A recent publication (Jagtap et al. Scr Mat 182:43, 2020) reported real-time observations of stress-driven whisker nucleation and growth in a thin film of Sn subjected to pressure on part of its surface. This paper describes a model of the experiment. Predicted whisker volume in the film adjacent to the loaded area is in good agreement with experimental measurements. The model also predicts correctly the variation of whisker density with distance from the punch at the end of the experiment but underestimates the rate of whisker nucleation during the first 50 h after the film is loaded. Comparison of model predictions with experiments provides clear evidence for a threshold stress for whisker growth, with an initial value of 17 MPa, which increases with whisker length to a saturated value of 22 MPa after whiskers reach a length of 19 µm. Implications for whisker growth in Sn films on Cu substrates are discussed. © 2021, The Minerals, Metals & Materials Society.