Abstract Background The benefits of hospital clinical pharmacy services are well established, though staff numbers required for its delivery have not been well described, leading to variation in pharmacy staffing between hospitals. The need to identify a consistent, objective method of determining staffing levels was recognised at a UK University Hospital and a Clinical Pharmacy Workforce Calculator (CPWC) was developed. Objective To report on the validation of the CPWC across acute hospital settings in Great Britain. Method Using the World Health Organisation’s Workload Indicators of Staffing Need (WISN) methodology, a two-round Delphi consensus study with an Expert Panel of UK hospital pharmacy managers was undertaken to develop the Activity Standard for pharmaceutical care and to identify the time unavailable for clinical work inherent in employing staff. Consenting Panel members then tested the CPWC by calculating the staff resource required for three staffing scenarios to determine whether it could be reliably used by different operators. Results Thirty-six participants consented to participate, and data was returned from 22 participants (61%) of whom 20 (56%) supplied analysable data. Consensus was achieved on the tasks required for pharmaceutical care delivery, the mean time each takes, the frequency of completion and the unavailable time in the employment of each grade of staff identified. The CPWC uses this data in an algorithm to calculate staffing requirements. Eleven participants (55%) tested the CPWC and analysis of their responses showed that 31 of 33 (94%) calculations were accurately completed using the CPWC. Conclusion This study has defined the WISN Activity Standard for pharmaceutical care delivery to hospital inpatients and validated the CPWC for acute medical and surgical hospital settings. The CPWC offers hospital pharmacy managers a useful tool to negotiate adequate staffing to deliver pharmaceutical care and its development methodology could be applied widely in pharmacy practice internationally.