Corrected QT Interval (QTc) Calculator

Estimate QTc from QT interval and heart rate using Bazett or Fridericia formulas.

Written by

Sid A. Abdala Balal

RGN · Medical & Nursing Content Author

Medically reviewed by

Dr. Adam Sayedi

MD · Emergency & Acute Medicine

⚠️ Safety note: QTc formula choice matters. Bazett may overcorrect at high heart rates and undercorrect at low heart rates; interpret with ECG context and local guidance.

Understanding this tool

Why nurses use it

Corrected QT (QTc) adjusts the QT interval for heart rate — essential when monitoring patients on QT-prolonging drugs, electrolyte disturbances, or arrhythmia risk.

When to use it

Use when reviewing ECG QT interval at a given heart rate — choose Bazett or Fridericia per local preference.

Calculate

How to use this calculator

Measure QT interval in ms from the ECG.
Enter heart rate and sex (for threshold comparison).
Select formula and compare QTc with sex-specific reference thresholds.

How the calculation works

Bazett: QTc = QT ÷ √RR. Fridericia: QTc = QT ÷ RR^(1/3). RR in seconds = 60 ÷ HR. Typical upper screening limits: ≤450 ms (male), ≤460 ms (female); >500 ms high TdP risk.

QTc (Bazett) = QT ÷ √RR; QTc (Fridericia) = QT ÷ RR^(1/3)

Formula used by this tool — verify units before applying at the bedside.

Worked example

QT 400 ms, HR 75: RR=0.8 s → Bazett QTc = 400/√0.8 ≈ 447 ms.

Limitations & clinical notes

Bazett overcorrects at high HR and undercorrects at low HR.
Single-lead QT automated measurements may differ from manual ECG review.

References

Key sources: CredibleMeds — QT prolongation.

Disclaimer: This calculator is for educational and bedside double-check purposes only. It does not replace institutional protocols, prescriber orders, pharmacist verification, or clinical judgment. Always follow your local policies — especially for high-risk medications, infusions, pediatrics, and critical care. If the patient is acutely unwell, escalate immediately rather than relying on any calculator output.