Though made only by infinitely small atoms, molecules have their own mass: learn how to calculate it with our molecular weight calculator.

Our tool is pretty intuitive, but if you're curious and eager to learn more, keep reading our article o learn:

  • What is the molecular weight?
  • How do I calculate the molecular weight?
  • The measurement techniques of the molecular weight
  • How to use our molecular weight calculator.

We will also show you how to find the molecular weight of water and of some more complex molecules.

What is the molecular weight?

The molecular weight or molecular mass (the terms are, quite incorrectly, used equivalently) is the mass of a molecule.

The molecular weight is a fundamental property of any substance, highly affecting many chemical and physical properties like viscosity, diffusion, and melting temperature.

In molecules like polymers, the molecular weight determines the nature of the final material. However, in this case, the molecular weight has a slightly different meaning: since each polymer molecule has a different length, we can define an average weight.

Calculating the molecular weight is not difficult at all: follow us in the next section.

How to calculate the molecular weight?

The molecular weight is calculated by summing the atomic masses of each component of a given molecule.

If you know the chemical formula of your molecule, simply sum the number of the same element till you reach the simplest version of the formula. For example, consider acetic acid, one of the components of vinegar. Its formula is CH3COOH\text{C}\text{H}_3\text{COOH}.

This formula gives us a bit of insight into the structure of the molecule. This is possible only for smaller molecules: it gets heavy rather quickly. We can reduce this formula by summing together the atoms "type-wise": C2H4O2\text{C}_2\text{H}_4\text{O}_2.

To calculate the molecular weight of the acetic acid, we just need to sum the atomic mass of two atoms of carbon, two of oxygen, and four of hydrogen.

What are the units of the molecular weight?

The measurement unit of molecular weight is the unified atomic mass unit, also known as dalton. This unit corresponds to 112\tfrac{1}{12} of the mass of an unbound, non-ionized atom of carbon-12.

Since the mass of an electron is negligible when compared to the one of a nucleon (the subatomic particles in the atom's nucleus), the unit of atomic mass approximates the one of a proton or a neutron.

The atomic mass of an element is similar to the number of particles in the nucleus. However, due to the presence of isotopes, the value we see on the periodic table is an average value computed, taking into account the abundance of lighter and heavier elements with equal atomic number.

The dalton is, similarly, the measurement unit for the molecular weight, too. In our calculator, you will see the accepted symbol for the atomic mass unit: u\text{u}.

How to measure the molecular weight

Now you know how to calculate the molecular weight. But how do we measure it? Researchers use many different techniques, from temperature-based ones (cryoscopy and ebulliometry) to ones that use scattered light.

The most elegant technique is, in our opinion, mass spectrometry. Injecting a charged molecule into a magnetic field with the correct upward or downward orientation causes a change in the trajectory thanks to Lorentz's force. The deviation, registered on a screen, is related to the mass to charge ratio, and by measuring the displacement and knowing the mass, it becomes possible to assess the mass of a molecule.

How to use our molecular weight calculator

Our molecular weight calculator allows you to insert up to six different elements. You can choose the number of atoms of each species in your formula.

🙋 The atoms of six different species (oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus) make up more than 99.95%99.95\% of all the atoms in a human body!

Initially, only two fields will be visible: as soon as you will select a second atom, we will make a third variable available, and so on.

At the bottom of the calculator, we will give you the result in atomic mass unit and the simplest chemical formula for your molecule.

An example: how to find the molecular weight of water

Let's calculate the molecular weight of water, one of the most common molecules on Earth. As you surely already know, the chemical formula of water is H2O\text{H}_2\text{O}: two hydrogen atoms and a single oxygen one.

Let's check the atomic masses of these two elements:

  • Hydrogen has atomic mass 1.007 u1.007\ \text{u};
  • Oxygen has atomic mass equal to 15.999 u15.999\ \text{u}.

We can approximate these values with, respectively, 1 u1\ \text{u}, and 16 u16\ \text{u}. To calculate the molecular weight of water, follow this formula:

2mH+mO=2 u1+16 u=18 u\footnotesize 2\cdot m_\text{H} + m_\text{O} = 2\ \text{u}\cdot 1+ 16\ \text{u} = 18\ \text{u}

The calculations for the molecular weight can soon become cumbersome: with heavy organic molecules, the number of atoms grows quickly. Luckily, in certain cases, the repetition of certain blocks gives us a hand: check out how with our protein molecular weight calculator.

Davide Borchia
1st Element
Hydrogen (H)
No. of atoms
2nd Element
No. of atoms
Your molecule:
Molecular weight: 1 u.
Chemical formula: H
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