One U.S. gallon of water weighs 8.33 pounds or 133 ounces, or 3.78 kilograms. However, one U.K. gallon of water weighs 10 pounds or 160 ounces, or 4.55 kilograms. These weights vary slightly depending on temperature.

The unit conversion for a gallon of water is not straightforward as the U.S. measuring system differs from that used in other parts of the world.

Is a U.S. gallon the same as a U.K. gallon or a Canadian gallon?

Furthermore, is a U.S. pound the same as a U.K. pound?

## Understanding a Gallon of Water Measurements

Why do you need to know: how much does a gallon of water weigh?

Does the weight of water have any practical implications for your life?

Water is essential to your health and wellbeing; water makes our blue planet perfect for life, and you use water every day for drinking, washing, cooking, and providing decorative features in your home and garden.

What are the times when you do need to know how much a gallon of water weighs?

Some of those may surprise you.

### What is Weight?

You know how much you weigh compared with a bag of flour or another person, but what do we mean when we talk about weight?

Back in High School, your science teacher explains that weight is a force that combines the mass (how much there is of a body) and that old standard gravity.

Because weight relates to mass, it also refers to volume.

When you know the volume of water, then you can work out the weight and vice versa.

Volume refers to how much space something occupies.

Water has the properties of changing its volume when it freezes, so cold winters make most homeowners worry about their plumbing.

Volume measurements are tricky as they depend on what system is popular in your country.

The metric standards aim to standardize measurements so everyone knows that a liter of liquid or gas is the same mass in every corner of the world.

Water is the defining standard for a kilogram – one liter of water weighs one kilogram. The metric system relates weight to volume.

### Is a Pound Weight the Same Everywhere?

On a universal scale, a pound weight does not weigh the same everywhere – on the moon, for example, with less gravity, that pound weighs a lot less.

Here on Earth, despite slight differences at Mt. Everest’s top, you can reliably treat a pound as the same weight.

When you compare the U.K. and the U.S. system of weights and measures, pounds and ounces are the same, but you get different interpretations of hundredweights and tons.

Avoid confusion, stick to pounds when comparing U.K. and U.S. weights, or use the metric scale.

The U.S. gallon and the U.K. gallon parted company around 1824.

The British chose to use a different volume measurement of one gallon, which changed to liters in 1995.

The U.S. gallon relies on a volume measurement of 231 cubic inches.

The result: the U.K. (and Canada) have a gallon that has one fifth more volume than the U.S. gallon. Plus, it weighs more.

## How Much Does a Gallon of Water Weigh?

At room temperature, a gallon of water weighs 8.33lbs (or 133 oz) in the U.S. system and 10lbs (or 160 oz) in the U.K.

The exact room temperature is not critical for most purposes. For accurate scientific measurements, you specify the water temperature.

The table below gives you a quick look at the weight of one gallon of water and the Liter unit for comparison.

If you want to know how much a gallon of water weighs in pounds and ounces, you need to clarify if you are talking U.S. or U.K. measurements.

Although a U.K. ounce is identical to a U.S. ounce and both systems have sixteen ounces in a pound, the gallon’s volume is different.

### Weight of a Gallon of Water in U.S. Measurements

Water in U.S. gallon converted into pounds, ounces, kilograms, and grams:

### Weight of a Gallon of Water in U.K./Canadian Measurements

Water in UK/CA gallon (also known as the Imperial gallon) converted into pounds, ounces, kilograms, and grams:

## How Does Temperature Affect the Weight of Water?

A given mass of water remains the same at all temperatures.

But when you look at the weight of a gallon of water, you compare a volume measure—the volume occupied by a fixed mass of water changes with temperature.

The volume of a liquid usually decreases with falling temperature.

The three basic phases of matter are gas, liquid, and solid.

The process is straightforward – energetic molecules and atoms move further apart using heat as their energy source. When matter gets cold, its atoms or molecules move closer together and usually are closest packed as a solid.

For most matter, the same volume of solid to liquid is heavier. Water is different, unique and its strange behavior is vital for life.

### Does the Weight of Water Change When Frozen?

Water behaves like any other liquid when cooled to 4˚C (39.2˚F); the colder water molecules get closer together, and they are at their smallest distance apart at this temperature.

