The American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP) risk calculator has been used to assist surgeons in predicting the risk of postoperative complications. This study aims to determine if the risk calculator accurately predicts complications in acute care surgical patients undergoing laparotomy. A retrospective review was performed on all patients on the acute care surgery service at a tertiary hospital who underwent laparotomy between 2011 and 2012. The preoperative risk factors were used to calculate the estimated risks of postoperative complications in both the original ACS NSQIP calculator and updated calculator (June 2016). The predicted rate of complications was then compared with the actual rate of complications. Ninety-five patients were included. Both risk calculators accurately predicted the risk of pneumonia, cardiac complications, urinary tract infections, venous thromboembolism, renal failure, unplanned returns to operating room, discharge to nursing facility, and mortality. Both calculators underestimated serious complications (26% vs 39%), overall complications (32.4% vs 45.3%), surgical site infections (9.3% vs 20%), and length of stay (9.7 days versus 13.1 days). When patients with prolonged hospitalization were excluded, the updated calculator accurately predicted length of stay. The ACS NSQIP risk calculator underestimates the overall risk of complications, surgical infections, and length of stay. The updated calculator accurately predicts length of stay for patients <30 days. The acute care surgical population represents a high-risk population with an increased rate of complications. This should be taken into account when using the risk calculator to predict postoperative risk in this population.