Peptide Molecular Weight Calculator

Inputs

Peptide sequence (one-letter codes)

Letters allowed: A R N D C E Q G H I L K M F P S T W Y V. Others ignored.

Mass model

Adduct for m/z

Used for positive-mode m/z across selected charge states.

N-terminus

C-terminus

Common residue modifications (applied to all matching residues)

Carbamidomethyl on C (+57.021464)

Oxidation on M (+15.994915)

Phosphorylation on S (+79.966331)

Phosphorylation on T (+79.966331)

Phosphorylation on Y (+79.966331)

Custom delta mass (Da) and count

Adds mass × count to the peptide (any sign allowed).

Charge states (comma-separated)

Positive integers, e.g., 1,2,3,4.

Results

Mass model: –

Termini: –

Length: 0 residues

Neutral mass (Da): –

Total delta from mods (Da): 0.000000

m/z by charge

Charge (z)	Adduct	m/z

Amino acid composition

Example data

These examples auto-calculate using current settings. Click a row to load the sequence.

Sequence	Length	Neutral mass (Da)	z=2 m/z (adduct)
`ACDE`	–	–	–
`PEPTIDE`	–	–	–
`MKWVTFISLLFLFSSAYSR`	–	–	–

What is the molecular weight of a peptide with the sequence ala-ala-arg-asn?

Lowercase names map to one-letter codes: ALA–ALA–ARG–ASN → AARN.

Monoisotopic neutral mass: 430.228831 Da
Average neutral mass: 430.464180 Da

Assumes free termini and no modifications; mass equals Σ residues + H₂O.

Amino acid residue masses reference

Residue masses exclude water; one H₂O is added for free termini.

Residue	Monoisotopic (Da)	Average (Da)

Supported modifications and delta masses

Modification	Target	Δ mass (Da)	Applied as

Phosphorylation adds 79.966331 per modified S, T, or Y residue.

Formula used

Two equivalent approaches exist. We use residue masses and add water:

Neutral mass = Σ(residue masses) + mass(H₂O) + Σ(fixed mods) + custom delta.

Residue masses already exclude water; adding one H₂O restores free termini.

mass(H₂O): 18.010565 (monoisotopic) or 18.01528 (average)
N-term acetylation: +42.010565; C-term amidation: −0.984016
Carbamidomethyl on C: +57.021464 each C
Oxidation on M: +15.994915 each M
Phosphorylation on S/T/Y: +79.966331 each modified residue

m/z for charge z with adduct mass A is: m/z = (Neutral mass + z·A) / z.

Adduct masses used: H⁺ 1.007276466, Na⁺ 22.989218, K⁺ 38.963158.

how to calculate molecular weight of peptide?

Select a mass model: Monoisotopic or Average.
Convert names to one‑letter codes and clean the sequence.
Sum residue masses from a residue mass table.
Add one H₂O to restore free termini.
Add fixed/variable modifications and any custom delta mass.
For m/z, add z adduct masses and divide by z.

Worked example (monoisotopic): `ACDE`

Residue	Mass (Da)
A	71.037114
C	103.009185
D	115.026943
E	129.042593
Σ residues	418.115835
+ H₂O	18.010565
Neutral mass	436.126400

Assumes free termini and no modifications. Use the checkboxes to add PTMs.

Key constants

H₂O (mono): 18.010565
H₂O (avg): 18.01528
H⁺: 1.007276466
Na⁺: 22.989218
K⁺: 38.963158

Common PTMs: CAM on C +57.021464, Oxidation on M +15.994915, Phospho on S/T/Y +79.966331.

How to use this calculator

Paste a peptide sequence using one-letter amino acid codes.
Select the mass model and choose termini options if needed.
Tick common modifications to apply across matching residues.
Add any custom delta mass and its count if applicable.
Enter charge states to compute m/z values for the chosen adduct.
Click Calculate to see mass, composition, and m/z table.
Use Download CSV or Download PDF to save results.
Click Shareable URL to encode current inputs in the address bar.

Tip: Click any example row to load that sequence into the form.