Peptide Molecular Weight Calculator
Paste a one-letter amino acid sequence to get molecular weight, pI, net charge, extinction coefficient and composition. Pure chemistry — runs in your browser.
How the numbers are calculated
- Molecular weight = Σ residue masses + H₂O (18.01528 Da average, 18.010565 Da monoisotopic). Acetylation adds 42.04 Da; C-terminal amidation subtracts 0.98 Da.
- pI and net charge use the Henderson–Hasselbalch equation with EMBOSS pKa values (N-term 8.6, C-term 3.6, Lys 10.8, Arg 12.5, His 6.5, Asp 3.9, Glu 4.1, Cys 8.5, Tyr 10.1).
- Extinction coefficient at 280 nm = 5,500 × Trp + 1,490 × Tyr + 125 × cystine (Pace et al., 1995).
- GRAVY is the average Kyte–Doolittle hydropathy; positive values suggest a more hydrophobic peptide.
Residue masses for every amino acid are listed in the amino acid chart. To turn a DNA or RNA sequence into a peptide sequence first, use the codon chart & translator.
FAQ
How is peptide molecular weight calculated?
Add the residue masses of every amino acid in the sequence, then add one water molecule (18.015 Da average) for the free N- and C-terminus. Terminal modifications such as C-terminal amidation (−0.98 Da) or N-terminal acetylation (+42.04 Da) are then applied.
What is the difference between average and monoisotopic mass?
Average mass uses natural isotope abundances and matches what you weigh on a balance. Monoisotopic mass uses only the most abundant isotope of each element and is what a high-resolution mass spectrometer reports as the first peak.
How is the isoelectric point (pI) estimated?
The calculator finds the pH at which the summed charges of the termini and ionizable side chains (D, E, C, Y, H, K, R) equal zero, using EMBOSS pKa values. Measured pI can differ by several tenths of a unit.
How do I convert mg of peptide to nmol?
Divide the mass by the molecular weight: nmol = mg ÷ MW (g/mol) × 1,000,000. Enter an amount in mg above and the calculator does it for you.
Does this account for disulfide bonds or salts?
No. Each disulfide bridge lowers the mass by about 2.016 Da, and counter-ions such as TFA or acetate add weight to a lyophilized powder. Neither is included.
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