AbstractWe evaluate the parameters that control the thickness of ductile shear zones that are generated by viscous heating. We employ two‐dimensional thermomechanical numerical models to simulate shear zone development under compression. Results show that the shear zone thickness is essentially independent on the numerical resolution and the initial size of a weak inclusion that triggers shear localization. A simple scaling law is derived which predicts the thickness with six physical parameters: far‐field stress and strain rate, thermal conductivity (both constant and temperature dependent), initial temperature, activation energy, and stress exponent. The scaling law is confirmed by numerical simulations for a wide range of parameters. For crustal deformation conditions typical temperature increase ranges between 50°C and 200°C, and the predicted thickness is between 1 km and 7 km. These thicknesses agree with natural crustal and lithospheric shear zones suggesting that shear heating is a process controlling crustal shear zone formation.