An ablation apparatus, comprising: a flexible catheter adapted for insertion into a heart of a living subject and having a distally disposed ablation electrode to be brought into contact with a target tissue in the heart; an ablator, operative to apply a dosage of energy to the target tissue at a power level so as to ablate the target tissue; a processor for predicting a lesion size according to a relationship between a given contact force between the ablation electrode and the target tissue, an amount of energy in the electrical current and a time interval during which the electrical current is passed through the ablation electrode; and a monitor linked to the processor, which is operative to display a visual indication of the predicted lesion size; characterised in that the processor is operative to execute iterations of the step of predicting a lesion size as a non-linear function of the contact force, the power level and the time interval, by varying the contact force and holding the power level and time interval at constant levels, until a saturation point is found, wherein an increase of the contact force fails to result in an increased predicted lesion size; and the ablation apparatus further comprises: an impedance measuring system, comprising a body surface electrode to be attached to the subject, having circuitry for passing an electrical current between the body surface electrode and the ablation electrode; control circuitry for operating the ablator, at values of the contact force, the power level and the time interval in a selected iteration of the step of predicting a lesion size. a flexible catheter adapted for insertion into a heart of a living subject and having a distally disposed ablation electrode to be brought into contact with a target tissue in the heart; an ablator, operative to apply a dosage of energy to the target tissue at a power level so as to ablate the target tissue; a processor for predicting a lesion size according to a relationship between a given contact force between the ablation electrode and the target tissue, an amount of energy in the electrical current and a time interval during which the electrical current is passed through the ablation electrode; and a monitor linked to the processor, which is operative to display a visual indication of the predicted lesion size; characterised in that the processor is operative to execute iterations of the step of predicting a lesion size as a non-linear function of the contact force, the power level and the time interval, by varying the contact force and holding the power level and time interval at constant levels, until a saturation point is found, wherein an increase of the contact force fails to result in an increased predicted lesion size; and the ablation apparatus further comprises: an impedance measuring system, comprising a body surface electrode to be attached to the subject, having circuitry for passing an electrical current between the body surface electrode and the ablation electrode; control circuitry for operating the ablator, at values of the contact force, the power level and the time interval in a selected iteration of the step of predicting a lesion size. an impedance measuring system, comprising a body surface electrode to be attached to the subject, having circuitry for passing an electrical current between the body surface electrode and the ablation electrode; control circuitry for operating the ablator, at values of the contact force, the power level and the time interval in a selected iteration of the step of predicting a lesion size.