peptide molecular weight calculator

What this peptide molecular weight calculator does

This calculator estimates the molecular weight of a peptide from its amino acid sequence. It supports both average mass and monoisotopic mass, and it can optionally account for common terminal changes (N-terminal acetylation and C-terminal amidation). It also reports a simple m/z value for your chosen charge state.

How peptide mass is calculated

A peptide is made of amino acid residues linked by peptide bonds. In mass calculations, we sum the residue masses and then add the mass of water to represent the full neutral peptide termini:

Peptide mass = Σ(residue masses) + H₂O + terminal modifications

The calculator uses standard residue masses for the 20 canonical amino acids. If you choose a charge state, the displayed m/z is computed as:

m/z = (M + z × proton mass) / z

Average vs monoisotopic mass

• Average mass uses isotope-weighted atomic averages and is useful for many routine estimates.
• Monoisotopic mass uses the lightest isotopes (e.g., ¹²C, ¹H) and is commonly used in high-resolution MS workflows.

How to use the tool

1. Paste or type your peptide sequence in one-letter format.
2. Select mass type (average or monoisotopic).
3. Choose terminal modifications if applicable.
4. Enter the charge state (z) for an m/z estimate.
5. Click Calculate.

Practical notes for lab work

1) Validate sequence symbols first

Most data entry errors come from unsupported letters or copy/paste artifacts. This calculator ignores spaces and line breaks, but it flags non-canonical residue codes so you can fix issues quickly.

2) Confirm modification assumptions

A peptide with free termini has a different mass than one that is acetylated or amidated. If your synthesis report includes caps or modifications, ensure those settings are selected before interpreting results.

3) Match mass type to instrument context

For many proteomics and analytical MS pipelines, monoisotopic values are preferred. For some formulation or theoretical calculations, average mass may be more convenient.

Common applications

• Quality checks after peptide synthesis
• Quick planning for MALDI or ESI-MS experiments
• Teaching peptide chemistry and sequence-to-mass relationships
• Cross-checking expected precursor masses in targeted assays

Limitations

This is a practical sequence calculator, not a full proteomics engine. It currently supports canonical amino acids and two terminal modifications only. It does not model isotopic envelopes, adduct series, internal modifications (e.g., oxidation, phosphorylation), or disulfide-linked multipeptide systems.

For advanced workflows, treat this tool as a fast first-pass estimate and then confirm with your preferred bioinformatics or vendor-specific mass spectrometry software.