Protein Molecular Weight Calculator

What Is Protein Molecular Weight and Why Does It Matter?

Protein molecular weight (also called molecular mass) is the mass of a protein or peptide calculated from the sum of its amino acid residues. It is a fundamental property used in biochemistry and molecular biology for techniques like Western blotting, gel electrophoresis, mass spectrometry, and protein purification. Knowing the precise molecular weight helps researchers identify proteins, predict their migration in gels, and design experiments.

How to Calculate Protein Molecular Weight

The molecular weight of a protein is calculated by summing the individual molecular weights of all amino acids in its sequence and subtracting the weight of water molecules lost during peptide bond formation:

$$MW_{protein} = \sum_{i=1}^{n} MW_{aa_i} - (n - 1) \times 18.0153$$

where $n$ is the number of amino acids in the sequence and $18.0153$ u is the molecular weight of water lost per peptide bond. Each time two amino acids join to form a peptide bond, a water molecule is released (dehydration synthesis).

For example, a dipeptide of Alanine (89.09 u) and Glycine (75.07 u):

$$MW = (89.09 + 75.07) - 1 \times 18.0153 = 146.14 \text{ u}$$

The result can be expressed in daltons (Da), unified atomic mass units (u), or kilodaltons (kDa), where 1 kDa = 1000 Da.

Using the Protein Molecular Weight Calculator

Build your protein sequence by selecting amino acids from the dropdown menus, one position at a time, up to 20 residues. The calculator instantly displays the molecular weight in both kDa and daltons, shows the one-letter and three-letter sequence notation, and provides a full breakdown of the calculation including the water loss from peptide bonds. You can remove individual amino acids using the x button or change them at any position.

This tool is ideal for students learning about protein structure, researchers planning Western blot experiments, and anyone working with peptides who needs quick molecular weight calculations.

Units: kDa, Da, and u

Molecular weight is expressed in daltons (Da) or unified atomic mass units (u), which are equivalent. One dalton is approximately the mass of a single proton or neutron. Kilodaltons (kDa) are commonly used for proteins since most proteins range from a few kDa to several hundred kDa. For example, an average amino acid has a molecular weight of about 110 Da (0.11 kDa).

After calculating molecular weight, use the Protein Concentration Calculator for concentration assays or the Avogadro Number Calculator to find the number of molecules in your sample.