Peptide Molecular Weight Calculator
Enter a single-letter amino acid sequence to get its molecular weight, computed both ways: monoisotopic (the mass of the most abundant isotope of each atom, used in mass spectrometry) and average (using standard atomic weights).
Single-letter amino acid codes only (A–Z, standard 20).
Results
- Monoisotopic mass
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- Average mass
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- Residues
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How this calculator works
Molecular weight is the sum of each residue's mass (its contribution once bonded into the chain) plus one water molecule for the free N- and C-termini. Residue masses come from a single constants table shared across every research calculator on this site.
How to calculate peptide molecular weight
- List each amino acid residue in the sequence.
- Look up each residue's mass (the free amino acid's mass minus one water molecule).
- Sum the residue masses across the whole sequence.
- Add one water molecule back, to account for the free N-terminal H and C-terminal OH.
Worked example: triglycine (Gly-Gly-Gly). Glycine's residue mass is about 57.05 Da, so three residues sum to 3 × 57.05 = 171.15 Da. Adding one water molecule (18.02 Da average) gives 171.15 + 18.02 = 189.17 Da — the average molecular weight of Gly-Gly-Gly.
Frequently asked questions
Is this the same as a peptide mass calculator?
Yes. "Molecular weight" and "mass" describe the same number here — the mass of one mole of the peptide, in g/mol (Da). Enter your sequence above to get both the monoisotopic and average mass.
What is peptide molecular weight?
It's the mass of one mole of the peptide, in grams per mole (g/mol or Da). It's computed as the sum of every amino acid residue's mass in the chain, plus one water molecule for the free N-terminus and C-terminus.
How do I calculate peptide molecular weight by hand?
List each residue in the sequence, look up its residue mass (the free amino acid's mass minus one water, since forming each peptide bond releases a water molecule), sum every residue, then add back one water molecule for the two free termini.
What's the difference between monoisotopic and average molecular weight?
Average molecular weight uses each element's standard atomic weight, which is a natural-abundance-weighted average across that element's isotopes. Monoisotopic mass uses only the single most abundant isotope of each element. High-resolution mass spectrometry reports monoisotopic mass, since it resolves individual isotope peaks rather than an averaged blend.
Does this calculator account for modifications like acetylation or amidation?
No — it computes molecular weight from the unmodified linear sequence only. Common modifications (N-terminal acetylation, C-terminal amidation, PEGylation, and others) each add or remove a known mass that isn't included here and would need to be added manually.
Where is molecular weight used in other calculations?
It's the input the peptide molarity calculator needs to convert a mass concentration into molarity, and the denominator the extinction coefficient calculator uses to convert ε280 into A(0.1%).
Is this a substitute for measuring mass by spectrometry?
No. This is a theoretical calculation from the sequence you enter. An actual synthesized or expressed peptide can differ from that theoretical mass because of modifications, incomplete synthesis, or adducts — mass spectrometry remains the standard for verifying a real sample.
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Educational tool only, not medical advice — verify with a licensed clinician. See the full medical disclaimer.