Calculator inputs
Use either a one-letter sequence or manual residue counts. When both are provided, sequence input takes priority.
Formula used
The calculator stores amino acid residue masses and then applies a water correction based on the chemical form you choose.
Average mass uses isotopic abundance averages. Monoisotopic mass uses the most abundant isotope for each element. For peptides, one water molecule remains after summing residue masses because the chain still has terminal groups.
How to use this calculator
Step 1: Paste a one-letter amino acid sequence, or leave it blank and enter manual counts.
Step 2: Choose average or monoisotopic mass, then select peptide chain total or free amino acid total.
Step 3: Set the decimal precision and click Calculate Molecular Weight.
Step 4: Review the result card above the form, inspect the residue contribution table, and use the CSV or PDF download buttons when needed.
Example data table
| Example input | Residues | Peptide average MW | Peptide monoisotopic MW |
|---|---|---|---|
| GAS | 3 | 233.2242 | 233.1012 |
| ACDE | 4 | 436.4370 | 436.1264 |
| PEPTIDE | 7 | 799.8328 | 799.3599 |
| MKWVTFISLL | 10 | 1237.5659 | 1236.6940 |
Frequently asked questions
1. What does this calculator measure?
It estimates molecular weight from a peptide sequence or amino acid composition. You can view peptide chain totals or the total mass of free amino acids before condensation.
2. What is the difference between average and monoisotopic mass?
Average mass uses natural isotope abundance. Monoisotopic mass uses the lightest common isotope set. Monoisotopic values are often preferred for high-resolution mass spectrometry.
3. Why is water added to peptide mass?
Residue masses represent amino acids after bond formation. Adding one water molecule restores the terminal groups of the finished peptide chain, giving the correct whole-molecule mass.
4. Can I paste lowercase letters or sequences with spaces?
Yes. The calculator normalizes lowercase text and ignores spaces or line breaks. Unsupported one-letter residue symbols still trigger a validation message.
5. Can I calculate from residue counts only?
Yes. Leave the sequence field empty and enter counts for the amino acids you want. This is useful when you know composition but not residue order.
6. Do leucine and isoleucine have different masses here?
No. Leucine and isoleucine are isomers, so they share the same residue mass in both average and monoisotopic calculations.
7. Does the calculator include modified residues?
No. It uses the 20 standard amino acids only. Post-translational modifications, protecting groups, salts, or adducts should be added separately if needed.
8. Why might an experimental mass differ from the result?
Measured masses can shift because of protonation state, counterions, oxidation, disulfide formation, incomplete sequence information, or instrument calibration differences.