A positive resection margin after breast-conserving surgery (BCS) is the most important risk factor for tumor recurrence. Re-excision after BCS often results in unnecessary wider excisions, or even mastectomies and poor cosmetic results, as well as increased medical costs and patients' anxiety. A nomogram for predicting positive resection margins may allow the surgeon to develop an individualized surgical plan. Data from 1,034 consecutive breast cancer patients with invasive or in situ breast cancer who initially underwent BCS between January 2008 and December 2009 were used to develop a nomogram for predicting positive resection margins. The nomogram was then validated independently using a cohort of 563 patients who underwent breast surgery in 2010. Multivariate logistic regression analysis showed that microcalcifications (OR 1.574, P = 0.034), grade 4 mammographic density (OR 4.515, P = 0.005), >0.5 cm difference in tumor size between magnetic resonance imaging and ultrasonography (OR 10.001, P < 0.0001), ductal carcinoma in situ (DCIS) on needle biopsy (OR 1.575, P = 0.044), and lobular component on needle biopsy (OR 3.985, P = 0.015) were independent predictors of positive resection margins. These significant variables were used to develop a nomogram for predicting positive resection margins after BCS; the AUCs of the study and the validation cohorts were 0.823 [95 % confidence interval (CI), 0.785-0.862] and 0.846 (95 % CI, 0.800-0.892), respectively. Our new nomogram using 5 variables that were determined before surgery to predict positive resection margins should aid the surgeon in developing individualized surgical plans for breast cancer patients who are scheduled for BCS.