Background <p>Large-scale estimation of effective dose (ED) for positron emission tomography (PET)/computed tomography (CT) remains limited. This study aimed to determine the ED of torso ¹⁸F-fluorodeoxyglucose (FDG) PET/CT using extensive real-world clinical data from a high-volume centre. Differences in ED were also evaluated according to PET/CT scanner type, patient sex, and body size.</p> Methods <p>A retrospective analysis was performed on PET/CT examinations from individuals aged ≥ 16 years and acquired between March 2019 and June 2022. Five PET/CT scanners were used during the study period. CT ED was calculated by multiplying the dose length product (DLP) by an ED/DLP conversion coefficient. PET ED was estimated using injected ¹⁸F-FDG activity and the conversion factor reported in International Commission on Radiological Protection Publication 106. Total PET/CT ED was computed as the sum of CT and PET ED values.</p> Results <p>A total of 51,784 examinations were analysed (mean age, 61.9 ± 12.6 years; 29,990 men). Mean CT ED was 3.7 ± 1.5 mSv in men and 3.7 ± 1.7 mSv in women. CT ED varied according to scanner type and protocol, with newer scanners and advanced image processing technologies associated with lower radiation exposure. A strong positive correlation was observed between volume CT dose index and body mass index across all scanners. Mean PET ED of PET was 6.6 ± 1.1 mSv in men and 5.6 ± 1.0 mSv in women. Mean total PET/CT ED was 10.2 ± 2.4 mSv in men and 9.3 ± 2.5 mSv in women.</p> Conclusions <p>The mean ED of torso ¹⁸F-FDG PET/CT derived from extensive real-world clinical data was comparable to that of conventional diagnostic CT. Both patient-related factors, including sex and body size, and scanner-specific characteristics, such as dose modulation algorithms, significantly influenced ED.</p>