mass peptide calculator

What this mass peptide calculator does

This tool estimates peptide molecular mass and expected m/z values for protonated ions. It is designed for quick planning during LC-MS, MALDI, and proteomics method development. You can switch between monoisotopic and average mass, include terminal modifications, and apply commonly used fixed/variable adjustments.

Under the hood, the calculator sums residue masses, adds water to account for peptide termini, then adds any user-selected modification masses. It then computes charge-state values using the standard formula for protonated ions.

How the mass is calculated

1) Neutral peptide mass

The neutral mass is estimated as:

Mass = (sum of residue masses) + H₂O + N-term mod + C-term mod + variable/fixed mods

The water term reflects peptide bond chemistry for a complete peptide chain. If you enable fixed carbamidomethylation on cysteine, each C residue gets the CAM mass added automatically.

2) Charge-state m/z

For a given charge state z, the protonated ion mass-to-charge is:

m/z = (M + z × proton_mass) / z

where M is the neutral peptide mass.

Step-by-step usage

• Paste your peptide in one-letter format (for example: PEPTIDEK).
• Select monoisotopic or average mass type.
• Optionally add N- and C-terminal modification masses.
• Set oxidized methionine count if needed.
• Choose whether cysteines should include fixed CAM.
• Set the highest charge state you want listed and click Calculate Mass.

Practical notes for proteomics workflows

Monoisotopic vs average mass

Use monoisotopic mass for high-resolution MS interpretation (Orbitrap/TOF workflows), especially when matching exact precursor peaks. Average mass can still be useful in lower-resolution contexts or for rough planning.

Common modifications to consider

• Carbamidomethylation (C): common fixed alkylation after IAA treatment.
• Methionine oxidation: often appears as a variable modification from sample handling.
• Terminal labels: isotopic tags, acetylation, amidation, or custom linker chemistry.

Example

If your peptide is ACDMK and you enable fixed CAM plus one methionine oxidation, the final mass includes: base residue sum + water + CAM on C + oxidation on M. The charge table then gives expected m/z values for 1+ through your selected max charge.

Troubleshooting tips

• If you get an invalid-sequence error, check for unsupported letters like B, J, O, U, X, or Z.
• If your observed precursor is off by ~1 Da, confirm monoisotopic vs average selection.
• If a measured peak is systematically higher, verify fixed modifications were included correctly.
• Always cross-check with instrument software for final reporting and regulatory documentation.

Important disclaimer

This calculator is for educational and research planning use. It does not replace validated vendor pipelines, controlled bioinformatics workflows, or clinical decision software.