In a retrospective, single-center cohort of 50,314 adult surgical patients using readily available preoperative clinical data, we modeled the number of major postoperative complications from none to three or more. We included acute kidney injury, prolonged stay (>48h) in an intensive care unit, need for prolonged (>48h) mechanical ventilation, severe sepsis, and cardiovascular, wound, and neurologic complications. Risk probability scores generated from the multinomial logistic models were used to develop an online calculator. We stratified patients based on the risk for having three or more postoperative complications. Patients older than 65 years (OR 1.5, 95% CI 1.4-1.6), males (OR 1.2, 95% CI 1.2-1.3), patients with a greater Charlson comorbidity index (OR 3.9, 95% CI 3.6-4.2), those requiring emergency surgery (OR 3.5, 95% CI 3.3.-3.7), and those admitted on a weekend (OR 1.4, 95% CI 1.3-1.5) were more likely to have three or more postoperative complications compared to having none. Patients in the medium and high risk categories were 3.7 and 6.3 times more likely to have three or more postoperative complications, and high risk patients were 5.8 and 4.4 times more likely to die within 30 and 90 days of admission respectively. Readily available, preoperative clinical and sociodemographic factors are associated with a greater number of postoperative complications and adverse surgical outcomes. We developed an online calculator that predicts probability for each number of complications after major surgery.