Carbon monoxide is the most abundant molecule after H_2_ and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission lines. In this paper an update and extension of the classical Mamon et al. (1988ApJ...328..797M) paper is presented; these authors provided the CO abundance profile, described by two parameters, as a function of mass-loss rate and expansion velocity. Following recent work an improved numerical method and updated H_2_ and CO shielding functions are used and a larger grid is calculated that covers more parameter space, including the strength of the ISRF. The effect of changing the photodissociation radius on the low-J CO line intensities is illustrated in two cases.