QTc Calculator — Corrected QT Interval by Bazett, Fridericia, Framingham & Hodges

With our QTc calculator you can correct the QT interval for your patient's heart rate. In the text below you will learn how to calculate QTc using Bazett's formula and other equations (Fridericia, Framingham, Hodges), what a normal QT interval is, what the risks of QT prolongation are, and which QT-prolonging drugs should be avoided in patients with a prolonged QT interval.

Are you interested in cardiology? We have many other calculators you may want to see — try the stroke volume calculator and our MAP calculator.

QT Interval

The QT interval is a part of the electrocardiographic (ECG) record of the electrical activity of the heart. It starts at the beginning of the QRS complex (caused by the depolarization of the ventricles) and ends at the end of the T wave (the repolarization of the ventricles). In other words, it is the time the ventricles need to activate and then recover.

Cardiologists often measure the QT interval because its prolongation correlates with ventricular arrhythmias such as torsades de pointes and is a risk factor for sudden cardiac death. The duration of the QT interval depends strongly on the heart rate: the faster the rhythm, the shorter the QT interval. Because of that, the QT interval has to be corrected for heart rate before it can be compared with normal values. The result is called QTc — the QT interval the heart would have at 60 beats per minute.

How to measure the QT interval well:

• use lead II or V5, where the end of the T wave is usually the clearest;
• mark the end of the T wave where it returns to the baseline (or where the steepest tangent of its downslope crosses the baseline) — do not include a separate U wave;
• average 3–5 beats; with atrial fibrillation, average more beats, because the RR interval changes from beat to beat;
• measure QT and RR on the same beat: the RR interval is the distance from the previous R wave to the R wave of the measured beat.

Bazett's Formula

The most common equation used for QT correction all around the world is the formula presented by the physiologist Henry Cuthbert Bazett in the article "An Analysis of the Time-Relations of Electrocardiograms", published in 1920 in the journal Heart. Bazett's formula looks like this:

QTc = QT / √RR

where:

• QTc — corrected QT interval;
• QT — duration of the QT interval; and
• RR — duration of the RR interval, in seconds (RR = 60 / heart rate).

Although very popular, Bazett's formula has its limitations — it is known for overcorrecting at high heart rates (the QTc comes out too long) and undercorrecting at low heart rates (the QTc comes out too short and can hide a prolonged QT).

Other QT correction formulas

Several other equations have been proposed. The calculator computes all of them at once and shows them side by side, so you can see whether they agree.

• Fridericia (Louis Sigurd Fridericia, 1920): QTc = QT / ∛RR. The cube root instead of the square root gives more consistent results at faster heart rates.
• Framingham (Sagie et al., 1992, from the Framingham Heart Study): QTc = QT + 0.154 × (1 − RR), with QT and RR in seconds — or QT + 154 × (1 − RR) with QT in milliseconds.
• Hodges (M. Hodges, 1983): QTc = QT + 1.75 × (heart rate − 60), with QT in milliseconds and the heart rate in beats per minute.

In a study published in 2016 in the Journal of the American Heart Association, Vandenberk and colleagues compared these methods in more than 6,000 patients and concluded that the Framingham and Fridericia equations provided the best rate correction and mortality prediction. Bazett's formula performed worst, yet it remains the one most ECG machines print.

At exactly 60 bpm the RR interval is 1 second, and every formula gives QTc = QT.

How to Calculate QTc?

1. Choose the units in which you measured the intervals: milliseconds, seconds, or small boxes on the ECG paper. One small box is 1 mm; at the standard paper speed of 25 mm/s (US and most countries) it is 40 ms, at 50 mm/s (used in parts of Europe) it is 20 ms. If you switch the units after typing, the values are converted for you.
2. Choose the sex — women have a longer normal QTc, so the limit for a prolonged QTc is different.
3. Enter the QT interval.
4. Enter either the heart rate in beats per minute or the RR interval — the calculator derives one from the other.
5. Optionally enter the QRS duration. If it is 120 ms or more, the calculator adds the Bogossian correction for a wide QRS (see below).
6. Choose the main formula and press Calculate. You will see the QTc by that formula with its category, a scale with the results of all four formulas, a comparison table, your measurements in every unit, every formula with your own numbers, and the reference ranges with your row highlighted.

Each QTc is rounded half up to a whole millisecond, and the category is taken from that rounded value, so the category always matches the number you see.

Worked example (the calculator's default values)

• QT = 400 ms, heart rate = 75 bpm → RR = 60 / 75 = 0.8 s.
• Bazett: 400 / √0.8 ≈ 400 / 0.8944 ≈ 447 ms.
• Fridericia: 400 / ∛0.8 ≈ 400 / 0.9283 ≈ 431 ms.
• Framingham: 400 + 154 × (1 − 0.8) = 400 + 30.8 ≈ 431 ms.
• Hodges: 400 + 1.75 × (75 − 60) = 400 + 26.25 ≈ 426 ms.

All four results are normal for both men and women.

When the formulas disagree

At a heart rate of 110 bpm with QT = 360 ms, Bazett gives 487 ms — prolonged — while Fridericia gives 441 ms, Framingham 430 ms and Hodges 448 ms, all normal for men. This is Bazett's overcorrection at a fast heart rate. The calculator tells you in words when the formulas put the same ECG into different categories.

Normal, Short and Prolonged QTc

These limits are for adults. Children and adolescents are assessed with other limits.

