Definition: a formula for correcting the observed QT interval in the electrocardiogram for cardiac rate (R-R interval): corrected QT = Q-T sec/√RR sec.

 
When entering the QT-interval and the RR-interval in millimeters, make sure the ECG is recorded at a speed of 25mm/sec. Normal values are between 300 and 450ms. At a longer corrected QT interval, the patient is at risk for torsade de pointes.