PPM Mass Error Calculator (Mass Spectrometry)
Calculate mass accuracy in ppm and mDa between a measured and a theoretical m/z for high-resolution mass spectrometry, plus the ± Da window for a ppm tolerance and a pass/fail check.
How it works
mass error (ppm) = (measured − theoretical) ÷ theoretical × 10⁶ · error (mDa) = (measured − theoretical) × 1,000 · tolerance window (Da) = theoretical × ppm ÷ 10⁶
High-resolution mass spectrometers (Orbitrap, FT-ICR, Q-TOF) report mass accuracy in parts per million because the same absolute error matters less at higher mass. Comparing a measured m/z with the exact monoisotopic mass of a proposed formula or peptide is the first filter for identification: modern instruments routinely achieve better than 3–5 ppm, and many journals ask for elemental compositions within 5 ppm. A tolerance in ppm also sets the extraction window for chromatograms and database searches.
Worked example
A peptide ion with theoretical m/z 524.2648 is measured at 524.2661. The error is 0.0013 Da = +1.30 mDa, and 0.0013 ÷ 524.2648 × 10⁶ = +2.48 ppm — within a ±5 ppm tolerance, whose window is ± 0.00262 Da (524.2622 – 524.2674).
Sources: Brenton AG, Godfrey AR. Accurate mass measurement: terminology and treatment of data. J Am Soc Mass Spectrom 2010;21:1821–35 · IUPAC — Definitions of terms relating to mass spectrometry (2013)
Frequently asked questions
How do I calculate ppm mass error?
Subtract the theoretical mass from the measured mass, divide by the theoretical mass and multiply by one million. A 0.001 Da error at m/z 500 is 2 ppm; the same error at m/z 1,000 is 1 ppm.
What mass accuracy is acceptable?
For confirming an elemental formula, within ±5 ppm is the common journal standard (some require ±3 ppm). Orbitrap and FT-ICR instruments often reach below 1–2 ppm with internal calibration; proteomics searches typically use 10–20 ppm precursor tolerance.
Should I use ppm or mDa?
ppm is relative and comparable across the mass range; mDa is absolute and is sometimes used for small molecules below about m/z 200, where a few mDa is a large ppm value.
Does the sign of the error matter?
The sign shows whether the measurement is high or low. A consistent sign across many ions points to a calibration drift that can be corrected with a lock mass.
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This calculator is for educational purposes. Double-check important results before acting on them.