The influence quantities for the uncertainty of a volumetric operation with glass instruments are calibration, repeatability and temperature. In the literature, measurement uncertainty budgets can be found, which count all three quantities separately although calibration and repeatability are merged in tabulated data to the Maximum Permissible Error. We propose that this error should be handled as a rectangular distribution in order to get a standard uncertainty. For the daily use in an analytical laboratory, the combined standard uncertainty of a volumetric operation is thus calculated from the Maximum Permissible Error plus the uncertainty of the temperature influence. Keywords Glass volumetric instruments Á Maximum Permissible Error Á Measurement uncertainty Á Measuring flasks Á Pipettes Á Single-volume glass instruments Á Uncertainty of volumetric operations