peptide mass calculator

What this peptide mass calculator does

This tool estimates the neutral peptide mass and predicted m/z values for protonated ions. It is designed for quick planning in proteomics, peptide synthesis, LC-MS method setup, and quality checks. Paste a peptide sequence, choose your output mode, and instantly get mass values for multiple charge states.

How the calculation works

1) Residue summation plus water

The calculator sums residue masses for each amino acid in your sequence, then adds one water molecule to represent the full peptide termini. This is standard for reporting intact peptide neutral masses.

• Monoisotopic mass: uses exact isotopic masses (best for high-resolution MS).
• Average mass: uses isotope-averaged atomic masses (common in broader chemistry references).

2) Optional terminal modifications

You can include common terminal changes directly in the calculator:

• N-terminal acetylation (adds mass)
• C-terminal amidation (slightly reduces mass relative to free acid terminus)

If your peptide has additional modifications, use the custom mass adjustment field to add (or subtract) a total delta mass in Daltons.

3) m/z by charge state

For each charge state z, the calculator reports:
m/z = (M + z × proton_mass) / z

This gives expected precursor values for protonated ions such as [M+H]⁺, [M+2H]²⁺, [M+3H]³⁺, and so on.

How to use this tool effectively

• Enter a clean sequence in single-letter format (for example, PEPTIDEK).
• Start with monoisotopic output for modern high-resolution instruments.
• Set maximum charge state to match your expected ionization behavior.
• Add known terminal or custom modifications before comparing against measured spectra.

Worked example

Suppose your sequence is ACDEFGHIK. The calculator sums each residue, adds water, then computes m/z values for z = 1 to z = 3 (or your chosen maximum). If you acetylate the N-terminus, all predicted masses shift upward by the acetyl delta.

Common pitfalls to avoid

• Using modified peptides without applying the relevant mass shifts.
• Comparing average masses against monoisotopic spectral peaks.
• Forgetting that charge state changes m/z spacing and apparent peak positions.
• Including non-standard residue letters without defining explicit masses.

Final notes

This calculator is ideal for fast peptide mass estimation and LC-MS targeting. For publication-grade structural annotation, isotopic envelope fitting, or fragmentation modeling, pair this output with your dedicated mass spectrometry software pipeline.