Protein Molecular Weight Calculator

How to use this calculator

1. Enter the protein or peptide sequence using standard one-letter amino-acid codes. Spaces and line breaks are ignored.
2. Set the number of identical separate chains if you want their combined mass.
3. Read the average molecular mass in daltons and kilodaltons. The molar mass has the same numerical value in g/mol.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Protein Concentration and DNA Concentration.

Why a sequence is not just a sum of free amino acids

Amino acids join through peptide bonds. Each joining reaction removes the equivalent of one water molecule, so a linear chain with n residues has n−1 peptide bonds. The first residue does not require a bond to a preceding residue. The correction therefore leaves the appropriate terminal groups in an unmodified, open chain.

The table used here contains average masses based on naturally occurring isotopes. These are different from monoisotopic masses commonly used in high-resolution mass spectrometry. Average-mass tables can differ slightly in their final digits because of the atomic weights and rounding used; the terminal-water convention is the same. Dalton and unified atomic mass unit are equivalent names for the molecular-scale unit; one kilodalton is one thousand daltons. Molar mass in grams per mole is numerically equivalent on this scale.

Know which chemical details are outside the sequence

The calculation assumes the 20 standard amino acids, unmodified termini, and a linear peptide. Disulfide bonds, phosphorylation, glycosylation, isotope labels, unusual residues, and other modifications change mass and are not inferred automatically. Do not compare a modified experimental species with this unmodified estimate without accounting for those differences.

Multiple separate identical chains are handled by multiplying each complete chain’s mass. No extra peptide bond is formed between those chains. If you want the mass of a single fused sequence, concatenate its residues and use one chain instead. Protein migration in a gel can also differ from ideal behavior, so this molecular-mass estimate alone does not guarantee an exact experimental band position.