Mass and Weight are difference | SI Units of Mass

Mass and weight

 Mass and Weight on an Object

Mass and weight seem interchangeable, but they are completely different. So what is mass? What is the weight? What are the differences, and why is it important to know them? How are they linked? We will answer these questions in this article. Please note that these explanations are simplified


Mass represents a quantity of matter.

Indeed, an object is made of a certain material, that is to say precisely with an assembly of atoms.

Even though an atom is infinitely small compared to what surrounds us, it has a certain mass which cannot change. Therefore, an object as a whole (except if it is broken and scattered) always has the same number of atoms and therefore the same amount of matter.

To measure this amount of material, a balance must be used.

Read also: PHYSICS | Fundamental SI Units of Measurement, SI Derived Units, Names and Symbols

Warning :
You have to know how to use the right scale with the right object (you will not measure the mass of sand carried by a truck with a kitchen scale!).

The unit of measure for mass is grams (g). Of course, it is useful to know the different sub-units of measurement:

Tonne (t)1 t = 106 g
Microgram (μg)1 μg = 10-6 g
Examples: The mass of an adult man is about 75 kg.
The mass of a cell phone is on average 200 g.
An average car has a mass close to 1 t.

Here is a table of the most commonly used units of measure for mass

​MassKilogram (kg)hectogram (hg)​decagram (dag)gram (g)decigram (dg)centigram (cg)milligram (mg)
Value equivalent to 1 g​0,001 kg​0,01 hg​0,1 dag​1 g​10 dg​100 cg​1000 mg
SI Units of Mass
Units of Mass
10 milligrams (mg)=1 centigrams (cg)
10 centigrams=1 decigrams (dg) = 100 milligrams
10 decigrams=1 gram (g)
10 decigrams=1000 milligrams
10 grams=1 dekagrams (dag)
10 dekagrams=1 hectogram (hg)
10 dekagrams=100 grams
10 hectograms=1 kilogram (kg)
10 hectograms=1000 grams
1000 kilograms=1 megagram (Mg) or 1 metric ton (t)

US Customary units of weight review (oz & lb)

The commonly used unit of mass is the pound (pound, abbreviated lbs), equal to 0.453 592 37 kg. The pound is divided into sixteen ounces (ounce, oz), with one ounce equivalent to about 0.028 kg.

There are also two larger units: tonnes. The long ton is worth 2,240 pounds, or about 1,016 kg, and the short ton (short ton) is worth 2,000 pounds, or about 907.2 kg.

Here is a summary of the most common units and conversions:

The capacities
– 1 U.S. gallon = 4 qt = 3,785 l
– 1 quart (qt) = 2 pt = 0.946 l
– 1 pint (pt) = 16 fl oz = 0.473 l
– 1 ounce liquid (fl oz) = 29.56 ml

The difference of mass and weight

The mass

Mass is the measure of the amount of matter an object is made of. Typically, mass is measured in grams or kilograms. A funny anecdote: In the Imperial system, mass is not measured in ounces or pounds, but with a unit called the slug. Mass is related to Force, because it can be measured by determining the amount of Force needed to change the direction or speed of an object. You probably know this equation: F (force) = m (mass) times a (acceleration). Mass is also related to gravity, which is essentially the attraction between two objects that have masses depending on the distance. However, they are not related; gravity has no effect on mass. Mass is also indestructible (like energy, and it’s also scalar), and can never be zero (think about it: mass is the measure of the amount of matter in an object. How an object can it be composed of zero matter?).

The weight

Here’s something you may not know: the metric unit for weight is Newton, not gram. The imperial unit for weight is the pound. Do you remember the connection between mass and gravity? Weight is the gravitational interaction between objects that have mass.

Weight depends on the effect of gravity. Weight increases or decreases with higher or lower gravity.

If you’ve researched the differences between weight and mass elsewhere, you’ve probably seen that your mass on Earth and on the Moon are the same, but you weigh less on the Moon than on Earth because the Moon’s mass doesn’t is not the same as that of Earth (so they have different gravity). The weight can be zero (it depends on gravity) and is a force (and therefore, it is a vector).

Let us find out the SI unit of weight

The SI unit of weight is Newton (N).

The newton is a derived unit in the SI

N = kg x m x s−2 = (kg x m) /S²


The SI unit of weight is that of force which is measured in newton. In other words, if we talk about SI units, then, weight is considered to be a force, and is hence, measured as kg m/s². However, if we talk about the legal standards for weights in markets, then, it is commonly measured in kilograms (kg).


Weight and mass are two important concepts in physics; important and related, but quite distinct

In everyday life, the terms mass and weight are often confused.

Weight is a force that is measured with a dynamometer and is graduated in Newton, while mass represents a quantity of matter and is measured with a scale graduated in grams or in one of its sub-units.

Mass, a quantity of matter

The mass of a body gives an indication of the amount of matter that constitutes it. Thus, the mass of an object is a directionless and constant magnitude – as long as the object does not undergo any alteration – which relates to the number and nature of the atoms that compose it.

In the international system, the unit of measure for mass is the kilogram.

Weight, a force linked to gravity

The weight of an object, on the other hand, relates to the action of the force of gravity on that body. In fact, the weight is a quantity always directed towards the center of the Earth (or of another celestial body) and which depends on:

  • the distance between the object and the Earth (or another celestial body);
  • of the mass of the bodies in question.

Thus, the weight of an object is higher on Earth than on the Moon.

In the international system, the unit of measurement for weight is Newton.

The mystery of the bathroom scale

However, it should not be concluded that a bathroom scale measures your mass and not your weight. Indeed, if you jump on the scale, the indication will, for a moment, greatly increase. This is a sign that the device is very sensitive to a force, and therefore to your weight. It was not until the late 1940s that the Newton was adopted as a unit of force. Previously, we used the kilogram-force, hence the confusion on the scale.

The link between mass and weight

If mass and weight are two different quantities, they are related by a simple expression:

weight = mass x g

where g represents the intensity of gravity, the value of which depends on where we are.

Formulas for weight in Physics

Where: g acceleration due to gravitym mass of object W weight of object. This is SI (International System) unit for weight is newton (N).

Sources: PinterPandai, Wired, Wikipedia

Photo credit: MikeRun / Wikimedia Commons

Photo explanation: same weights (1N) on springs scales on three different celestial bodies (Earth, Moon and Mars), showing the difference between weight.

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