We have recently revised the carbon dioxide calibration scale used by numerous laboratories that measure atmospheric CO₂. The revision follows from an improved understanding of the manometric method used to determine the absolute amount of CO₂ in an atmospheric air sample. The new scale is 0.18 μmol mol⁻¹ (ppm) greater than the previous scale at 400 ppm CO₂. While this difference is small in relative terms (0.045 %), it is significant in terms of atmospheric monitoring.