Corrected QT Interval (QTc)

What this tool does

It applies two common heart-rate corrections to the measured QT. Bazett over-corrects at extreme rates; Fridericia is often preferred at fast HRs in research, but neither replaces manual adjudication or ECG software standards.

Formulas

RR (s) = 60 / HR. Bazett: QTc = QT / √RR. Fridericia: QTc = QT / RR^(1/3). (QT in ms, same numeric divisor.)

Equivalently QTc_ms = QT_ms / √(60/HR) for Bazett when RR is in seconds.

References

Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370. Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen. Z Gesamte Exp Med. 1920;13:208-212.

Limitations

• Bazett is inaccurate at very slow or very fast heart rates; clinical decisions use institution-specific ECG criteria and drug lists.
• U waves, fusion, poor baseline, and automated QT measurement errors are common; do not use this page alone to start or stop medications.

Bazett QTc bands (rough teaching)

• Bazett QTc under 350 ms is unusually short on this rough screen—verify measurement and clinical context.
• Bazett QTc roughly 350–449 ms is a common “screening” band; sex-specific and institutional cutoffs differ.
• Bazett QTc 450–499 ms is often flagged as prolonged for many adult men; many use higher thresholds for women—confirm locally.
• Bazett QTc ≥ 500 ms is a high-risk teaching threshold for drug monitoring and inherited channelopathy workup when clinically appropriate.

Drug safety & follow-up (teaching)

• Many institutions maintain QT-prolonging drug lists and pharmacist review when starting high-risk agents.
• Electrolyte repletion (K⁺, Mg²⁺), drug interactions, and genetic risk may change monitoring intensity.
• Marked QT prolongation or symptoms warrants clinician-directed evaluation—not delayed based on this calculator.