What is Weight? (All the Basic Definitions & Units Explained)

When we try and go deeper into understanding the definition of weight, it becomes clear that weight is more than just how heavy something is. There are many invisible forces acting in front of our very eyes that determine the weight of something. This post will have a look at the deeper meaning of weight and how it differs from other similar concepts such as mass. 

Strictly speaking, weight is the force exerted on a body by gravity and is measured in newtons. Another way to define it would be the force needed to support a given object. These are the most common definitions found in introductory physics textbooks around the world. It differs from mass in that mass measures the amount of matter something contains. 

If you can’t remember the information from that high school class, you’re in luck. Because this post will go into detail about the difference between weight and mass and the invisible force that ties it all together. Hopefully, without putting you to sleep.

Weight is All About Gravity

The first thing to understand is that the weight of something is determined by gravity. Or rather, the force that gravity has on an object. The more force that gravity pulls on an object with, the heavier that object is. 

What this means is that the mass of something is consistent. The mass of a 10 kilogram weight is the same on Earth, the Moon, or Jupiter. 

The weight of something, however, changes based on the gravitational force present. So something that weighs 10 kilograms on Earth will weigh less on the moon because the gravitational force of the moon is less than that of the Earth. The same applies to other planets in our solar system. 

Gravity is weight

So if somebody believes that they’re overweight, the truth is that gravity is simply pulling them down with more force compared to somebody who is lighter than them. 

For the sake of clarity, weight in this blog post will refer to the weight of an item on Earth.

Weight Measurement Units Explained

Over the years, there have been a huge amount of units of measurement for weight that have been invented. The baseline measurement unit, and the one that all the rest are based off of, is the measurement of force. In the International System of Units (SI), this is the newton. 

The newton measures the gravitational pull on an object, and is a good starting point for measuring the weight of things. In fact, the newton is the only unit of measurement that measures weight. All the other units that we have come to know as measuring weight are actually measuring mass. 

It’s no coincidence that the unit of measurement for the force of gravity is named after the person that discovered gravity in the first place, Mr Isaac Newton. 

The very first recorded unit of measurement for weight is the grain. The grain was used to weigh out silver and gold in the distant past. As you may have guessed, this was based on the weight of a grain of barley or wheat. Since then, times have changed somewhat, and the grain as a unit of measurement has fallen out of use. 

With that said, we still have plenty of different units that we can use to measure weight. We can measure things from as light as a feather, to as heavy as a building, and we need different units of weight to give us an accurate understanding of exactly how heavy something is. 

Shortly we’ll take a look at some of the different units of measurement that we use to determine how heavy something really is. But first, let’s go back in time. 

Which Came First, Metric or Imperial?

metric vs imperial

Officially, the metric system was first explained by it’s creator in 1618. Although it’s been around for longer than 400 years, the adoption of the metric system has not been around for that long. In some parts of the world (looking at you USA), the metric system has still not been adopted. 

The metric system was officially adopted by France in 1795. Close to 200 years after it’s initial inception, it was finally adopted as the official method of weights and measures in a major country. 

The imperial system was adopted by Britain in 1824, although elements of this system go back way further than that. For example, the Roman Empire used the mile as a unit of measurement as far back as 0AD. 

Over the century and a half that followed, regular corrections to this measurement system meant that it was constantly evolving. In 1963, weights and measures were redefined to fit into the metric system. Eventually, in 1965, Britain adopted the new system of weights and measures and officially switched over to the metric system. 

It is interesting to note that Britain originally followed the Imperial system and only officially adopted the metric system just over 50 years ago. 

As you can see, elements of the Imperial system have been in use for way longer than the metric system. These elements have changed and been refined over the centuries as our lives and the world has changed around us. 

Even though the metric system has been around for centuries, worldwide adoption has been slow and is still not complete. This can lead to plenty of confusion and miscommunication. So let’s take a look at some of the different units we use to measure weight. 

The Metric System’s Units

Metric units


In the metric system, the lightest measure of weight is the milligram. This is equal to 1/1000th of a gram. 


Going up (or heavier) from there, we get the gram which is equal to 1000 milligrams. The word ‘gram’ comes from the Late Latin word ‘gramma’. This word, in turn, comes from the French word ‘gramme’ which means a small weight. The symbol for gram is g. 


One gram is still a small amount of weight. The next unit up the scale is the kilogram and 1000 grams equal one kilogram. The word ‘kilogram’ finds its origins in the French word ‘kilogramme’. Which itself comes from the Greek word for a thousand, which sounds similar to ‘kilo’. Therefore, the word kilogram, when broken down into its original parts, literally means a thousand grams. 

