INTRODUCTION Laparoscopic surgery has become the standard for colorectal cancer resection in the UK but it can be technically challenging in patients who are obese. Patients whose body fat is mainly inside the abdominal cavity are more challenging than those whose fat is mainly outside the abdominal cavity. Abdominal fat ratio (AFR) is a simple parameter proposed by the authors to aid identification of this subgroup. MATERIALS AND METHODS All 195 patients who underwent elective, laparoscopic colorectal cancer resections from March 2010 to November 2013 were included in the study. For patients who were obese (body mass index greater than 30), preoperative staging computed tomography was used to determine AFR. This was assessed by two different, blinded observers and compared with conversion rate. RESULTS Of the 195 patients, 58 (29.7%) fell into the obese group and 137 (70.3%) into the non-obese group. The median AFR of the obese group that were converted to open surgery was significantly higher at 5.9 compared with those completed laparoscopically (3.3, P = 0.0001, Mann-Whitney). There was no significant difference in conversion rate when looking at body mass index, tumour site or size. DISCUSSION Previous studies have found body mass index, age, gender, previous abdominal surgery, site and locally advanced tumours to be associated with an increased risk of conversion. This study adds AFR to the list of risk factors. CONCLUSION AFR is a simple, reproducible parameter which can help to predict conversion risk in obese patients undergoing colorectal cancer resection.