Research tool · Free
Peptide molecular weight & property calculator
Paste an amino-acid sequence and get molecular weight, formula, net charge, isoelectric point, hydropathy, and extinction coefficient instantly. The same numbers ExPASy ProtParam gives, in one screen, with no signup.
Length
15
residues
Avg molecular weight
1,419.56
g/mol (Da)
Monoisotopic mass
1,418.70
Da
Theoretical pI
3.88
isoelectric point
Net charge
-2.02
at pH 7.0
GRAVY
-0.693
hydrophilic
Extinction ε₂₈₀
0
M⁻¹cm⁻¹ (Cys reduced)
Abs 0.1% (280 nm)
0.000
1 g/L solution
Molecular formula
C62H98N16O22
Charge at pH 7: -2.02. The curve crosses zero at the isoelectric point.
Peaks above the line are hydrophobic stretches; dips are hydrophilic.
Sequence, colored by side chain
Reads the sequence, not the finished product
These are the properties of the unmodified linear free-acid peptide. C-terminal amidation, N-terminal acetylation, disulfide bonds, cyclization, and fatty-acid conjugation all change the real mass. In particular, the modified GLP-1 drugs (semaglutide, tirzepatide, retatrutide) weigh far more than their bare sequence, so this tool is for unmodified research peptides.
How it is calculated
- Mass and formula:atoms are summed from each residue's composition, removing one water per peptide bond. Average mass uses standard atomic weights; monoisotopic mass uses the principal isotope of each element.
- Net charge and pI: Henderson-Hasselbalch on the EMBOSS pKa set (N-term 8.6, C-term 3.6, Asp 3.9, Glu 4.1, His 6.5, Cys 8.5, Tyr 10.1, Lys 10.8, Arg 12.5). The pI is the pH where net charge is zero.
- GRAVY:the mean Kyte & Doolittle (1982) hydropathy across all residues. Negative is hydrophilic, positive is hydrophobic.
- Extinction coefficient: Pace et al. (1995): 5500 per Trp + 1490 per Tyr, assuming all cysteines reduced. Abs 0.1% is that value divided by molecular weight.
Questions
What does this peptide calculator compute?+
From an amino-acid sequence it returns the molecular formula, average and monoisotopic molecular weight, net charge at pH 7, theoretical isoelectric point (pI), GRAVY hydropathy index, and the molar extinction coefficient at 280 nm. Masses are summed from per-residue atomic composition, pI and net charge use the EMBOSS pKa set, GRAVY uses Kyte and Doolittle (1982), and the extinction coefficient uses Pace et al. (1995).
Does it work for semaglutide or tirzepatide?+
No. This tool describes the free-acid, unmodified amino-acid sequence. Semaglutide, tirzepatide, and similar GLP-1 drugs carry fatty-acid chains and non-standard residues, so their true molecular weight is much higher than the bare sequence. Use the calculator for unmodified research peptides such as BPC-157, Selank, Semax, Epithalon, and DSIP.
What is the isoelectric point (pI)?+
The pI is the pH at which the peptide carries no net electrical charge. It is found by solving the charge equation for zero net charge across pH 0 to 14. It predicts how a peptide behaves in techniques such as ion-exchange chromatography and isoelectric focusing.
Can I paste one-letter or three-letter codes?+
Both. Paste a one-letter string (GEPPPGKPADDAGLV) or hyphen/space-separated three-letter codes (Gly-Glu-Pro-Pro-Pro). Non-amino-acid characters are ignored, and unrecognized tokens are flagged.
Does it account for amidation, acetylation, or disulfide bonds?+
No. The result is the unmodified linear free acid. C-terminal amidation, N-terminal acetylation, disulfide bonds, cyclization, and conjugation all shift the mass and are not modeled, so treat the output as the sequence baseline, not a finished product's certificate value.
For research and educational purposes. Values are theoretical estimates from sequence; verify against a certificate of analysis or mass spectrometry for any real material.