QTc Calculator
Correct the QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas, with sex-specific interpretation thresholds.
Enter the QT interval in milliseconds plus the heart rate or RR interval, choose a correction formula, and get the QTc with an interpretation against sex-specific thresholds. Compare formulas when the heart rate is very fast or very slow, since Bazett overcorrects at the extremes.
Why correct the QT interval?
The QT interval is heart-rate dependent: it shortens as the heart beats faster and lengthens as it slows. A QT of 460 ms means something very different at 50 beats per minute, where it may be entirely physiological, than at 110 beats per minute, where it signals genuinely delayed repolarization. Correction formulas standardise the measurement to a heart rate of 60, producing the QTc, a single number that can be compared against population thresholds and tracked over time in the same patient.
The clinical stakes are set by torsades de pointes. When repolarization is sufficiently delayed, early afterdepolarizations can trigger a polymorphic ventricular tachycardia that is often self-terminating but sometimes degenerates into ventricular fibrillation and sudden death. The risk climbs with the QTc, becoming substantial above 500 ms, and with the change from baseline: an increase of more than 60 ms after starting a drug is itself a warning sign even if the absolute value stays below 500. Congenital long QT syndrome, bradycardia, hypokalaemia and hypomagnesaemia amplify the risk further.
The four formulas
Bazett's formula, published in 1920, divides the QT by the square root of the RR interval in seconds. It remains the default on most ECG machines and in most clinical documentation, which is an argument for knowing it even when it is not the best choice. Its weakness is systematic: it overcorrects at fast heart rates, labelling tachycardic patients as prolonged when they are not, and undercorrects at slow rates, missing genuine prolongation in bradycardia.
Fridericia's formula uses the cube root of the RR interval instead, and it tracks the true rate-dependence of repolarization more faithfully across the physiological range. It is the preferred formula in drug safety studies, in regulatory guidance, and among electrophysiologists, and it is the one to reach for whenever the heart rate is below 60 or above 100. The Framingham linear formula adds 154 times one minus the RR interval, and the Hodges formula adds 1.75 times the heart rate minus 60; both are linear corrections that behave well at extremes and are occasionally used in research settings. This calculator offers all four so the formulas can be compared directly on the same ECG.
How to interpret the result
Thresholds differ by sex because women have longer baseline QTc intervals than men. A QTc up to 430 ms in men and 450 ms in women is generally considered normal. The borderline zone runs from 430 to 450 ms in men and 450 to 470 ms in women. Above 450 ms in men and 470 ms in women, the QTc is prolonged and deserves review of drugs, electrolytes and family history. Above 500 ms in either sex, the QTc is markedly prolonged and high risk: QT-prolonging drugs should be stopped or substituted where possible, potassium and magnesium repleted, and cardiology involved.
Measurement quality matters as much as the number. The QT should be measured in the lead with the longest, clearest T wave, the end of the T wave identified carefully, and U waves excluded. Atrial fibrillation, bundle branch block and ventricular pacing all complicate measurement, and published adjustments exist for wide QRS. When the QTc sits near a threshold, remeasuring by hand and averaging several beats is worth the minute it takes.
Drugs, electrolytes and acquired long QT
Drug-induced QT prolongation is the commonest cause of a newly prolonged QTc in hospital. The list is long and includes antiarrhythmics such as sotalol and dofetilide, macrolides, fluoroquinolones, haloperidol and other antipsychotics, methadone, ondansetron and several antidepressants; CredibleMeds maintains the authoritative categorized list. Risk is dose-dependent and additive across drugs, and it rises steeply when hypokalaemia, hypomagnesaemia or bradycardia coexist, which is why electrolytes should be checked and corrected in every patient with a prolonged QTc.
When a necessary drug prolongs the QTc, the response should be proportionate. A small increase that stays well within normal limits may only need documentation and rechecking. Crossing into prolonged territory calls for dose reduction, substitution, or enhanced monitoring depending on how essential the drug is. Crossing 500 ms, or lengthening more than 60 ms from baseline, should prompt stopping the drug if at all possible and continuous ECG monitoring until the QTc recovers, because torsades risk at those values is no longer theoretical.
