Background: Various methods have been introduced for estimating deep inferior epigastric artery perforator (DIEP) flap volume based on computed tomography or magnetic resonance angiographic images. However, when radiologic images cannot be obtained, estimations are subjective. The purpose of this study was to develop a prediction model for estimating DIEP flap weight using the pinch test. Methods: The pinch test was performed at three paraumbilical sites using a skin-fold caliper in 107 consecutive patients who underwent DIEP flap breast reconstruction. Multiple linear regression analysis was used to develop a formula to estimate flap weight. Predictor variables included body mass index (BMI, kg/m2), flap height (H, cm), flap width (W, cm), and flap thickness (mm) measured by the pinch test at the following three paraumbilical sites: 5 cm right (R), left (L), and inferior (I) of the umbilicus. The model accuracy was tested using leave-one-out cross-validation. Results: A prediction model was developed from the multiple regression analysis (R2 = 89.03%, P <.001); flap weight, g = −1308 + 24.57 × BMI + 6.80 × (R + L)/2 + 7.89 × I + 20.51 × H + 32.55 × W. The formula was implemented in a smartphone application, DIEP-W version 2.0, for real-time use. The mean absolute percentage error in the cross-validation was 12.15%. Conclusions: DIEP flap weight can be estimated by the pinch test with the developed prediction model in an easy, cost-effective, and relatively accurate manner. This method will improve surgical planning and allow surgeons to provide better counselling for patients when radiologic images are not available.