A quick rule of thumb for estimating whether the QTc is normal is to look for it to be half the preceding RR interval (see figure below from ECG Interpretation). However, note that this rule of thumb is not accurate at heart rates < 62 bpm. Also, it is conservative at heart rates > 66 bpm. An excellent thorough explanation can be found at Dr. Smith’s ECG blog here. For heart rates < 62 bpm, Dr. Smith suggests using QT < 40% of the preceding RR as rule of thumb. Also, be careful – computer calculated QTc are often incorrect – may need to get those calipers out! (or just count little boxes – each one is 0.04 sec, or 40msec). The most common formula used to correct QT is the Bazett formula QTc = QT / sqrt(RR), although this formula produces false positives at high heart rates. Here is a calculator at MedCalc that will do the math for you!

QT half RR ecg-interpretation.blogspot.com