Corrected QT

Q: How does the calculator get the RR interval used in the Bazett formula?

You never measure the RR interval yourself. The calculator converts the heart rate you enter into an RR interval in seconds by dividing 60 by the heart rate, then applies QTc = QT / sqrt(RR). Enter the QT interval measured on the ECG in milliseconds, the heart rate in beats per minute, and the patient's sex, because the upper limit of normal is 450 ms in men and 460 ms in women.

Q: Why can Bazett correction mislead at fast or slow heart rates?

Bazett's correction does not track heart rate evenly: at fast rates it inflates the QTc, and at slow rates it deflates it, so the number can look more abnormal or more normal than true repolarization warrants. That is a property of the equation rather than a data-entry error. The calculator notes that Fridericia and Framingham corrections may suit some clinical settings better, and that QTc never replaces clinical assessment of arrhythmia risk.

Q: What does a QTc of 500 ms or higher indicate?

A QTc at or above 500 ms is classified as marked prolongation and is associated with increased risk of Torsades de Pointes, a potentially fatal ventricular arrhythmia. Values between 450 and 499 ms in men or 460 and 499 ms in women are reported as prolonged. Because QT measurement varies between observers and the tracing must be readable, the result should be weighed with the patient's medications, electrolyte status, and clinical picture rather than used as a stand-alone diagnosis.

Corrects the QT interval for heart rate using the Bazett formula.

Endocrinology EndocrinologyEvidence-BasedUpdated 2026-07

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

QTc is used to assess prolongation risk, particularly in patients with electrolyte abnormalities or medication exposure.

Calculator Information

Specialty

Endocrinology

📋 Reference Ranges

Normal Range

< 460 ms in women and < 450 ms in men

Reference ranges for this calculator
Classification	Range
Normal QTc	<450 ms (men) / <460 ms (women)
Prolonged QTc	450–499 ms (men) / 460–499 ms (women)
Markedly prolonged QTc	≥500 ms

What This Calculator Measures

Corrects the QT interval for heart rate using the Bazett formula to assess QT prolongation risk.

How to Use

Measure the QT interval from the ECG (from the start of the QRS complex to the end of the T wave).
Enter the QT interval in milliseconds.
Enter the heart rate in beats per minute.
Select the patient's sex (thresholds differ between men and women).
Review the corrected QTc value and its interpretation.

How the Calculation Works

The Bazett formula corrects the QT interval for heart rate: QTc = QT / sqrt(RR), where RR (in seconds) = 60 / heart rate.

Variables Explained

QT Interval

Measured QT interval in milliseconds, divided by the square root of RR.

Heart Rate

Heart rate in beats per minute, converted to the RR interval in seconds (60 / heart rate).

Sex

Biological sex, used to apply sex-specific upper limits of normal for QTc.

Interpretation Guide

QTc <450 ms (men) or <460 ms (women) is normal. 450–499 ms (men) or 460–499 ms (women) indicates prolongation. ≥500 ms indicates marked prolongation and increased risk of Torsades de Pointes.

Sex-specific interpretation

Clinical Significance

QT prolongation is a risk factor for Torsades de Pointes, a potentially fatal ventricular arrhythmia. Identifying prolonged QTc allows clinicians to address modifiable risk factors such as electrolyte abnormalities and QT-prolonging medications.

When It Is Useful

Assessment of QT prolongation risk
Patients with electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia)
Medication review for QT-prolonging drugs
Pre-operative cardiac risk assessment
Evaluation of syncope or family history of sudden cardiac death

When It Is Not Useful

As a standalone risk assessment without clinical context
When the ECG tracing quality is poor and QT measurement is unreliable

Common Mistakes

Entering the QT interval in seconds instead of milliseconds produces a QTc far below the 450 ms and 460 ms sex-specific limits.
Applying the 450 ms male limit to a woman, or the 460 ms female limit to a man, classifies the result against the wrong sex-specific threshold.
Reading Bazett over-correction at a fast heart rate as true prolongation ignores a limitation that Fridericia or Framingham correction may address.

Limitations

The Bazett formula tends to over-correct at high heart rates and under-correct at low heart rates.
QT measurement itself has significant inter-observer variability.
Does not replace clinical assessment of arrhythmia risk.
Other correction formulas (Fridericia, Framingham) may be more appropriate in certain clinical settings.

Worked Example

A 45-year-old woman has a QT interval of 460 ms and a heart rate of 72 bpm.

Inputs:

qt: 460

heartRate: 72

sex: 2

Expected Result: QTc ≈ 504 ms (using Bazett formula). This is markedly prolonged (≥500 ms) and indicates increased risk of Torsades de Pointes.

Evidence & References

Source:: Original Bazett Description / AHA ECG Guidelines
Content updated:: 2026-08

Bazett HC. Heart. 1920;7:353-370.

References

Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370. (Original Description)
AHA ECG Guidelines for the Interpretation of the Resting 12-Lead ECG. (Guideline)

Frequently Asked Questions

How does the calculator get the RR interval used in the Bazett formula?

You never measure the RR interval yourself. The calculator converts the heart rate you enter into an RR interval in seconds by dividing 60 by the heart rate, then applies QTc = QT / sqrt(RR). Enter the QT interval measured on the ECG in milliseconds, the heart rate in beats per minute, and the patient's sex, because the upper limit of normal is 450 ms in men and 460 ms in women.

Why can Bazett correction mislead at fast or slow heart rates?

Bazett's correction does not track heart rate evenly: at fast rates it inflates the QTc, and at slow rates it deflates it, so the number can look more abnormal or more normal than true repolarization warrants. That is a property of the equation rather than a data-entry error. The calculator notes that Fridericia and Framingham corrections may suit some clinical settings better, and that QTc never replaces clinical assessment of arrhythmia risk.

What does a QTc of 500 ms or higher indicate?

A QTc at or above 500 ms is classified as marked prolongation and is associated with increased risk of Torsades de Pointes, a potentially fatal ventricular arrhythmia. Values between 450 and 499 ms in men or 460 and 499 ms in women are reported as prolonged. Because QT measurement varies between observers and the tracing must be readable, the result should be weighed with the patient's medications, electrolyte status, and clinical picture rather than used as a stand-alone diagnosis.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. QTc interpretation should always be performed in the context of the individual patient's clinical presentation, medication list, and ECG quality.