Why correct QT for heart rate?
The QT interval naturally shortens as heart rate increases and lengthens as it slows. QTc ("corrected QT") removes that heart-rate dependence so the value can be compared against a fixed reference range and tracked over time — for example, before and after starting amiodarone, methadone, or an antipsychotic.
Bazett vs. Fridericia
Bazett's formula (QTc = QT / √RR) is the most widely taught and used at the bedside, but it over-corrects at fast heart rates and under-corrects at slow ones. Fridericia's formula (QTc = QT / RR^⅓) is generally more accurate outside the normal heart-rate range and is preferred in research settings. This calculator shows both so you can see where they diverge.
Interpreting the result
- Normal: ≤450 ms (male), ≤460 ms (female)
- Borderline / caution: up to 500 ms
- High risk for Torsades de Pointes: >500 ms, or an increase of >60 ms from baseline
Common causes of QT prolongation include antiarrhythmics, antipsychotics, certain antibiotics (e.g. macrolides, fluoroquinolones), methadone, hypokalemia, hypomagnesemia, hypocalcemia, and congenital long QT syndrome.
See also: ECG Interpretation Basics & STEMI Criteria.