Protein Molecular Weight Calculator
Estimate average and monoisotopic protein mass from a standard one-letter amino acid sequence.
Your result, clearly explained
Paste a sequence or load the example, then run the tool. Your sequence stays in this browser.
How to use
- Enter a protein
Paste standard one-letter amino acids or one protein FASTA record.
- Calculate mass
Read average mass, monoisotopic mass and the residue count.
- Check assumptions
Use the estimate for a single unmodified linear chain and export the report.
Why ToolerWork?
Sequence calculations run inside your browser.
Strand direction, code table and approximation limits are explained.
Related Tools
View AllTranslate DNA into amino acids in any of six reading frames using the standard genetic code.
Find ATG-start open reading frames on both DNA strands, with coordinates and translated proteins.
Look up a DNA or RNA codon to find its amino acid, abbreviation and stop-codon status.
Why use this tool?
- Use residue-specific masses instead of a single average per amino acid.
- Compare average and monoisotopic neutral mass in Da and kDa.
- Reject unknown residues that would make the result falsely precise.
Method & assumptions
Mass equals the sum of ExPASy amino acid residue masses plus one H₂O molecule for a linear chain with free termini. Average water mass is 18.01524 Da; monoisotopic water mass is 18.01056 Da. The estimate excludes disulfide-bond mass loss, post-translational modifications, bound molecules and ion charge. A single terminal * is removed; internal stops and ambiguous residues are rejected.
References: NCBI genetic codes · ExPASy residue masses
Explore all 10 biology tools →Frequently asked questions
What is the difference between average and monoisotopic mass?
Average mass uses naturally averaged isotope masses. Monoisotopic mass uses the lightest common isotope of each element. Both outputs describe the neutral chain.
Why is water added only once?
Residue masses already account for the water lost during peptide-bond formation. Adding one water supplies the terminal groups of a linear peptide.
Can I enter X, B, Z, U or O?
This calculator supports the 20 standard amino acids only. Resolve ambiguous residues and use a specialized calculator for selenocysteine, pyrrolysine or modified residues.
Are disulfide bonds and modifications included?
No. The result assumes an unmodified linear chain with reduced cysteines and free termini. Disulfide bonds, glycosylation and other modifications change the mass.
Why can the result differ from an experimental measurement?
Measurements can reflect modifications, processing, ions or bound molecules. This sequence-based estimate does not model those effects or predict gel migration.