Protein Molecular Weight Calculator Updated Aug 11, 2026

A free protein molecular weight calculator. Paste an amino acid sequence in single-letter code and the tool returns the molecular weight in daltons (Da) and kilodaltons (kDa), the residue count, the average mass per residue, and the amino acid composition. It uses standard average residue masses, the same method behind tools like ExPASy ProtParam.

How to use this protein molecular weight calculator

1. Paste your sequence. Enter the protein or peptide in single-letter amino acid code, for example MKTAYIAKQR. FASTA headers, spaces, numbers, and line breaks are ignored by default.
2. Click Calculate. The tool sums the average residue mass of every valid amino acid and adds one water molecule for the free N- and C-termini.
3. Read the result. You get the molecular weight in daltons and kilodaltons, the residue count, and the average mass per residue, plus the amino acid composition.

The Copy button saves the result for a lab notebook, methods section, or spreadsheet. Everything runs in your browser, so your sequence is never uploaded.

The protein molecular weight formula

A protein is a chain of amino acids joined by peptide bonds. Every time two amino acids join, the reaction releases one water molecule. So the molecular weight is not the sum of the free amino acid masses; it is the sum of their residue masses (each free amino acid minus one water), with a single water added back for the two free ends of the finished chain:

This calculator uses average residue masses, which reflect the natural mix of isotopes and match the values used by the widely cited ExPASy ProtParam tool. Average masses are the standard choice for reporting a protein's overall molecular weight. Mass spectrometry sometimes uses monoisotopic masses instead, which are slightly lower.

Amino acid average residue masses

These are the average residue masses this calculator sums. A residue mass is the amino acid mass after losing one water in the peptide bond, so it is what each amino acid contributes to the chain:

Glycine is the lightest residue and tryptophan the heaviest. The average across all twenty amino acids is close to 110 Da, which is where the "110 Da per residue" rule of thumb comes from.

Quick estimate: the 110 Da per residue rule

If you only need a rough figure and know the length, multiply the number of amino acids by about 110 Da:

Common uses for a protein molecular weight calculator

• Reading an SDS-PAGE gel — comparing a band's position against the calculated molecular weight to confirm a protein's identity.
• Planning experiments — converting between mass and moles for buffers, dilutions, and molar concentrations.
• Writing methods sections — reporting the molecular weight of a construct or peptide in a paper or thesis.
• Biology and biochemistry coursework — checking homework answers and understanding how residue masses build a protein.
• Ordering synthetic peptides — confirming the expected mass before a supplier makes the peptide.

What this calculator does and does not include

This tool calculates the molecular weight of the plain amino acid chain. It gives an accurate figure for a standard, unmodified protein or peptide, but a few real-world features shift the true mass:

• Disulfide bonds each remove about 2 Da (two hydrogens), so a protein with several bridges weighs slightly less than the sum shown.
• Post-translational modifications such as glycosylation, phosphorylation, or acetylation add mass that a sequence alone cannot predict.
• Terminal modifications such as C-terminal amidation change the end groups and shift the mass by a few daltons.
• Non-standard residues outside the twenty common amino acids are skipped, so include only standard single-letter codes for an exact result.

Protein molecular weight calculator FAQs

Related calculators

This protein molecular weight calculator from Anchor AI Tools provides estimates for general and educational use. It computes the unmodified amino acid chain and does not account for disulfide bonds, post-translational modifications, or non-standard residues. Confirm critical values with a dedicated laboratory tool.