Mean wind pressure coefficients (Cp) are key input parameters for air infiltration and ventilation studies. However, building energy simulation and stand-alone airflow network programs usually only provide and/or use a limited amount of Cp data, which are based on several assumptions. An important assumption consists of using surface-averaged Cp values instead of local Cp values with a high resolution in space. This paper provides information on the uncertainty in the calculated airflow rate due to the use of surface-averaged Cp data. The study is performed using published empirical data on pressure coefficients obtained from extensive wind tunnel experiments. The uncertainty is assessed based on the comparison of the airflow rate (phi) calculated using the surface-averaged Cp values (phi(AV)) and the airflow rate calculated using local Cp values (phi(LOC)). The results indicate that the uncertainty with a confidence interval of 95% is high: 0.23 phi(AV) < phi(LOC) < 5.07 phi(AV). In cases with the largest surface-averaged Delta Cp, the underestimation or overestimation is smaller but not negligible: 0.52 phi(AV) < phi(LOC) < 1.42 phi(AV). These results provide boundaries for future improvements in Cp data quality, and new developments can be evaluated by comparison with the uncertainty of the current methods.