And herein lies the beauty of the metric system. 

All units are designed to fit neatly into the next unit up on the scale. This makes it easy to calculate in everyday life. For example, if you need half a kilogram of something, you can quickly calculate that this means you need 500 grams of whatever it is you’re talking about. 

A good trick to help simplify the metric system is to remember that the word ‘kilo’ means one thousand. So one kilogram is one thousand grams. Or one kilometre is one thousand metres. 

The Imperial System’s Units

imperial units

There is no equivalent for a milligram in the imperial system. In fact, there is not even an equivalent for a gram in the imperial system.


The smallest unit of weight in the imperial system is an ounce, which equals about 28 grams. This is roughly the weight of one slice of bread. 

The word ‘ounce’ comes from the Latin word uncia, a unit that was one twelfth of the Roman pound (also known as a libra). The abbreviation for ounce (oz) has a different origin. It came later from the Italian word onza (now spelled oncia). 

If you wish to weigh something lighter than an ounce, you need to switch over to the metric system. Unless you want to be dealing with fractions of an ounce which can get confusing very quickly. 


On the other end of the scale, if you start stacking up those ounces, you will get to a pound once you have 16 ounces. This shows us how units of measurement have changed over time. One ounce used to equal one twelfth of a Roman pound. Whereas in modern times it equals one sixteenth of a pound.

Going back to the Roman pound, we can see the origins of the word behind this unit of measurement. It used to be called the libra, the Latin word for ‘scales’ or ‘balance’. 12 oncia made up one libra. We can still see evidence of the origin of the pound in the abbreviation we still use today: lb. This is shorthand for libra. 

As you can see from this example, the imperial system does not fit together logically like the metric system does. You need to learn the particulars of each unit of measurement and memorize how many of one thing make up the next unit in the scale of measure. 

What About The Different Tonnes?

Next up, we have the ton. In some countries, a ton refers to 2000 pounds of mass. This is known as a short ton and clocks in at 907 kilograms. This is also known as a US ton. 

In other countries, a ton clocks in at 1016 kilograms. This is known colloquially as a long ton and it is 2240 pounds. 

The origin of the word ton is interesting and may give us a glimpse into the priorities of life in the old days. The word that we use today is derived from the word tun. Tun describes a large barrel used in the wine trade. 

The French came up with this word, which is derived from the word ‘tonnerre’, which means thunder in English. This is named after the rumbling sound this barrel of wine would produce when it was rolled. 

From this origin story we can imagine huge barrels of wine, weighing a ton each, being rolled around olden day France and making such a loud noise that they named a unit of measure after it. 

To make our lives difficult, there is no easy conversion between the imperial and metric systems. This is something that you need to learn off by heart if it is important to you. Unless of course every country could simply come to an agreement that we all should use the metric system, as it’s so much easier to understand. But who am I to change the world.

More Weight vs Mass Talk


What we have all come to know as weight is, in fact, incorrect. As discussed above, weight is the measure of the force that gravity has on an object. So the weight of something is subjective to the force of gravity that is being exerted upon it. 

In order to find the weight of something, we need to know the mass of the given object first. Once we know the mass of the object, we can multiply that by the acceleration of gravity to find the weight of that particular object. 

The weight is then measured by a unit of measurement known as a newton. 

To understand this further, let’s imagine a 10 kilogram weight floating through space. We know the mass of this object is 10 kilograms. The mass will be the same no matter where the object is located. Even floating through space. 

However, because this weight is floating through space and there is no gravitational force acting on this object, the object is weightless. 

If this object drifts it’s way over to the moon and settles down on a scale, the weight measured would be less than 10 kilograms. This is due to the fact that the gravitational force on the moon is less than it is on the Earth. 

The mass of the object is still 10 kilograms. The mass of the object will always be 10 kilograms unless we modify the object. Does this make sense yet? 

To add to the complications, the gravitational force on the Earth is not consistent everywhere in the world. Scales actually need to be calibrated to their location to accommodate these changes in gravitational force over the Earth. 

So the way that we use weight in our everyday lives is in fact incorrect. We should be discussing the mass of things if we want to be more accurate. 

In Conclusion

If we want to get technical, the way that the word ‘weight is used today in everyday language is incorrect. We actually mean the mass of an object. Because the weight of something refers to the force that gravity has on a given object, and the force of gravity changes depending on what planet you’re on. 

On the other hand, the mass of something is consistent no matter where you are in the universe. As if this was not confusing enough, there are two different systems with completely different units of measurement and which one you use is mostly determined by which country in the world you live in.

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