QT Prolongation

A prolonged QT interval means that the ventricles take longer than usual to recover after each beat. During this vulnerable period an early extra beat can trigger torsades de pointes — a polymorphic ventricular tachycardia that may cause fainting, seizures or, if it turns into ventricular fibrillation, sudden cardiac death. The risk grows with the QTc: an increase of the QTc by more than 60 ms from the baseline, or a QTc of 500 ms or more, is considered a high risk.

QT prolongation can be:

• congenital — long QT syndrome (LQTS), caused by variants in genes for cardiac ion channels (most often KCNQ1, KCNH2 and SCN5A); or
• acquired — much more common, caused by medicines, low potassium, magnesium or calcium, a slow heart rate, heart disease (heart failure, myocardial ischemia), hypothyroidism, hypothermia, anorexia nervosa or intracranial events.

Risk factors for drug-induced torsades de pointes include female sex, older age, hypokalemia, hypomagnesemia, bradycardia, heart failure, kidney or liver failure (higher drug levels), and taking two or more QT-prolonging drugs at the same time.

Wide QRS: the Bogossian correction

The QT interval includes the QRS complex. When the QRS is wide — 120 ms or more, for example with bundle branch block or ventricular pacing — the QT is longer without any change in repolarization, and the QTc overstates the risk. Bogossian et al. (2014) proposed to subtract half of the QRS duration from the QT first: QTm = QT − 0.5 × QRS, and then to correct the QTm with the usual formula. For example, QT = 480 ms and QRS = 140 ms at 75 bpm give a Bazett QTc of 537 ms, but a QTmc of only 458 ms.

Short QT

A very short QT interval is rare. A QTc of 320 ms or less meets the criterion for short QT syndrome in the 2022 ESC guidelines — an inherited condition that can cause atrial fibrillation and sudden cardiac death. A QTc between 321 and 360 ms suggests it only together with other findings. Before worrying, check the measurement: the end of the T wave is easily placed too early.

QT-Prolonging Drugs

Many commonly used medicines can prolong the QT interval. In a patient with a prolonged QTc, these drugs should be avoided or used with care — with ECG monitoring and correction of potassium and magnesium. Examples (not a complete list):

• Antiarrhythmics: amiodarone, sotalol, dofetilide, dronedarone, quinidine, procainamide, disopyramide, flecainide;
• Antibiotics: macrolides (azithromycin, clarithromycin, erythromycin) and fluoroquinolones (levofloxacin, moxifloxacin, ciprofloxacin);
• Antifungals: fluconazole and other azoles;
• Antimalarials: chloroquine, hydroxychloroquine, quinine;
• Antipsychotics: haloperidol (especially intravenous), chlorpromazine, thioridazine, pimozide, sertindole;
• Antidepressants: citalopram, escitalopram, tricyclic antidepressants;
• Antiemetics and prokinetics: ondansetron, domperidone, droperidol;
• Opioids: methadone;
• Others: arsenic trioxide, some antihistamines (terfenadine, astemizole — withdrawn in many countries), cocaine.

The most widely used reference is the CredibleMeds (crediblemeds.org) list, which sorts drugs into "known", "possible" and "conditional" risk of torsades de pointes. Never stop or change a prescribed medicine on your own — discuss it with your doctor.

Units: Milliseconds, Seconds and ECG Boxes

The QT and RR intervals are usually given in milliseconds (ECG machines in the US and Europe) or in seconds (the unit used by the original formulas). On paper you count the small boxes (1 mm each) and convert them with the paper speed:

• at 25 mm/s (the standard in the US and most countries): 1 small box = 40 ms, 1 large box (5 mm) = 200 ms;
• at 50 mm/s (used in parts of Europe): 1 small box = 20 ms, 1 large box = 100 ms.

For example, a QT of 10 small boxes is 400 ms at 25 mm/s but only 200 ms at 50 mm/s — always check the paper speed printed on the ECG. The results show every interval in all units.

FAQs

What is a normal QTc?

For adults, a QTc below 450 ms in men and below 460 ms in women is normal, and above 360 ms it is not short.

Which QTc formula is the best?

Studies favor Fridericia and Framingham. Bazett's formula is the most widely used but overcorrects at heart rates above about 90 bpm and undercorrects below 60 bpm. When the formulas disagree, look at Fridericia and Framingham first.

What QTc is dangerous?

A QTc of 500 ms or more — or an increase of more than 60 ms from the patient's baseline — carries a high risk of torsades de pointes and needs prompt medical review.

Why is the QTc limit higher for women?

After puberty, women have a longer QT interval than men, partly because testosterone shortens ventricular repolarization. Women are also more prone to drug-induced torsades de pointes.

Can I calculate QTc at a fast heart rate?

Yes, but above 100 bpm every correction formula becomes less accurate, and the T wave may merge with the next P wave. If possible, repeat the ECG when the heart rate is slower.

How do I convert RR to heart rate?

Heart rate (bpm) = 60 / RR (in seconds). An RR interval of 0.8 s is 75 bpm; 1 s is 60 bpm.

References

1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
2. Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Medica Scandinavica. 1920;53:469–486.
3. Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). American Journal of Cardiology. 1992;70(7):797–801.
4. Hodges M, Salerno D, Erlien D. Bazett's QT correction reviewed: evidence that a linear QT correction for heart rate is better. Journal of the American College of Cardiology. 1983;1:694.
5. Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? Journal of the American Heart Association. 2016;5(6):e003264.
6. Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, Part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250.
7. Zeppenfeld K, Tfelt-Hansen J, de Riva M, et al. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. European Heart Journal. 2022;43(40):3997–4126.
8. Bogossian H, Frommeyer G, Ninios I, et al. New formula for evaluation of the QT interval in patients with left bundle branch block. Heart Rhythm. 2014;11(12):2273–2277.
9. Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060.