Proteins make up most of what is not water in our bodies: our protein molecular weight calculator will tell you everything you need to know about the fundamental LEGO blocks of biological matter.

Here you will learn:

  • What is a protein, and what are amino acids;
  • Why there are essential amino acids;
  • What is a protein's molecular weight;
  • How to calculate the molecular weight from the amino acid sequence.
  • An example: calculate the kDa protein's weight for the smallest protein known to scientists.

What are proteins?

Proteins are the building blocks of biology. Composing over 20%20\% of our bodies, they are second in weight only to water.

Back to the main topic, proteins are fundamental components of living beings, with functions ranging to:

  • Giving structure to organs and tissues;
  • Regulating and controlling the biological processes inside and outside cells (from simple maintenance to our entire metabolism);
  • Allowing for the vast majority of the chemical reaction in our bodies (think of enzymes);

And much more.

Proteins are sequences of repeated smaller units, biomolecules called amino acids. Amino acids are characterized by three groups:

  • An ammino group: —NH3+\text{---NH}_3^+;
  • A carboxylic acid group: —CO"H\text{---CO}_"\text{H}; and
  • An R\text{R} functional group.

Different functional groups give origin to more than 500500 known amino acids. Of them, only about 2020 appear with regularity in biological tissues. Moreover, every species has a specific set of essential amino acids, which are fundamental to maintaining the functionality of the organism.

Humans need nine essential amino acids but use a total of 2121 for all of the required biological processes. Why is there such a distinction? Our bodies can't synthesize those nine amino acids: we have to integrate them with our diets. A deficit of them would cause serious issues: careful about what you eat!

Apart from a healthy diet, you should also think about proper air humidity to breathe. If you're intrigued, the dew point calculator explains what relative humidity is, which is the main parameter describing the water content of the air.

What is the molecular weight of a protein?

We can identify proteins by the various "levels" of their structures: from complex three-dimensional arrangements to simple sequences of amino acids. The most basic structure is called the primary structure, and it is nothing but the sequence of amino acids making up a specific protein. For longer chains, we call this sequence polypeptide.

Every polypeptide is unique to a protein, and this allows us to use the molecular weigth of a protein to identify them. Once we know what we are analyzing, we can use the molecular weight to track:

  • Stages of biological processes;
  • Changes in the protein structure;
  • Immunological responses;

And more.

Calculate a protein's molecular weight

To calculate the molecular weight of a protein, we simply have to add the molecular weight of each individual amino acid making up the polypeptide.

You can easily find tables with the weights of the most common — essential — amino acids online, but here it is:


Weight (kDa\text{kDa})









































The weight of a protein is calculated in kDa, kilodaltons: the dalton is a measurement unit used to quantify the weight of molecules and atoms: a dalton equals the mass of 112\tfrac{1}{12} of an unbound atom of carbon-12 (the most common isotope of the element). It roughly (very roughly) corresponds to the mass of a nucleon (the subatomic particles in the nucleus); thus, the mass of a molecule in daltons is a good approximation of its number of protons and neutrons.

How to use our protein molecular weight calculator

Our peptide molecular weight calculator allows you to calculate the weight of a sequence of up to 2020 amino acids.

Start by selecting the first amino acid of the desired sequence: we left the first fundamental amino acid as the default for the first variable. As soon as you fill in the second variable, another field will appear.

We will calculate the protein kDa weight and also the weight in atomic units, u\text{u}. Now that you know how to calculate the molecular weight of a peptide, check a practical example!

Calculate the molecular weight of a peptide: an example of how to determine the molecular weight of a protein

The smallest "functional" protein known to man comes from the saliva of the Gila monster, a lizard from Northern-Central America. The protein is made up of only 20 amino acids!

The sequence of this peptide is:

Asn,Leu,Tyr,Ile,Gln,Trp,Leu,Lys,Asp,Gly,Gly,Pro,Ser,Ser,Gly,Arg,Pro,Pro,Pro,Ser\begin{align*} &\text{Asn},\text{Leu},\text{Tyr},\text{Ile},\text{Gln},\\ &\text{Trp},\text{Leu},\text{Lys},\text{Asp},\text{Gly},\\ &\text{Gly},\text{Pro},\text{Ser},\text{Ser},\text{Gly},\\ &\text{Arg},\text{Pro},\text{Pro},\text{Pro},\text{Ser} \end{align*}

Let's build this small protein in our protein molecular weight calculator: choose all the amino acids making up the sequence. If you don't want to check every name, here is the corresponding sequence of letters:


Using the table above, or inserting the amino acids in our protein molecular weight calculator, you will be able to determine the molecular weight from the amino acid sequence. Compute the sum: the resulting molecular weight is 2.165 kDa2.165\ \text{kDa}, or 2164.5 u2164.5\ \text{u}.

If you are still interested in biochemistry, check DNA concentration calculator: you will learn one of the most important laboratory techniques in the field!

Davide Borchia
You can add up to 20 amino acids.
1st Amino acid
Alanine (Ala, A)
2nd Amino acid
Molecular weight: 0.089 kDa = 89.09 u
Your protein: Ala
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