What is the QT interval and why does it matter?

The QT interval represents the time from the start of ventricular depolarization to the end of repolarization, measured on an ECG. A prolonged QT interval increases risk of Torsades de Pointes (TdP), a potentially fatal ventricular arrhythmia.

Why do we need to correct the QT interval?

The QT interval naturally shortens at faster heart rates. Without correction, comparing QT intervals at different heart rates would be inaccurate. QTc (corrected QT) normalizes the measurement to a heart rate of 60 bpm.

Which formula should I use: Bazett or Fridericia?

Bazett is historically most common but over-corrects at high HR and under-corrects at low HR. Fridericia is more accurate at extreme heart rates. Most literature and drug studies use Bazett, but Fridericia is increasingly preferred for clinical decisions.

What drugs commonly prolong QT?

Common QT-prolonging drugs include: antiarrhythmics (amiodarone, sotalol, dofetilide), antipsychotics (haloperidol, ziprasidone), antibiotics (fluoroquinolones, macrolides), antiemetics (ondansetron, droperidol), and methadone. Always check www.crediblemeds.org.

What are risk factors for Torsades de Pointes?

Risk factors include: QTc >500ms, female sex, hypokalemia, hypomagnesemia, bradycardia, recent QTc increase >60ms, congenital long QT syndrome, heart failure, multiple QT-prolonging drugs, and drug-drug interactions affecting metabolism.

How do I measure QT interval accurately?

Measure from the beginning of the QRS complex to the end of the T wave in leads II or V5-V6. Use the lead with the longest QT. Avoid measuring if prominent U waves are present. Use multiple beats and average. Automated measurements may be inaccurate.

QTc Calculator

Calculate corrected QT interval to assess risk of arrhythmias. Includes Bazett and Fridericia formulas.

ECG Data

QT Interval

Measured QT interval from ECG (in milliseconds). Measure from start of QRS to end of T wave. Best measured in leads II or V5-V6. Use the lead with the longest QT.

Heart Rate

bpm

Heart rate at time of ECG. Calculated RR interval: {rrInterval.toFixed(3)} seconds. QT correction accounts for heart rate effects on QT.

Why sex matters: Females have longer normal QTc values than males. Upper normal limits differ: 450ms (male), 470ms (female).

QTc (Bazett)

— ms

Enter ECG data

QTc (Fridericia)

— ms

More accurate at extreme heart rates

Risk Assessment

High Risk (TdP)

Reference Ranges

Category	Male	Female	Action
Normal	<450 ms	<470 ms	No action
Borderline	450-470 ms	470-480 ms	Review meds, electrolytes
Prolonged	>470 ms	>480 ms	Cardiology consult
High Risk	>500 ms		Discontinue offending drugs, monitor

The Formulas

Bazett Formula

QTc = QT / √RR

= 400 / √0.833 = ms

Fridericia Formula

QTc = QT / ∛RR

= 400 / ∛0.833 = ms

RR interval = 60 / Heart Rate = 60 / 72 = 0.833 seconds

Frequently Asked Questions

How to Use

About the QTc Calculator

What it calculates

This qtc calculator applies a validated clinical scoring method used in healthcare settings. Corrected QT interval (Bazett/Fridericia).

When to use it

Use as a clinical decision support aid alongside professional judgment — not as a standalone diagnosis.

Example

Adjust the inputs above to model your specific scenario with the QTc Calculator.

Explore More

QTc Calculator

ECG Data

— ms

Reference Ranges

The Formulas

About the QTc Calculator

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