How to Use This Calculator
Type or paste the sequence using one-letter codes, for example G for glycine and A for alanine, or pick a common peptide from the preset list. Spaces and lowercase letters are fine. Click Calculate to see the molecular weight in g/mol, the number of residues and how many nanomoles are in 1 mg. Any letter that is not one of the 20 standard amino acids is flagged.
Molecular Weights of Common Research Peptides
These values are for the plain linear sequence, the same way this calculator works them out. Peptides with an amide end, a disulfide bond or another change weigh slightly differently in real life.
| Peptide | Sequence | Residues | Molecular weight | nmol per mg |
|---|---|---|---|---|
| BPC-157 | GEPPPGKPADDAGLV | 15 | 1,419.55 g/mol | 704 |
| TB-500 fragment | LKKTETQ | 7 | 846.98 g/mol | 1,181 |
| Selank | TKPRPGP | 7 | 751.88 g/mol | 1,330 |
| Semax | MEHFPGP | 7 | 813.93 g/mol | 1,229 |
| GHK | GHK | 3 | 340.38 g/mol | 2,938 |
| Epitalon | AEDG | 4 | 390.35 g/mol | 2,562 |
| Glutathione | ECG | 3 | 307.32 g/mol | 3,254 |
| Oxytocin (linear) | CYIQNCPLG | 9 | 1,010.19 g/mol | 990 |
Why Molecular Weight Matters for Dosing
Two peptides can weigh the same in milligrams but contain very different numbers of molecules. A milligram of a small peptide like GHK holds about four times as many molecules as a milligram of BPC-157. When a study reports a dose in nanomoles or micromoles, you need the molecular weight to turn it into milligrams you can weigh or draw. Our guide on why molar dosing matters goes into this in more depth.
Average Mass or Monoisotopic Mass?
Certificates of analysis usually give the average mass, worked out from the standard atomic weight of each element. Mass spectrometry reports often give the monoisotopic mass, which uses only the most common isotope of each element. The two are close but not the same. This calculator gives the average mass, which is the one to use for weighing, mixing and dosing.
Who Should Use This
Researchers converting between mass and molar amounts, and anyone checking a supplier's stated molecular weight against the sequence.
What This Calculator Does
The Peptide Molecular Weight Calculator adds up the average residue masses of a sequence entered in single-letter code, plus one water molecule, to give the average molecular weight in g/mol. It also shows the number of residues and how many nanomoles are in 1 mg.
Formula Used
Molecular weight (g/mol) = Sum of average residue masses + 18.015 (water)nmol per mg = 1,000,000 / Molecular weightnmol in a dose = Dose (mcg) x 1,000 / Molecular weightExample Calculation
Limitations
The result is the average mass of the linear peptide with a free acid end. It does not include C-terminal amides, disulfide bonds, acetylation, salt forms or other changes unless you adjust for them using the note under the calculator.
Frequently Asked Questions
How do you calculate the molecular weight of a peptide?
Add up the residue mass of each amino acid in the sequence, then add 18.02 g/mol for the one water molecule that belongs to the whole chain. The calculator does this from the sequence you paste.
What is the molecular weight of BPC-157?
Worked out from its sequence, GEPPPGKPADDAGLV, BPC-157 is about 1,419.6 g/mol as a linear peptide with a free acid end.
How do I convert mcg to nmol?
Multiply the amount in mcg by 1,000, then divide by the molecular weight. 250 mcg of BPC-157 is 250 x 1,000 / 1,419.6, which is about 176 nmol.
Why is my supplier's molecular weight different?
The supplier may list the peptide as a salt, such as an acetate, or with an amide end or a disulfide bond. Each of these changes the mass. This calculator gives the plain linear peptide, so small differences are normal.
Is g/mol the same as Da?
Yes, for this purpose. A molecular weight of 1,419.6 g/mol is the same as 1,419.6 Da (daltons).
What is the difference between peptide mass and molecular weight?
In this calculator they are the same number: the average mass in g/mol. Mass spectrometry reports may give the monoisotopic mass instead, which is slightly different (see the section above on average and monoisotopic mass).
Can I use this as a protein molecular weight calculator?
Yes, for any sequence made of the 20 standard amino acids. The method is the same: add the residue masses and one water. It does not include disulfide bonds, sugars, amides or other changes.
How do I find the molecular weight of a polypeptide?
Enter the polypeptide sequence in one-letter code and click Calculate. The result is the sum of the residue masses plus one water molecule, in g/mol.