Limitations
No formula perfectly captures the QT to rate relationship in every patient, and Bazett in particular misleads at heart rate extremes, which is why the calculator offers alternatives. Automated ECG machine measurements of QTc are convenient but frequently inaccurate, especially with abnormal T wave morphology; manual verification is recommended before acting on a borderline value. The thresholds are population-derived and do not account for individual modifiers such as congenital long QT mutations, where events can occur at modest QTc values. Finally, the calculator interprets a single ECG snapshot; trend over time and change from the patient's own baseline carry at least as much meaning as any absolute cutoff.
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469-486.
- Al-Khatib SM, Stevenson WG, Ackerman MJ, et al. 2017 AHA/ACC/HRS guideline for management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Circulation. 2018;138(13):e272-e391.
- Tisdale JE, Chung MK, Campbell KB, et al. Drug-induced arrhythmias: a scientific statement from the American Heart Association. Circulation. 2018;138(15):e574-e598.
Key takeaways
- The QTc standardises the QT interval to a heart rate of 60 beats per minute so it can be interpreted against fixed thresholds.
- Bazett's formula is the most common but overcorrects at fast rates and undercorrects at slow rates; Fridericia is more accurate at the extremes.
- Prolonged QTc is over 450 ms in men and over 470 ms in women; over 500 ms in either sex is markedly prolonged and high risk.
- An increase of more than 60 ms from baseline after starting a drug is a warning sign even below 500 ms.
- Check and correct potassium and magnesium, review all QT-prolonging drugs, and consider congenital long QT syndrome in young patients.
- Verify borderline machine readings by hand before acting on them.
Frequently asked questions
What is the QTc interval?
The QTc is the QT interval corrected for heart rate. The raw QT interval shortens as heart rate rises, so a correction formula standardises it to what it would be at 60 beats per minute, allowing comparison across heart rates. Prolonged QTc reflects delayed ventricular repolarization and is associated with risk of torsades de pointes, a potentially fatal polymorphic ventricular tachycardia.
Which QTc formula should I use?
Bazett's formula, QT divided by the square root of the RR interval in seconds, is the most widely used and the default on most ECG machines, but it overcorrects at fast heart rates and undercorrects at slow ones. Fridericia's cube-root formula is more accurate across heart rate extremes and is preferred in drug safety studies and by many electrophysiologists. Framingham and Hodges are linear alternatives with similar advantages at extreme rates.
What QTc is considered prolonged?
Thresholds are sex-specific because women have longer baseline QTc: prolonged is generally over 450 ms in men and over 470 ms in women, with over 500 ms considered markedly prolonged and high risk in either sex. Borderline ranges are roughly 430 to 450 ms in men and 450 to 470 ms in women. A QTc over 500 ms, or an increase of more than 60 ms from baseline after starting a drug, should prompt urgent review.
Can I use heart rate or RR interval?
Either. The RR interval in seconds equals 60 divided by the heart rate in beats per minute, so the two carry the same information. This calculator accepts heart rate directly, or the RR interval if the ECG reports it, and converts internally before applying the chosen formula.
What drugs prolong the QT interval?
Many commonly used drugs prolong QT, including certain antiarrhythmics such as amiodarone, sotalol and dofetilide, macrolide and fluoroquinolone antibiotics, antipsychotics such as haloperidol and ziprasidone, methadone, ondansetron, and some antidepressants. Risk rises with higher doses, combinations of QT-prolonging drugs, hypokalaemia, hypomagnesaemia, bradycardia, and congenital long QT syndrome. CredibleMeds maintains the standard reference list.
Does a prolonged QTc always need treatment?
Not always, but it always needs evaluation. A mildly prolonged QTc with a clear reversible cause, such as hypokalaemia or a newly started drug, may resolve with correction. Persistent prolongation, values over 500 ms, syncope, or a family history of sudden death warrant cardiology referral to consider congenital long QT syndrome and preventive measures including beta blockade.