Peptide Calculator

How to Use the Peptide Calculator

1. Enter your peptide sequence using standard single-letter amino acid codes (e.g. ACDEFGHIKLM).
2. Select N-terminus and C-terminus modifications if applicable (e.g. acetylation, amidation) — these are common in synthetic peptides.
3. Click Calculate Peptide Properties.
4. Results include: MW in Da and kDa, length, isoelectric point (pI), net charge at pH 7.4, GRAVY hydrophobicity index, extinction coefficient at 280 nm, and full amino acid composition.
5. Use the Charge at a Specific pH field to look up net charge at any pH.

About Peptide Physicochemical Properties

Molecular weight is calculated from residue masses of each amino acid plus one water molecule (18.015 Da) for the termini. N-acetylation adds 42.011 Da; C-amidation subtracts 0.984 Da.

Isoelectric point (pI) is the pH at which the peptide has zero net charge, calculated using Henderson-Hasselbalch equations with standard pKa values for each ionisable group.

GRAVY index (Grand Average of Hydropathy) is the sum of hydropathy values for all residues divided by sequence length, using the Kyte-Doolittle scale. Positive values indicate hydrophobic peptides; negative values indicate hydrophilic peptides.

Extinction coefficient at 280 nm is calculated from Trp (5500), Tyr (1490), and Cys disulfide bonds (125) contributions, following the Pace et al. 1995 method.

N-terminus and C-terminus Modifications

• Free NH₂ / COOH (default): Unmodified termini — the standard form for most recombinant proteins and natural peptides.
• N-acetylation (+42.011 Da): Common modification that neutralises the positive charge of the N-terminus, increasing metabolic stability. Used in many therapeutic peptides.
• C-amidation (−0.984 Da): Replaces the C-terminal carboxyl group with an amide, neutralising the negative charge and improving membrane permeability and protease resistance.
• Many bioactive peptides (e.g. oxytocin, vasopressin) are C-terminally amidated in vivo.