Protein Molecular Weight Calculator — Compute Protein MW from Amino Acid Sequence in kDa, Da & g/mol

Why Do We Need the Protein Weight Calculator?

If you are a student who is just learning about proteins and wants to run some quick calculations, then this peptide molecular weight calculator is just the tool for you. There is a more specific use, however — Western Blot, a special technique used to differentiate proteins in a sample.

In the Western Blot technique, a mixture of different proteins is placed onto a special gel and then separated using an electric field. All the substances in the mixture move by an amount determined by their protein molecular weight (MW). The lower the protein MW, the greater the distance it'll travel through the gel.

Adding the mixture to the starting point — heavy elements will stay near the starting point, while light contents will travel far away. Protein MW allows us to predict the exact location our proteins will move to — so we know where to look.

Amino acids are one of the most common buffers in the human body.

How to Use the Protein Molecular Weight Calculator?

The protein mass calculator is the simplest tool you could imagine — just choose the desired combination of amino acids using the ready-to-go list. All the elements are enumerated with their full name and both their 3- and 1-letter codes.

Our protein kDa calculator will show you your results in both unified atomic mass unit (u) and kilodaltons (kDa), as well as in grams per mole (g/mol) for metric use, and ounces per mole (oz/mol) and pounds per mole (lb/mol) for the American system.

How to Calculate Protein Molecular Weight from the Sequence?

It's just as simple as it seems — add together all the molecular weights of the amino acids present in your sequence. You also need to subtract the molecular weight of water for every bond created.

Why Do We Need to Subtract the Water's Weight? 💧

With every peptide bond formed between two amino acids, a molecule of water is created and expelled. It no longer constitutes a part of the protein.

We use the following formula in our amino acid calculator for molecular weight:

For example:

• If you use 2 amino acids, you subtract 1 × 18.02 u = 18.02 u.
• If you use 45 amino acids, you subtract 44 × 18.02 u = 792.88 u.

A Word About Units — kDa, u, g/mol

What is a Dalton (Da)?

A Dalton (Da) or unified atomic mass unit (u) is defined as 1/12 of the mass of a carbon-12 atom. It is the standard unit for expressing the mass of atoms and molecules. For proteins, this unit is particularly useful because protein masses are on the scale of thousands to millions of Da.

What is kDa?

A kilodalton (kDa) is simply 1,000 Da. Most proteins range from a few kDa (small peptides) to several hundred kDa (large proteins). For example:

• Insulin: ~5.8 kDa
• Hemoglobin subunit: ~16 kDa
• Antibody (IgG): ~150 kDa

What is g/mol?

The unit g/mol (grams per mole) is used in chemistry for molar mass. It is numerically equal to Da: a protein with MW of 50,000 Da has a molar mass of 50,000 g/mol. This means one mole of that protein weighs 50 kg.

American Units (oz/mol, lb/mol)

For users familiar with the American customary system, the same molar mass can be expressed in oz/mol or lb/mol:

• 50,000 g/mol × 0.035274 = 1,763.7 oz/mol
• 50,000 g/mol × 0.002205 = 110.2 lb/mol

The 20 Standard Amino Acids and Their Molecular Weights

Molecular weights shown are average isotopic masses (Da/u). Water molecular weight = 18.0153 Da.