Energy levels affect the treatment efficacy and efficiency of electroconvulsive therapy (ECT). United States devices require preselection of energy dosage before stimulating patients, and two strategies have been proposed to determine an effective dosage, i.e., based on the patient's age and measured estimates of seizure threshold. The age method is criticized for overstimulation and an assumed association with increased cognitive effects. Threshold determination strategy is cumbersome and requires multiple stimulations, possibly placing patients at increased cardiovascular risk. In 35 patients, we examined an energy estimate for bilateral electrode placement at half the patient's age in "percent of energy" or joules delivered by the Thymatron and MECTA devices. Each patient required one stimulation in the first treatment to elicit motor seizures averaging 51.7 s. Subsequently, 20 patients were randomly assigned to either threshold titration followed by half-age for the first two treatments or the opposite (AB-BA design). Half-age energy was 7 J (30%) higher than titration estimates, corresponding to 55% energy of the age method. The relation of half-age and titration estimates were confirmed in energy dosing records from two independent centers. Energy dosing by half-age calculation in bilateral ECT is simple, practical, avoids overdosing and repeat stimulation, and is a useful substitute for the more complex strategy based on threshold estimation.