QTc Calculator (Bazett, Fridericia, Framingham) | QTc Interval

Why Calculate QTc?

The QT interval represents the time it takes for the heart's ventricles to contract and recharge. However, this interval naturally shortens when the heart beats faster and lengthens when it beats slower. To determine if the QT interval is truly "long" or "short" regardless of the heart rate, we must "correct" it. This corrected value is known as the QTc.

An accurately calculated QTc is vital for assessing the risk of Torsades de pointes, a specific type of ventricular tachycardia that can lead to sudden cardiac arrest.

Bazett vs. Fridericia Formulas

Several formulas exist to correct the QT interval. Choosing the right one depends on the heart rate context.

Bazett Formula

The most common formula in clinical practice. It is accurate for normal heart rates (60-100 bpm) but over-corrects at fast heart rates, potentially showing false positives.

Fridericia Formula

Considered more accurate at extremes (High/Low HR). The FDA prefers Fridericia for clinical drug trials. If HR > 100 bpm, this is the preferred method to avoid false alarms.

Drugs that Prolong QT

"Acquired" Long QT Syndrome is often drug-induced. Hundreds of common medications can block the potassium channels in the heart, prolonging the recharge time. Common culprits include:

Antibiotics
Macrolides (Azithromycin), Fluoroquinolones (Levofloxacin)
Antidepressants
SSRIs (Citalopram, Escitalopram)
Antipsychotics
Haloperidol (Haldol), Quetiapine (Seroquel)
Anti-emetics
Ondansetron (Zofran)

Frequently Asked Questions

What is considered a dangerous QTc?

A QTc greater than 500 ms is universally considered the "Danger Zone." At this level, the risk of developing Torsades de pointes (a fatal heart rhythm) rises exponentially. Doctors will usually stop QT-prolonging medications immediately if the number crosses this threshold.

Why does the Bazett formula overestimate?

The Bazett formula was created in 1920. It "over-corrects" at high heart rates (tachycardia), making the QTc look dangerously high when it might actually be normal. If your Heart Rate is over 80 bpm, check the Fridericia result for a more accurate assessment.

Can anxiety cause a long QT?

Indirectly. Anxiety raises your heart rate and adrenaline. While anxiety itself doesn't change the electrical recharge time of the heart cells, the rapid heart rate can make the uncorrected QT interval shorter, while the stress might expose underlying genetic ion channel issues (Long QT Syndrome).

How do I measure QT on an ECG strip?

Find a lead where the T-wave is clearly visible (usually Lead II or V5). Measure from the beginning of the Q-wave (the first dip) to the end of the T-wave (where it returns to the baseline). Do not include the U-wave if present. Count the small boxes (each is 40ms) and multiply.

Is Long QT Syndrome hereditary?

Yes, Congenital Long QT Syndrome is a genetic disorder affecting sodium or potassium channels. However, Acquired Long QT is more common and is caused by medications or electrolyte imbalances (low potassium/magnesium).

References

American College of Cardiology (ACC). QT Interval Guidelines.
CredibleMeds. List of QT Prolonging Drugs.
Life in the Fast Lane (LITFL). ECG Library: QT Interval.
Mayo Clinic. Long QT Syndrome Symptoms.

Tools & Data Verified by the EverydayCalculators Medical Research Team.

Last updated: January 2026.

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