Peptide molecular weight calculator.
Paste a peptide sequence in one-letter code to get its molecular formula, average molecular weight, monoisotopic mass, net charge at pH 7, isoelectric point and GRAVY hydropathy. Everything runs in your browser; nothing is sent anywhere.
Sequence in, mass out.
| Length | - |
|---|---|
| Molecular formula | - |
| Average molecular weight | - |
| Monoisotopic mass | - |
| Net charge at pH 7.0 (estimate) | - |
| Isoelectric point (estimate) | - |
| GRAVY hydropathy | - |
The math behind the numbers.
Each amino acid in a chain is a residue: the free amino acid minus one water lost to the peptide bond. The calculator adds up the residue formulas, listed in the amino acid chart, and puts back one water for the free ends, which gives the molecular formula. Average molecular weight uses standard atomic weights and is the number you would weigh out; monoisotopic mass uses the most abundant isotope of each element and is the number a mass spectrometer reports for the lightest peak.
An N-terminal acetyl group (see N-terminus vs C-terminus) adds C2H2O. A C-terminal amide replaces the final OH with NH2, which lowers the mass by about 0.98. D-amino acids weigh the same as their L forms, so a D-Ala counts as A.
The charge estimate sums the fraction of each ionizable group that is charged at pH 7.0, using the EMBOSS set of pKa values, and the isoelectric point is the pH at which that sum reaches zero. Published pKa sets differ, and the choice shifts both results, so treat them as estimates.
GRAVY is the grand average of hydropathy: the mean Kyte-Doolittle value of the residues in the sequence. Positive values lean hydrophobic, negative values hydrophilic.
What it does not cover.
- Non-standard residues such as Aib, 2-naphthylalanine or dimethyltyrosine, which have no one-letter code; the amino acid chart lists their residue masses.
- Other modifications: disulfide bonds (subtract 2 hydrogens, about 2.02, per bond), cyclization, lipidation or PEG.
- Salt forms. Synthetic peptides are usually supplied as acetate or trifluoroacetate salts, so the weight of a powder includes its counterions and some water, which is why lab reports measure the peptide content directly.
Checked against published values.
| Compound | Sequence | Modifications | Formula | Calculated | Published |
|---|---|---|---|---|---|
| BPC-157 | GEPPPGKPADDAGLV | none | C62H98N16O22 | 1419.54 g/mol | 1419.5 g/mol (PubChem) |
| TB-500 | LKKTETQ | N-acetyl | C38H68N10O14 | 889.01 g/mol | 889.0 g/mol (PubChem) |
| MOTS-c | MRWQEMGYIFYPRKLR | none | C101H152N28O22S2 | 2174.59 g/mol | 2174.6 g/mol (PubChem) |
| GHK (without copper) | GHK | none | C14H24N6O4 | 340.38 g/mol | 340.4 g/mol (PubChem) |
| CJC-1295, no DAC | YADAIFTQSYRKVLAQLSARKLLQDILSR | C-amide | C152H252N44O42 | 3367.90 g/mol | 3367.9 g/mol (calculated) |
Calculated with the tool above. Ipamorelin, SS-31 and tesamorelin contain non-standard residues or caps and are left out.
From sequence to tested vial.
Helion's compound pages list each sequence, formula and molecular weight, and every published lab report states identity by mass spectrometry, HPLC purity and the measured content of the lot. See the research library, the guide to how peptides are tested and the COA Portal.
Specs and storage by compound.
Questions (8)
How do you calculate the molecular weight of a peptide?
Add the residue masses of every amino acid in the sequence, then add one water (18.02 g/mol) for the free N- and C-termini. Adjust for modifications such as an N-terminal acetyl (+42.04) or a C-terminal amide (-0.98).
What is the difference between average and monoisotopic mass?
Average molecular weight uses standard atomic weights, which reflect the natural mix of isotopes; it is the value used for weighing. Monoisotopic mass uses only the most abundant isotope of each element; it is what a mass spectrometer reports for the lightest peak.
Does the calculator include salts like acetate or TFA?
No. It calculates the peptide itself. Synthetic peptides are usually supplied as salts and contain some water, so the weight of a powder is higher than the peptide content alone.
How does C-terminal amidation change the mass?
Replacing the terminal OH with NH2 lowers the molecular weight by about 0.98 g/mol and adds one nitrogen and one hydrogen while removing one oxygen.
How accurate is the net charge estimate?
It is an estimate from one set of pKa values (EMBOSS) at pH 7.0. Different pKa sets give somewhat different answers, and neighboring residues and structure shift real pKa values.
What is the isoelectric point of a peptide?
The pH at which its net charge is zero. The calculator finds it from the same EMBOSS pKa set used for the charge estimate, so it is an estimate rather than a measured value.
What does GRAVY mean?
Grand average of hydropathy: the average Kyte-Doolittle hydropathy value across the sequence. Values above zero suggest a more hydrophobic peptide, values below zero a more hydrophilic one.
Can I use it for peptides with non-standard amino acids?
Only the 20 standard amino acids are supported. Residues such as Aib or dimethyltyrosine need their own masses added by hand; the amino acid chart lists them.
Published studies (4)
- Characterization of Synthetic Peptide Therapeutics Using Liquid Chromatography-Mass Spectrometry: Challenges, Solutions, Pitfalls, and Future Perspectives
- IPC - Isoelectric Point Calculator
- A simple method for displaying the hydropathic character of a protein
- Synthetic pharmaceutical peptides characterization by chromatography principles and method development
Citations from PubMed for reference. They describe the compound in the literature, not any Helion lot, and nothing here is guidance on use.