Convert between ppm and Da mass error for mass spectrometry. Calculate observed or theoretical m/z.
Enter any two of the four values (theoretical m/z, observed m/z, ppm error, Da error) and the calculator solves for the other two. The "Quick Converter" at the bottom lets you see what a ppm tolerance window looks like in Da at a given m/z — useful for setting database search tolerances.
Mass accuracy in mass spectrometry is the difference between the measured (observed) mass and the true (theoretical) mass. It's expressed either in absolute terms (Da or mDa) or relative terms (ppm).
ppm = ((m/z_obs − m/z_theo) / m/z_theo) × 10⁶
Da = m/z_obs − m/z_theo
Worked example: You observe a peptide at m/z 1045.5392, and the theoretical monoisotopic mass is 1045.5344. Error = (1045.5392 − 1045.5344) / 1045.5344 × 10⁶ = 4.59 ppm, or 0.0048 Da (4.8 mDa).
Typical instrument accuracies: Orbitrap instruments achieve <3 ppm (often <1 ppm with internal calibration). Q-TOF instruments typically achieve 2–5 ppm. Triple quadrupoles give 0.1–0.5 Da accuracy (not useful for ppm calculations). Tighter mass accuracy reduces false positive identifications in proteomics and metabolomics database searches.
Important: ppm error is mass-dependent — 10 ppm at m/z 500 is 0.005 Da, but 10 ppm at m/z 2000 is 0.02 Da. This is why ppm is preferred over Da for reporting mass accuracy across a range of masses.