Understand the relationship
The reasoning behind the result
A chain is residues plus terminal water
MW = Σ residue masses + H₂O
Residue masses represent amino acids after peptide-bond condensation. One water molecule completes the free termini of one unmodified linear chain.
A multi-chain complex must be represented chain by chain with its actual linkages.
Composition determines the sequence total
The twenty standard residues have different average masses, so residue count alone gives only a rough approximation.
The contribution ledger makes the exact entered composition auditable.
Average and monoisotopic masses differ
The implemented coefficients are average isotopic residue masses. Mass spectrometry may use monoisotopic masses and explicit charge/adduct states instead.
The displayed Da and g/mol values are numerically equivalent molecular-weight conventions for this model.
Sequence statistics do not predict function
Acidic, basic, aromatic and other composition counts do not establish folding, localization, activity or stability.
Post-translational processing and modifications require explicit residue and chemistry records.
Follow the numbers
Reconcile a peptide molecular weight
- Enter ACDEFGHIK as one unmodified linear chain.
- Count each of the nine standard residues.
- Multiply each count by its average isotopic residue mass.
- Sum the residue contributions.
- Add one 18.01528 Da water term for the free termini.
The result is the unmodified average molecular weight of the entered chain, not a measured mass-spectrum peak.
Quick guide
How to use this calculator
- Enter the exact chain after any cleavage that you intend to model.
- Review every residue count and mass contribution.
- Treat modifications, crosslinks and charge as separate chemical models.
Calculation method
Calculation and interpretation
Keep sequence-derived mass and composition together while leaving charge and pI to an explicit ionization model.
Average molecular weight = Σ average isotopic residue masses + 18.01528 Da for one water molecule completing a linear chain.
Worked example
Reconcile a peptide molecular weight
The result is the unmodified average molecular weight of the entered chain, not a measured mass-spectrum peak.
Average molecular weight = Σ average isotopic residue masses + 18.01528 Da for one water molecule completing a linear chain.
Supported inputs
Precision and limits
Twenty standard residues
Ambiguous, nonstandard and modified residue symbols are excluded from this exact sequence model.
One linear chain
Crosslinks, disulfide mass changes, cyclic peptides and multi-chain complexes need explicit chemistry.
Unmodified mass
Glycosylation, phosphorylation, labels, prosthetic groups, adducts and cleavage outside the entered sequence are excluded.
No structure or function
Composition does not predict folding, activity, localization, stability or gel migration.
Numerical support
Complete mass and composition statistics support up to 100,000 standard residues.
Continue calculating
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