Peptide Molecular Weight Calculator
Paste a sequence in one-letter code (GEPPPGKPADDAGLV) or hyphenated three-letter code (Gly-Glu-Pro-…) and the calculator returns the average molecular weight, the monoisotopic mass, the singly protonated [M+H]⁺ ion, the molecular formula and the residue composition. Checkboxes add N-terminal acetylation and a C-terminal amide, and a field sets the number of disulfide bonds. It covers the 20 standard amino acids; the average MW it reports is the figure to carry into the molarity calculator.
Results
- Average molecular weight
- 1419.54 g/mol
- Monoisotopic mass
- 1418.7042 Da
- [M+H]⁺ 1419.7114 m/z
- Molecular formula
- C62H98N16O22
- Length
- 15 residues
- Ala 2 · Asp 2 · Glu 1 · Gly 3 · Lys 1 · Leu 1 · Pro 4 · Val 1
Provided for laboratory calculation convenience only. All compounds sold by Australian Peptide Lab are for in-vitro research use only — not for human or veterinary use.
A peptide's mass follows from its formula, and its formula follows from its sequence. The calculator adds up residue formulas, applies the selected modifications and converts the total to masses using standard element values.
How the calculation works
Each residue is counted as its amino acid minus one water, because a water is lost for every peptide bond. One water (H₂O) is then added back for the free N- and C-termini. Average mass uses standard atomic weights (C 12.0107, H 1.00794, N 14.0067, O 15.9994, S 32.065); monoisotopic mass uses the lightest stable isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). [M+H]⁺ is the monoisotopic mass plus one proton, 1.007276.
| Modification | Formula change | Average Δ | Monoisotopic Δ |
|---|---|---|---|
| N-terminal acetylation | + C₂H₂O | +42.04 | +42.0106 |
| C-terminal amide (–OH → –NH₂) | + N + H − O | −0.98 | −0.9840 |
| Each disulfide bond | − 2H | −2.02 | −2.0157 |
Worked examples
These outputs agree with the formulas and masses held by PubChem for each compound.
| Peptide | Entered as | Options | Formula | Average MW | Monoisotopic | [M+H]⁺ |
|---|---|---|---|---|---|---|
| BPC-157 | GEPPPGKPADDAGLV | none | C₆₂H₉₈N₁₆O₂₂ | 1419.54 | 1418.7042 | 1419.7114 |
| Selank | TKPRPGP | none | C₃₃H₅₇N₁₁O₉ | 751.87 | 751.4341 | 752.4413 |
| Semax | MEHFPGP | none | C₃₇H₅₁N₉O₁₀S | 813.92 | 813.3480 | 814.3552 |
| Epithalon | AEDG | none | C₁₄H₂₂N₄O₉ | 390.35 | 390.1387 | 391.1460 |
| GHK | GHK | none | C₁₄H₂₄N₆O₄ | 340.38 | 340.1859 | 341.1932 |
| Oxytocin | CYIQNCPLG | amide, 1 disulfide | C₄₃H₆₆N₁₂O₁₂S₂ | 1007.19 | 1006.4365 | 1007.4437 |
Oxytocin shows why the options matter: without the amide and disulfide it would read 1009.20.
Average, monoisotopic or [M+H]⁺
Use the average MW for weighing and molarity, because a weighed sample contains the natural isotope mixture. Use the monoisotopic mass and its ions to identify peaks in a high-resolution spectrum. As peptides grow, the all-light isotopic form becomes a smaller share of the molecules: with 62 carbons at about 1.07% ¹³C, only around half of BPC-157 molecules contain no ¹³C at all.
Input rules that change the answer
- Type residues only. Terminal groups typed as text are misread: "Ac-GHK" is parsed as Ala-Cys-Gly-His-Lys (514.60) instead of acetyl-GHK (382.41), and "-NH2" after a one-letter sequence adds Asn and His. Use the checkboxes.
- Avoid three-letter spacing in one-letter code. One-letter text split into groups of exactly three ("GEP PPG KPA") is read as three-letter codes and rejected. Remove the spaces.
- Read the warning line. Unrecognised characters are listed and left out of the mass. Compare the residue count with the expected sequence length every time.
