An ablation apparatus, comprising: a flexible catheter, having a proximal segment, a distal end configured to be inserted into a body cavity of a living subject and a distal tip, which is disposed at the distal end of the catheter and is configured to be brought into contact with a tissue in the body cavity; a pressure detector configured to sense a mechanical force against the distal tip when the distal tip engages the tissue; an ablator, configured to apply a given dosage of energy to the tissue so as to ablate the tissue; a controller, configured to be responsive to a signal produced by the pressure detector to determine a magnitude of the mechanical force, and to compute an ablation volume using the given dosage of energy and the magnitude of the mechanical force; and a monitor linked to the controller, which is operative to display a map of the tissue and an indication of the computed ablation volume in the tissue, characterised in that the pressure detector comprises: a resilient member configured to couple the distal tip to the proximal segment of the catheter and configured to deform in response to engagement of the distal tip with the tissue; and a position sensor within the catheter configured to sense a position of the distal tip relative to the proximal segment of the catheter, and generate signals responsively to changes in response to deformation and orientation of the resilient member; and wherein the controller is operative to determine deformation and orientation coordinates of the distal end of the catheter using the signals generated by the position sensor, the controller further operative to determine, responsively to the signals the magnitude of the mechanical force. a flexible catheter, having a proximal segment, a distal end configured to be inserted into a body cavity of a living subject and a distal tip, which is disposed at the distal end of the catheter and is configured to be brought into contact with a tissue in the body cavity; a pressure detector configured to sense a mechanical force against the distal tip when the distal tip engages the tissue; an ablator, configured to apply a given dosage of energy to the tissue so as to ablate the tissue; a controller, configured to be responsive to a signal produced by the pressure detector to determine a magnitude of the mechanical force, and to compute an ablation volume using the given dosage of energy and the magnitude of the mechanical force; and a monitor linked to the controller, which is operative to display a map of the tissue and an indication of the computed ablation volume in the tissue, characterised in that the pressure detector comprises: a resilient member configured to couple the distal tip to the proximal segment of the catheter and configured to deform in response to engagement of the distal tip with the tissue; and a position sensor within the catheter configured to sense a position of the distal tip relative to the proximal segment of the catheter, and generate signals responsively to changes in response to deformation and orientation of the resilient member; and wherein the controller is operative to determine deformation and orientation coordinates of the distal end of the catheter using the signals generated by the position sensor, the controller further operative to determine, responsively to the signals the magnitude of the mechanical force. a resilient member configured to couple the distal tip to the proximal segment of the catheter and configured to deform in response to engagement of the distal tip with the tissue; and a position sensor within the catheter configured to sense a position of the distal tip relative to the proximal segment of the catheter, and generate signals responsively to changes in response to deformation and orientation of the resilient member; and wherein the controller is operative to determine deformation and orientation coordinates of the distal end of the catheter using the signals generated by the position sensor, the controller further operative to determine, responsively to the signals the magnitude of the mechanical force.