# Corrected QT (QTc) Calculator

Calculate corrected QT interval (QTc) using Bazett's and Fridericia's formulas. Enter QT interval and heart rate to assess QT prolongation risk.

## What this calculates

The QT interval on an electrocardiogram (ECG) varies with heart rate, so it needs to be corrected for accurate clinical assessment. Enter your measured QT interval and heart rate to calculate the corrected QT using both Bazett's and Fridericia's formulas.

## Inputs

- **QT Interval** (ms) — min 100, max 700 — Measured QT interval from ECG in milliseconds
- **Heart Rate** (bpm) — min 30, max 220 — Resting heart rate in beats per minute

## Outputs

- **RR Interval** — Calculated RR interval from heart rate
- **QTc (Bazett)** — Corrected QT using Bazett's formula
- **QTc (Fridericia)** — Corrected QT using Fridericia's formula
- **Interpretation** — formatted as text — Clinical interpretation of the corrected QT interval

## Details

The QT interval represents the time it takes for the heart's ventricles to depolarize and repolarize, measured from the start of the Q wave to the end of the T wave on an ECG. Because this interval naturally shortens at faster heart rates and lengthens at slower rates, raw QT values are not directly comparable between patients with different heart rates. Corrected QT (QTc) formulas adjust for this rate dependence.

Bazett's formula (QTc = QT / sqrt(RR)) is the most widely used correction and remains the standard in most clinical settings. However, it tends to overcorrect at high heart rates and undercorrect at low heart rates. Fridericia's formula (QTc = QT / RR^(1/3)) handles extreme heart rates more accurately and is now preferred by the FDA for drug safety studies and by many electrophysiologists.

A prolonged QTc (generally >470 ms in men or >480 ms in women) increases the risk of a dangerous ventricular arrhythmia called torsades de pointes. Many medications, electrolyte imbalances, and inherited conditions (long QT syndrome) can prolong the QT interval. Monitoring QTc is a routine part of cardiac care and drug safety assessment.

DISCLAIMER: This calculator is for educational reference only. QT interval measurement and clinical interpretation should be performed by qualified healthcare professionals. Do not make treatment decisions based solely on this tool.

## Frequently Asked Questions

**Q: What is a normal QTc interval?**

A: A normal QTc is generally considered to be between 350 and 440 milliseconds, though the upper limit varies slightly by sex. Some references use 450 ms for men and 460 ms for women as the upper normal limit. Values above 500 ms are associated with a significantly increased risk of life-threatening arrhythmias and typically require urgent clinical attention.

**Q: Why are there different QTc formulas?**

A: Different formulas were developed because no single correction perfectly accounts for the heart rate-QT relationship across all rates. Bazett's formula (1920) is the most commonly used but overcorrects at high heart rates. Fridericia's formula (1920) performs better at extreme rates. Other formulas include Framingham and Hodges, each with their own strengths. Many clinicians report multiple corrections for comparison.

**Q: What medications can prolong the QT interval?**

A: Many drug classes can prolong the QT interval, including certain antiarrhythmics (sotalol, amiodarone), antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, ziprasidone), antidepressants (citalopram), and antiemetics (ondansetron). The website CredibleMeds.org maintains a comprehensive list. Always check for QT-prolonging drug interactions, especially when combining multiple medications.

**Q: When should I be concerned about QTc prolongation?**

A: A QTc above 470 ms in men or 480 ms in women warrants medical attention. QTc above 500 ms is considered high-risk for torsades de pointes regardless of sex. Other concerning signs include a QTc increase of more than 60 ms from baseline, symptoms like unexplained fainting, or a family history of sudden cardiac death. Always consult a healthcare provider for proper evaluation.

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Source: https://vastcalc.com/calculators/health/corrected-qt
Category: Health & Fitness
Last updated: 2026-04-08