Non-standard residues
Only the 20 standard amino acids are recognised. Aib, norleucine, ornithine, naphthylalanine, pyroglutamate, phosphorylation, lipidation, PEG and metal ions are outside its table, so the copper in GHK-Cu is not included. The workaround is to calculate the standard portion and add the remaining residue formulas by hand. For ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH₂):
| Part | Formula | Average mass |
|---|---|---|
| His-Phe-Lys with C-terminal amide (calculator, D-Phe entered as F) | C₂₁H₃₁N₇O₃ | 429.52 |
| + Aib residue | C₄H₇NO | 85.10 |
| + 2-naphthylalanine residue | C₁₃H₁₁NO | 197.23 |
| Ipamorelin | C₃₈H₄₉N₉O₅ | 711.85 |
The total matches PubChem's formula for ipamorelin.
Free peptide, salt and net peptide content
The calculator returns the neutral free peptide. A lyophilised powder also carries counter-ions and water, so the weighed mass is more than the peptide mass. If all three basic sites of GHK (N-terminal amine, His and Lys) carried trifluoroacetate, the salt would weigh 682.45 g/mol and be only 49.9% peptide. Molar calculations should pair the free-peptide MW from this tool with a mass corrected for net peptide content; the TFA versus acetate guide explains how salt forms arise, and the guide to reading peptide sequences covers the notation for modified and non-standard residues.
Frequently asked questions
Why doesn't the result match the molecular weight on a certificate of analysis?
Check the formula before the number. Common causes are a salt form quoted instead of the free peptide, a terminal modification left unticked, a non-standard residue the calculator skipped, or a different atomic-weight table, which shifts larger peptides in the second decimal place. The certificates of analysis guide shows where the expected mass appears on a certificate of analysis.
Which figure should be compared with a mass spectrum?
On an instrument that resolves isotopes, compare the monoisotopic mass, or [M+H]⁺ for a singly charged ion. Electrospray often produces multiply charged ions, where m/z = (M + z × 1.007276) ÷ z; for BPC-157, [M+2H]²⁺ falls at 710.36. Where the isotope cluster is not resolved, the peak centroid sits nearer the average mass. Mass spectrometry covers the ion types.
Can the calculator handle D-amino acids?
Indirectly. A D-amino acid has exactly the same formula and mass as its L-form, so entering the ordinary one-letter code gives the correct mass. Neither this calculator nor a mass spectrum distinguishes the two; stereochemistry needs chiral chromatography or other methods. Non-standard side chains are a different matter, because their formulas differ.
How is a head-to-tail cyclic or lactam-bridged peptide calculated?
There is no checkbox for it. Calculate the linear sequence, then subtract one water (18.02 average, 18.0106 monoisotopic) for each new amide bond formed by cyclisation or side-chain lactam bridging. Disulfide bonds have their own field because they remove two hydrogens rather than water.
Why can't semaglutide or ipamorelin be calculated from their sequences here?
Both contain residues outside the standard 20. Semaglutide carries Aib at position 8 and a lysine derivatised with a fatty-acid moiety through a linker [1]; ipamorelin contains Aib and D-2-naphthylalanine. Use the published formula, or calculate the standard part here and add the extra residue formulas by hand, as shown above.
References
- 1.Lau J, Bloch P, Schäffer L, et al. Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide. J Med Chem. 2015. PubMed 26308095
Related guides
How to Read Peptide Sequences: Codes, Termini, Modifications and Cyclic Peptides
Peptide notation decoded: one- and three-letter codes, N→C direction, Ac- and -NH2, D-amino acids, Aib, Nal, lactams and disulfides, with worked examples.
6 min read →Testing & QualityUnderstanding Certificates of Analysis for Research Peptides: HPLC, Mass Spec and Red Flags
How to read a peptide certificate of analysis: the HPLC and mass-spec sections, what each proves and misses, and how to spot a fabricated or recycled COA.
7 min read →Lab TechniquePeptide Molarity Calculations: From Milligrams to Micromolar, Corrected for Net Peptide Content
Peptide molarity maths with worked examples: mass to moles, stock and working solutions, net peptide content, counter-ions, UV checks and serial dilutions.
6 min read →Testing & QualityTFA vs Acetate Peptide Salts: Counter-Ions, Assay Interference and the Mass Maths
Why peptides arrive as TFA salts, how trifluoroacetate can interfere with cell and receptor assays, how salt exchange works, and how it shifts mass.
6 min read →More lab tools
Peptide Reconstitution Calculator
Vial size and diluent volume in; concentration and the volume holding a given amount out.
Peptide Molarity Calculator
Convert between peptide mass, molecular weight and molar concentration (mM, µM).
Peptide Dilution Calculator
How much stock solution and diluent to combine for a working concentration.