In radiation therapy treatment planning, margins are added to the tumour volume to ensure that the correct radiation dose is delivered to the tumour in the presence of geometrical uncertainties. The van Herk margin formula (VHMF) was developed to calculate the minimum margin on the target to provide full coverage by 95% of the prescribed dose to 90% of the population. However, this formula is based on an ideal dose profile model that is not realistic for lung radiotherapy. The purpose of this study was to investigate the validity of the VHMF for lung radiotherapy with accurate dose calculation algorithms and respiratory motion modeling. Ultimately, the VHMF ensured sufficient target coverage, with the exception of small lesions in soft tissue; however, the derived PTV margins were larger than necessary. A novel planning approach using the VHMF was tested indicating the need to account for tumour motion trajectory and plan conformity.