A cardiac rhythm management device predicts defibrillation thresholds without any need to apply defibrillation shocks or subjecting the patient to fibrillation. Intravascular defibrillation electrodes are implanted in a heart. By applying a small test energy, an electric field near one of the defibrillation electrodes is determined by measuring a voltage at a sensing electrode offset from the defibrillation electrode by a known distance. A desired minimum value of electric field at the heart periphery is established. A distance between a defibrillation electrodes and the heart periphery is measured, either fluoroscopically or by measuring a voltage at an electrode at or near the heart periphery. Using the measured electric field and the measured distance to the periphery of the heart, the defibrillation energy needed to obtain the desired electric field at the heart periphery is estimated. In an example, the device also includes a defibrillation shock circuit and a stimulation circuit.