A method, consisting of ablating tissue for a timeperiod, measuring a contact force applied during the timeperiod, and measuring a power used during the time period.The method further includes ceasing ablating the tissue whena desired size of a lesion produced in the tissue, asestimated using an integral over the time period of aproduct of the contact force raised to a first non-unityexponent and the power raised to a second non-unityexponent, is reached.