Unlike other substances, water is still liquid at this point, and it does not become solid until the freezing point of 0˚C (32˚C).

On its journey from liquid to solid, water’s crystalline structure involves the molecules moving apart to create ice.

This remarkable behavior means that ice occupies 9% more volume than water and is lighter than liquid water – similar to how a gallon of frozen milk weighs less than a gallon of milk in liquid form.

Ice floats, and ponds freeze from the top down, which is fantastic if you are a fish or a frog because it means that the pond or lake’s bottom can stay liquid all through the winter.

Imagine if water behaved like all other liquids and the ice sank to the bottom of the pond?

Bad news for fish and the rest of life on Earth. But water is amazing, and its unique properties make our life possible even at the frozen poles.

## When Does the Weight of Water Matter?

For most purposes, no one worries about the weight of water. The volume tends to be the crucial measurement.

When the weight of water matters, it is handy to know the answer to the question: how much does a gallon of water weigh?

### Personal Hydration

Depending on your build and activity levels, you may need up to a gallon (U.S.) of water per day.

Typically, you don’t need to worry about that water’s weight until you are in the position of having to carry your daily or weekly water supply.

If you are going for a day trek in a hot, dry area, you need a minimum of one gallon (U.S.) per person for drinking and hydration.

When you are lifting a gallon of water into your car, those 8.33lbs are manageable. Going on a walk with the equivalent of 16 eating apples in your pack means making some choices about what you pack in your backpack.

### Aquariums

If you buy a fish tank with a matching stand, someone else has calculated the water’s weight and the stand’s necessary strength.

But if you want to use furniture as a base or to repurpose an item as an aquarium?

Now you need to consider the water weight as part of your project.

The first step to calculating the weight of water is to calculate the tank’s volume using the simple formulas for blocks and cylinders.

The volume of a block = Length x Width x Height.

The volume of a cylinder = π (pi) x Radius x Radius x Height.

This calculation gives you the volume in cubic inches or cubic centimeters. Then you can convert it into gallons or liters using the following ratios:

• 231 cubic inches in a US Gallon.
• 1,000 cubic centimeters in a liter.

When you know the volume, you can calculate the weight of water, and this allows you to stress test your aquarium arrangement by using test weights or books or bottled water to check the structural integrity of your project.

### Swimming Pools

Swimming pools sunk into the ground allow the Earth to support the weight of water.

Above ground pools apply weight to the bottom of the pool and the sides.

For safety and longevity reasons, you need a stable, flat surface to support your pool’s weight and the impact of people swimming and splashing water around.

A small slope of two to three inches across your standalone pool can be enough to destabilize the sidewalls and cause swimming pool failure.

An average size swimming pool holds 15,000 gallons of water, a mass weighing 125,000 pounds.

If you want to put the pool on your roof, you need an architect or structural engineer to calculate the maximum load you can put on your roof.

It is not an impossible dream because if your current roof is not suitable, you can install reinforcing to deal with water weight for your pool. But when you want a rooftop pool, the weight of water is crucial to your installation.

### Building Dams

The construction of a functioning and safe dam requires an appreciation of the powerful nature of water’s weight.

The Hoover Dam, for example, relies on compression and weight of concrete to hold back a maximum of 9.2 trillion gallons of water, weighing a staggering 77 trillion pounds of water or roughly 5,500 million of the largest African elephants on the planet.

### International Space Station

Water is vital to human life, and the International Space Station (ISS – source) holds around 530 gallons of water in reserve.

Regular deliveries of 400 gallons of water every three to six months maintain the reserve, and the ISS rigorously harvest and recycle as much water as possible.

The rockets that carry supplies and cargo to the ISS can carry 5,000 pounds, and the water resupply occupies two-thirds of the payload.

Space travel is one of the situations where water weight is a vital factor in planning operations.

## Last Drop

Like air and Earth, we take water for granted and seldom consider how much a gallon of water weighs or the volume it occupies as we go about our daily lives.

But water is essential to life, and as more people move to creative and sustainable building practices, the weight of water has a more significant impact on daily life.

References:

https://en.wikipedia.org/wiki/Comparison_of_the_imperial_and_US_customary_measurement_systems

https://inquiryproject.terc.edu/curriculum/curriculum5/3_ice/inv_11/index.html