π« QTc Calculator
Bazett and Fridericia corrected QT, side by side.
For informational purposes only, not medical advice. Consult a healthcare professional.
The QT interval shortens as heart rate rises, so raw QT values cannot be compared across heart rates. Correction formulas normalize QT to a heart rate of 60: Bazett divides by the square root of the RR interval in seconds, Fridericia by the cube root. Prolonged QTc, over 450 ms in men and 470 in women, raises the risk of torsades de pointes, a dangerous arrhythmia, and values over 500 mark high risk. This calculator gives both corrections with the standard thresholds for ECG reading practice and clinical reference.
How to Use This Calculator
Enter the QT interval in milliseconds and the heart rate from the same ECG. The calculator corrects with both Bazett and Fridericia and applies sex-specific thresholds.
Measure QT from the start of Q to the end of T in the lead with the clearest T wave, using the same beat as the heart rate.
Enter QT in ms and heart rate in bpm. The calculator derives the RR interval automatically.
Read both corrections. Fridericia is steadier at fast and slow rates; Bazett overcorrects at extremes.
How the Math Works
Convert heart rate to RR interval: 60 divided by bpm gives seconds, so 70 bpm is 0.857 s. Bazett QTc equals QT divided by the square root of RR; Fridericia divides by the cube root. With QT 400 ms at 70 bpm, Bazett gives 400 divided by 0.926, about 432 ms, and Fridericia gives 400 divided by 0.950, about 421 ms. Thresholds: prolonged above 450 ms in men and 470 in women, borderline 440 to 460, and high torsades risk above 500. Bazett overcorrects at fast rates and undercorrects at slow ones, which is why electrophysiologists prefer Fridericia or Hodges at extremes.
Worked Example
A QT of 400 ms at 70 bpm gives a Bazett QTc of 432 ms and a Fridericia QTc of 421 ms. Both sit under the Over 450 ms (male) threshold the calculator shows, so this ECG reads as normal repolarization. At 120 bpm the same 400 ms QT would correct far higher by Bazett, which is exactly the overcorrection problem Fridericia avoids.
Reading Your Result
Under 440 ms is normal for most adults. The 440 to 470 band is borderline: check medications, electrolytes, and repeat the ECG. Above 450 in men or 470 in women is prolonged and deserves a medication review plus potassium, magnesium, and calcium checks. Above 500 is high risk for torsades: QT-prolonging drugs should stop and monitoring intensifies. A lengthening QTc on serial ECGs matters even within the normal range, since the trend can precede the threshold.
| QTc | Band | Action |
|---|---|---|
| Under 440 ms | Normal | Routine |
| 440-470 ms | Borderline | Review meds, recheck |
| Over 450/470 ms | Prolonged | Electrolytes, med review |
| Over 500 ms | High risk | Stop culprits, monitor |
Limitations
Correction formulas approximate a complex rate relationship; no formula is exact at all heart rates. Measurement error dominates: T-wave end is often ambiguous, and U waves confuse the reading. The calculator cannot see the ECG, so mismeasured inputs produce confident-looking wrong answers. Drug and electrolyte context decides management, not the number alone.
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For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is QTc?
The QT interval corrected for heart rate: what the QT would be at 60 beats per minute. It lets ECGs at different heart rates be compared. Prolonged QTc signals delayed ventricular repolarization and arrhythmia risk.
What is the difference between Bazett and Fridericia?
Both correct QT for rate; Bazett uses the square root of RR and Fridericia the cube root. Bazett is traditional but overcorrects at fast heart rates and undercorrects at slow ones. Fridericia stays more accurate across rates and is preferred when rates are extreme.
What QTc is considered prolonged?
Above 450 ms in men and 470 ms in women by common thresholds, with 440 to 470 often called borderline. Above 500 ms marks high risk for torsades de pointes. Thresholds vary slightly between guidelines, so note which your institution uses.
What causes prolonged QT?
Medications top the list: certain antibiotics, antipsychotics, antiarrhythmics, and antiemetics. Also low potassium, magnesium, or calcium, bradycardia, congenital long QT syndromes, and hypothermia. Reviewing the drug list is always step one.
What is torsades de pointes?
A polymorphic ventricular tachycardia associated with prolonged QT, where the QRS complexes appear to twist around the baseline. It can self-terminate or degenerate into ventricular fibrillation. Magnesium IV is the acute treatment alongside correcting the cause.
Can I calculate QTc by hand?
Yes: divide 60 by the heart rate for RR in seconds, take the square root for Bazett or cube root for Fridericia, and divide the QT in ms by it. For 400 ms at 75 bpm: RR 0.8 s, square root 0.894, QTc about 447 ms by Bazett.
Why does sex matter for QTc?
Women average slightly longer QTc than men, hence the higher 470 threshold versus 450. Hormonal influences on repolarization drive the difference. Some guidelines use a single 460 to 470 line, but sex-specific cutoffs remain common.
Do smartwatches measure QTc?
Some single-lead ECG wearables estimate QT, but lead placement and measurement differ from 12-lead ECGs and the readings are not validated for clinical decisions. Treat them as curiosity data; prolonged readings need a real ECG.
What drugs prolong QT most notoriously?
Class III antiarrhythmics like sotalol and dofetilide, some antipsychotics such as haloperidol and ziprasidone, macrolide and fluoroquinolone antibiotics, and the antiemetic ondansetron. CredibleMeds maintains the reference list clinicians check.
Should QTc be monitored on these drugs?
When risk factors stack: baseline prolonged QTc, electrolyte disturbances, bradycardia, or multiple QT-prolonging drugs together. A baseline and follow-up ECG is standard practice in those situations. Low-risk single agents in healthy people often need no monitoring.