What this calculator returns
- Molecular weight (peptide mass) — the average mass of the peptide in daltons.
- Isoelectric point (pI) — the pH at which the peptide carries no net charge.
- Net charge at pH 7 — useful for predicting solubility and handling.
- Extinction coefficient — for estimating concentration from UV absorbance at 280 nm.
How molecular weight is calculated
The molecular weight is the sum of each residue's average mass plus one water molecule (18.015 Da). For example, a short chain like BPC-157 (GEPPPGKPADDAGLV) returns roughly 1419 Da. Paste your own sequence above to see the exact value.
This is sometimes called the peptide mass calculator, peptide MW calculator, or peptide molecular mass calculator. All four phrasings refer to the same average-mass calculation. The result is in daltons (Da), occasionally written as g/mol.
Peptide MW for common research peptides
The molecular weight of common research peptides is a useful sanity check when comparing a synthesised batch against the theoretical sequence. Standard values for widely-used peptides:
- BPC-157 (15 residues): ~1419 Da
- TB-500 fragment (Ac-SDKP): ~488 Da (full TB-500 is ~4963 Da)
- Semaglutide (modified GLP-1 analog): ~4114 Da
- Tirzepatide (modified GIP/GLP-1 analog): ~4814 Da
- Retatrutide (modified GLP-1/GIP/glucagon analog): ~4731 Da
- Ipamorelin (5 residues, modified): ~711 Da
- CJC-1295 (with DAC): ~3647 Da
- GHK-Cu (tripeptide + copper): ~340 Da (without copper) or ~404 Da (with Cu²⁺)
- Melanotan II (cyclic heptapeptide): ~1024 Da
Note that compound-specific modifications (acetylation, fatty acid conjugation, cyclisation) shift the mass beyond what a standard residue sum will return. The calculator above handles standard 20-residue sequences accurately; modified peptides need a specialised tool or the manufacturer's technical data sheet for exact values.
Why peptide molecular weight matters
In a peptide dosing context, molecular weight is the connection between mass measurement (mg, mcg) and moles, which matters for any concentration calculation expressed in molar units. For most peptide users working in mg/mL or mcg/mL, the calculator's MW output is sufficient.
In a research or characterisation context, the calculated MW is what gets compared against mass spectrometry results. A measured mass that differs from the calculated mass by 18 Da suggests a missing or extra water; by a multiple of a specific modification suggests partial modification of the peptide. The calculated value is the theoretical baseline.
For dosing math directly — turning a vial into a measured draw — you usually do not need molecular weight at all. The reconstitution calculator works directly from vial mass and water volume.
Other common phrasings for this calculator
Different communities call this tool different things — they all mean the same calculation:
- Peptide molecular weight calculator — the most common phrasing in pharmaceutical contexts.
- Peptide mass calculator — common in research and proteomics.
- MW calculator for peptide — compact form used in supplier catalogues.
- Peptide molecular mass calculator — academic phrasing, identical meaning.
- Mass calculator for peptide — variant phrasing, same calculation.
All of these return average mass in daltons. If you arrived here looking for any of those tools, you are in the right place.