Peptide Molecular Weight Calculator
Calculate the molecular weight of any peptide sequence. Free online tool with instant results and amino acid composition breakdown.
Molecular Weight Calculator
Enter a peptide sequence using one-letter amino acid codes (G, A, V, L, I, P, F, W, M, S, T, C, Y, H, D, E, N, Q, K, R)
How to Use This Calculator
- Enter your peptide sequence using one-letter amino acid codes
- Accepted codes: G, A, V, L, I, P, F, W, M, S, T, C, Y, H, D, E, N, Q, K, R
- Click Calculate to see the molecular weight in Daltons (Da)
- View the residue count and amino acid composition breakdown
The Formula
The molecular weight of a peptide is calculated by summing the residue weights of each amino acid:
Where n is the number of amino acid residues and 18.015 Da is the mass of water lost during each peptide bond formation (condensation reaction).
Example Calculations
Glutathione (GSH)
Gly-Ser-His
75.03 + 105.09 + 155.16 − 2 × 18.015 = 299.30 Da
Oxytocin (truncated)
CYIQNCPLG
9 residues, disulfide-bonded cyclic nonapeptide = 1007.19 Da
Tripeptide AAA
Ala-Ala-Ala
89.09 × 3 − 2 × 18.015 = 231.23 Da
Vasopressin (truncated)
CYFQNCPRG
9 residues, cyclic nonapeptide = 1056.22 Da
Amino Acid Residue Weights
| Code | Amino Acid | Residue MW (Da) |
|---|---|---|
| G | Glycine | 75.03 |
| A | Alanine | 89.09 |
| V | Valine | 117.15 |
| L | Leucine | 131.17 |
| I | Isoleucine | 131.17 |
| P | Proline | 115.13 |
| F | Phenylalanine | 165.19 |
| W | Tryptophan | 204.23 |
| M | Methionine | 149.21 |
| S | Serine | 105.09 |
| T | Threonine | 119.12 |
| C | Cysteine | 121.16 |
| Y | Tyrosine | 181.19 |
| H | Histidine | 155.16 |
| D | Aspartic Acid | 133.10 |
| E | Glutamic Acid | 147.13 |
| N | Asparagine | 132.12 |
| Q | Glutamine | 146.15 |
| K | Lysine | 146.19 |
| R | Arginine | 174.20 |
Frequently Asked Questions
What is the molecular weight of a peptide?
The molecular weight of a peptide is the sum of the molecular weights of its constituent amino acid residues minus the water molecules lost during peptide bond formation. For a peptide with n residues, (n−1) water molecules (18.015 Da each) are removed.
How do you calculate peptide molecular weight manually?
Sum the molecular weights of all amino acid residues in the sequence, then subtract 18.015 Da for each peptide bond (number of residues minus 1). For example, for GSH (Gly-Ser-His): 75.03 + 105.09 + 155.16 − 2 × 18.015 = 299.30 Da.
What units are used for peptide molecular weight?
Peptide molecular weight is typically expressed in Daltons (Da) or atomic mass units (AMU), which are equivalent to g/mol.
Does this calculator account for disulfide bonds?
The basic calculator computes the linear peptide molecular weight. Disulfide bonds between cysteine residues result in loss of 2 hydrogen atoms (2.016 Da per bond), which should be subtracted for cyclic peptides.