Background: In order to effectively plan the delivery of immunization services, manage stock and supply levels and target interventions, national immunization programmes (NIP) must have an estimate of the target population they serve. To overcome challenges with target population estimation, some NIPs apply “rule-of-thumb” conversion factors to total population estimates. We compare these proportionate target population values with those from an external source. Methods: Using data reported by national immunization programmes in sub-Saharan Africa, we computed the proportionate target population as the number of births, surviving infants and children under 5 years of age, respectively, as a proportion of the total population size. We compared these values with those estimates computed from United Nations Population Division (UNPD) data. We then recomputed NIP target population sizes using the proportionate target population values from the UNPD applied to the total population size reported by NIP. Results: Data were available from 47 sub-Saharan Africa countries. Births as a proportion of the total population were greater within reports from NIP (median, 0.0400; IQR: 0.350 - 0.0437) compared to values from UNPD estimates (median, 0.0364; IQR: 0.0332 - 0.0406). Similar patterns were observed for surviving infants (median: NIP, 0.0360; UNPD, 0.0337) and children under 5 years of age (median: NIP, 0.1735; UNPD, 0.1594). The percent difference in proportionate target population ratios between reports from NIPs and the UNPD was >10% in 23 countries for births, in 18 countries for surviving infants, in 15 countries for children under 5 years of age. After re-computing target populations using UNPD proportionate target population values applied to NIP reported total population, recomputed administrative coverage levels for the third dose of DTP containing vaccine were higher in 32 of the 47 countries compared to reported administrative coverage levels. Conclusion: Because childhood immunization-related target populations are among the more difficult ones to accurately estimate and project, immunization programmes in sub-Saharan Africa are encouraged to include a critical assessment of the target population values, in conjunction with their national statistics system, as part of the on-going programme monitoring process.