Appendicectomy is a common emergency operation. The aim of this analysis was to study the effect of preoperative delay on disease progression, and whether a novel scoring system (Atema score) could be useful in predicting complicated appendicitis. Patients with uncomplicated acute appendicitis on CT and who underwent appendicectomy in 2014-2015 were analysed for patient characteristics, preoperative delay and outcomes. Of 837 patients with uncomplicated appendicitis on CT, 187 (22.3 per cent) were found to have complicated appendicitis at surgery. The median time estimate for perforation was 25.4 h after CT, with an hourly rate of perforation of 2 per cent. Patients with an Atema score of 6 or less and those with no appendicolith on CT and a C-reactive protein level below 51 mg/l were the slowest to develop perforation, reaching a perforation rate of 5 per cent in 7.1 and 7.6 h respectively. A substantial proportion of patients with uncomplicated acute appendicitis on CT have complicated appendicitis at surgery. However, in patients with no risk factors, surgery can be postponed safely for up to 7 h.