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QTc Calculator (Bazett, Fridericia, Framingham, Hodges)

Correct the ECG QT interval for heart rate with the Bazett, Fridericia, Framingham or Hodges formula, from the heart rate or the RR interval, and read it against the prolongation thresholds for men and women.

Last updated: October 2026

Corrected QT (QTc)

438.18 ms

This calculator gives an estimate for general information only. It is not medical advice and does not replace a doctor, pharmacist or other qualified health professional. Always check with them before acting on the result, especially for medication doses, children, pregnancy or a medical condition, and follow the guidelines of your local health authority.

Formula below · 4 sources (doi.org, database.ich.org, escardio.org, litfl.com) · Updated Oct 2026

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About this calculator

The QT interval on an ECG runs from the start of the QRS complex to the end of the T wave and measures ventricular depolarization plus repolarization. It shortens as the heart rate rises, so a raw QT cannot be compared between patients or between ECGs at different rates. A rate-corrected QT (QTc) estimates what the QT would be at 60 bpm, where the RR interval is 1 second.

RR interval. RR (seconds) = 60 ÷ heart rate (bpm). Enter whichever you have; 72 bpm is an RR of 833 ms.

The four formulas, with RR in seconds and QT in ms:

• Bazett (1920): QTc = QT ÷ √RR. The most used formula and the one most drug labels and automated ECG machines report. It over-corrects at fast rates (above about 90 bpm it reads too long) and under-corrects at slow rates. • Fridericia (1920): QTc = QT ÷ ∛RR. The cube root flattens the rate dependence; it is more accurate at fast and slow rates and is the formula regulators prefer in thorough-QT drug studies. • Framingham (Sagie 1992): QTc = QT + 154 × (1 − RR). A linear correction fitted to the Framingham Heart Study population. • Hodges (1983): QTc = QT + 1.75 × (heart rate − 60). Also linear, written in heart rate rather than RR.

All four agree at 60 bpm and diverge as the rate moves away from it. The tiles show every formula so the spread is visible; when they disagree by more than 20-30 ms, the rate is far from 60 and Bazett is usually the outlier.

Thresholds. The 2009 AHA/ACCF/HRS recommendations for ECG interpretation call a QTc of 450 ms or more prolonged in men and 460 ms or more in women. 500 ms is the level at which the risk of torsades de pointes rises clearly, and the ICH E14 guidance treats a QTc above 500 ms, or an increase of more than 60 ms over baseline, as a signal of concern in drug trials. A QTc of 360 ms or less raises the question of short QT syndrome under the 2022 ESC guidelines on ventricular arrhythmias, though that diagnosis needs more than one number.

Measurement matters more than the formula. Measure the longest QT in leads II or V5, exclude the U wave, and in atrial fibrillation average several beats. With a wide QRS (bundle branch block, pacing) the QT is inflated and a modified correction is used. This calculator gives a corrected interval for information; the interpretation of an ECG rests with the clinician.

How to use

Measure the QT in milliseconds (each small ECG square is 40 ms at 25 mm/s) and enter the heart rate or the RR interval, then pick the formula your service uses and the patient's sex. Example (the defaults): QT 400 ms at 72 bpm, Bazett, male. RR = 60 ÷ 72 = 0.833 s; √0.833 = 0.913; QTc = 400 ÷ 0.913 = 438 ms, within the normal range for a man. The other formulas give Fridericia 400 ÷ ∛0.833 = 400 ÷ 0.941 = 425 ms, Framingham 400 + 154 × (1 − 0.833) = 426 ms and Hodges 400 + 1.75 × (72 − 60) = 421 ms. Example at a fast rate: QT 340 ms at 110 bpm, female. RR = 0.545 s. Bazett gives 340 ÷ 0.739 = 460 ms, exactly at the female threshold, while Fridericia gives 340 ÷ 0.817 = 416 ms and Framingham 340 + 154 × 0.455 = 410 ms. The Bazett figure is an over-correction at this rate; a repeat ECG once the rate settles, or the Fridericia value, is the better guide.

Frequently asked questions

What is a normal QTc?

Below 450 ms in men and below 460 ms in women by the 2009 AHA/ACCF/HRS recommendations. Some references use 440 ms as the upper limit for men. A QTc of 500 ms or more is markedly prolonged for anyone and carries a clearly increased risk of torsades de pointes.

Which QTc formula should I use?

Bazett is what most ECG machines, drug labels and hospital protocols report, so it is the default here. Fridericia is more accurate when the heart rate is below 60 or above 90-100 bpm and is preferred by regulators for drug studies. Framingham and Hodges are linear corrections that behave like Fridericia at fast rates. When the formulas disagree widely, the heart rate is far from 60 and Bazett is the one most likely to mislead.

How do I measure the QT interval?

On a standard 25 mm/s recording each small square is 40 ms and each large square 200 ms. Measure from the earliest start of the QRS to the end of the T wave, where the tangent to the steepest part of the descending T wave meets the baseline, in lead II or V5, taking the longest value. Exclude a U wave. Automated machine QT values are usually reliable but should be checked when the result is near a threshold.

What makes the QTc longer?

Many drugs (some antiarrhythmics, antipsychotics, antidepressants, macrolide and fluoroquinolone antibiotics, antiemetics such as ondansetron, methadone), low potassium, magnesium or calcium, bradycardia, hypothermia, myocardial ischaemia, raised intracranial pressure and congenital long QT syndrome. A new prolongation should prompt a review of medicines and electrolytes before anything else.

Can I enter the RR interval instead of the heart rate?

Yes. Choose "RR interval" and enter it in milliseconds (the distance between two consecutive R waves, 1 large square = 200 ms). The calculator converts it to seconds for the formulas and shows the equivalent heart rate in the tiles.

Does the calculator handle a wide QRS or atrial fibrillation?

Not specifically. With a bundle branch block or paced rhythm the QT includes the widened QRS, and corrections such as subtracting the QRS excess over 120 ms (or JT interval methods) are used; in atrial fibrillation the QT and RR should be averaged over 5-10 beats before entering them. Those situations need a clinician's judgement.

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