What is the Protein Molecular Weight Calculator?

Calculate the average molecular weight of a protein from its one-letter amino acid sequence. Get mass in daltons (Da) and kilodaltons (kDa), residue mass breakdown, and amino acid composition. Uses standard average residue masses (ExPASy ProtParam). Supports FASTA input. Free, private, and instant in your browser.

How to use the Protein Molecular Weight Calculator

1. Paste a protein sequence using one-letter amino acid codes.
2. Stop symbols (*) are ignored; FASTA headers are stripped automatically.
3. Read molecular weight in Da and kDa.
4. Review amino acid counts and mass contributions in the table.
5. Copy the summary for lab notes or protein chemistry homework.

Common use cases

• Estimating MW of a translated ORF before lab work
• Checking peptide mass after removing a stop codon symbol
• Comparing composition of two protein sequence variants

Frequently asked questions

How is protein molecular weight calculated?

The tool sums average residue masses for each amino acid and adds 18.015 Da for the terminal H and OH of the free peptide chain (standard ProtParam approach).

Is this monoisotopic or average mass?

Average molecular weight using tabulated mean residue masses, not monoisotopic exact mass.

Are modifications included?

No. Phosphorylation, glycosylation, disulfide bonds, and non-standard residues are not included.

What about ambiguous codes like X or B?

Only the 20 standard amino acids (A, C, D, E, F, G, H, I, K, L, M, N, P, Q, R, S, T, V, W, Y) are supported.

Is my sequence uploaded?

No. All calculations run locally in your browser.