Peptide Molecular Weight Calculator
Paste a sequence to get its molecular weight, formula, monoisotopic mass, isoelectric point, net charge at any pH, extinction coefficient and GRAVY. Then turn milligrams into moles, or a target concentration into the milligrams to weigh.
Non-standard residues (Aib, D-amino acids, lipidation as in semaglutide) are not supported; use the vendor COA mass. That rules out ipamorelin (Aib, D-2-Nal, D-Phe), semaglutide and cyclic Melanotan II.
Net charge vs pH
Drag to any pH from 0 to 14Mass, moles & concentration
Uses the average MW above (1,419.54 g/mol).
To make 2 mL at 2.50 mg/mL you need 5.00 mg of peptide.
Dissolving 5.00 mg of peptide in 2 mL gives 2.500 mg/mL, which is 1,761 µM.
Amino-acid composition
| Residue | Count | Mol % | Share |
|---|---|---|---|
| P Pro Proline | 4 | 26.7 | |
| G Gly Glycine | 3 | 20.0 | |
| A Ala Alanine | 2 | 13.3 | |
| D Asp Aspartic acid | 2 | 13.3 | |
| E Glu Glutamic acid | 1 | 6.7 | |
| L Leu Leucine | 1 | 6.7 | |
| K Lys Lysine | 1 | 6.7 | |
| V Val Valine | 1 | 6.7 |
What This Calculator Works Out
Paste a sequence and every number updates at once. The tool reads one-letter or three-letter code, adds the end groups you pick, and returns the molecular formula, average and monoisotopic mass, isoelectric point, net charge across the pH scale, extinction coefficient at 280 nm, GRAVY score and a residue count.
It opens on BPC-157 (GEPPPGKPADDAGLV), a 15-residue peptide with an average mass of 1,419.54 g/mol. The preset chips load TB-500, GHK, Semax, Selank and oxytocin with their usual end groups already set.
The extinction coefficient matters if you check a solution on a UV spectrophotometer. It counts only tryptophan (5,500), tyrosine (1,490) and cystine (125) per Pace 1995. BPC-157 has none of the three, so it reads 0 at 280 nm and needs another method.
How Peptide Molecular Weight Is Computed
Each peptide bond releases one molecule of water. The calculator adds up the residue formulas (each amino acid minus H₂O), then adds one water back for the two free ends. A 15-mer has 14 peptide bonds, so it weighs about 252 Da less than its 15 loose amino acids.
End groups change the total. N-terminal acetylation adds C₂H₂O, or 42.04 Da. A C-terminal amide swaps the –OH of the acid for –NH₂, which makes the peptide 0.984 Da lighter. Each disulfide bond removes two hydrogens, so closing the ring between oxytocin's two cysteines costs 2.016 Da.
Masses come straight from the formula. Average mass uses C 12.0107, H 1.00794, N 14.0067, O 15.9994 and S 32.065. Tables built on H 1.008 differ in the second decimal, which is why BPC-157 is listed as 1,419.53 on one site and 1,419.55 on another.
Why the Vendor COA Mass Is Different
Your vial holds more than just peptide. Synthetic peptides are cleaved and purified with trifluoroacetic acid, so the dry powder is a salt: a TFA counter-ion sits on each basic group unless the maker exchanged it for acetate. The powder also holds water.
So a "5 mg" vial is 5 mg of peptide, salt and water unless the COA says net peptide. Net peptide content is often 70–90%. Type it into the Net peptide field and the nanomole count and the amount to weigh both correct themselves.
The mass spec line is a third number. It reports the measured ion, usually [M+H]⁺, so expect it about 1 Da above the monoisotopic mass shown here, not near the average MW.
Isoelectric Point and Net Charge, Plainly
Every peptide carries charges that shift with pH. Lysine, arginine, histidine and a free N-terminus are positive in acid. Aspartate, glutamate, a free C-terminus, cysteine and tyrosine turn negative as the pH climbs. The isoelectric point (pI) is the pH where the two cancel out. Acetyl and amide caps remove the charge at their end.
A peptide is usually least soluble near its pI. BPC-157 has a pI of 3.88 and a charge of about −2 at pH 7.4, so it dissolves easily in neutral bacteriostatic water. Selank (pI 11.65) sits at the other end of the scale.
The calculator uses the EMBOSS pK set: N-terminus 8.6, C-terminus 3.6, Lys 10.8, Arg 12.5, His 6.5, Asp 3.9, Glu 4.1, Cys 8.5, Tyr 10.1. ExPASy ProtParam uses the Bjellqvist set and gives BPC-157 a pI of 4.03. Both are estimates, so read the pI as a guide.
From Milligrams to Moles
Moles tell you how many molecules you have. Divide the mass by the molecular weight: 5 mg of BPC-157 at 1,419.54 g/mol is 3,522 nmol, or 3.52 µmol. The other direction works too. Two millilitres at 1 mM needs 2.84 mg.
The Make a solution box does this for any volume and target, in µM or mg/mL. It also adds the extra powder to weigh when net peptide content is below 100%.
Once you know the mass, the peptide calculator turns it into units on the syringe. For hormones labelled in IU, the mg to IU conversion page lists the factors.
Peptide Molecular Weight FAQ
01How do you calculate the molecular weight of a peptide?
Add the residue mass of each amino acid (the amino acid minus one water), then add one water (18.015 Da) back for the free ends. Then adjust for end groups: +42.04 Da for an N-terminal acetyl, −0.984 Da for a C-terminal amide and −2.016 Da per disulfide bond. For BPC-157 with free ends that gives C62H98N16O22 and 1,419.54 g/mol.
02Why is the mass on my COA different from the calculated MW?
There are two usual reasons. The mass spec line reports the measured ion, usually [M+H]+, which sits about 1 Da above the monoisotopic mass. And the vial weight includes TFA or acetate counter-ions plus water, so net peptide content is often 70–90%. A "5 mg" vial may hold closer to 4 mg of actual peptide.
03What is the difference between average and monoisotopic mass?
Average mass uses the natural mix of isotopes and is the number for weighing and molarity. Monoisotopic mass uses only the lightest stable isotope of each element (12C, 1H, 14N, 16O, 32S) and matches the first peak a mass spectrometer sees. For BPC-157 they are 1,419.54 and 1,418.7042 Da.
04What is a peptide's isoelectric point used for?
The pI is the pH at which the net charge is zero. Solubility is usually lowest there, so a peptide near its pI is more likely to cloud or clump. Mixing at a pH one or two units away from the pI keeps it charged and dissolved. This calculator uses the EMBOSS pK set, so expect small differences from ExPASy.
05How many nanomoles are in a 5 mg vial of BPC-157?
At 100% net peptide, 5 mg ÷ 1,419.54 g/mol = 3,522 nmol, or 3.52 µmol. If the COA lists 80% net peptide content, the vial holds 4 mg of peptide, which is 2,818 nmol. Enter the COA percentage in the Net peptide field to see the number for your own vial.
06Can the calculator handle ipamorelin, semaglutide or Melanotan II?
No. Ipamorelin contains Aib and D-amino acids, semaglutide has Aib plus a fatty-acid side chain, and Melanotan II is a cyclic peptide with norleucine and D-phenylalanine. None of these are among the 20 standard residues the tool knows. Use the molecular weight printed on the vendor COA for those.
All tools are for educational purposes only. Not medical advice.