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Background: A short QT interval, within which an increased risk for atrial fibrillation and/or fatal cardiac arrhythmias occurs, has been difficult to define. Methods: The lower percentiles of a new QTc formula were determined, using a precise mathematical fitting of the QT-heart rate relationship from the ECGs of 13,600 individuals from the NHANES II and III surveys. Results: The QTc interval for persons in the lower fifth percentile, second (2.5th) percentile and first percentile, were calculated. Conclusions: Based on the new spline formula, a short QTc is defined at the first percentile, and is less than 380 ms in both men and women.