The QT interval shortens as heart rate rises, so a raw QT cannot be compared between ECGs or against a fixed limit. The corrected QT (QTc) estimates what the QT would be at 60 beats per minute. This calculator uses the Fridericia formula by default and shows Bazett, Framingham and Hodges alongside it, because the formula used changes the answer — sometimes enough to move a result across the prolonged threshold.
- What it estimates
- QT interval corrected to a heart rate of 60 bpm
- Inputs
- Measured QT (ms), heart rate (bpm), sex
- Default formula
- Fridericia — QT / RR^(1/3)
- Prolonged
- ≥ 450 ms in men, ≥ 460 ms in women
- High risk
- ≥ 500 ms
- Thresholds from
- AHA/ACCF/HRS ECG recommendations, 2009
Formula
Fridericia (default); Bazett, Framingham (Sagie 1992) and Hodges shown for comparisonRR = 60 / HR; QTc (Fridericia) = QT / RR^(1/3); QTc (Bazett) = QT / √RR; QTc (Framingham) = QT + 154 × (1 − RR); QTc (Hodges) = QT + 1.75 × (HR − 60)
- QT
- — Measured QT interval, in milliseconds
- HR
- — Heart rate, in beats per minute
- RR
- — Interval between successive R waves, in seconds (60 ÷ HR)
- QTc
- — Corrected QT interval, in milliseconds
Bazett is the formula most often printed on ECG reports, but it overcorrects at fast heart rates and undercorrects at slow ones, which the Framingham investigators and the ICH E14 drug-safety guideline both point out. In a 6,609-patient hospital cohort, Fridericia and Framingham corrected for rate best and Bazett worst. Fridericia is the default here for that reason; all four are always shown so you can match whichever your ECG report used.
QTc Calculator examples
| Input | Result | Category |
|---|---|---|
| Man, QT 400 ms, 75 bpm | 431 ms | Normal |
| Woman, QT 440 ms, 70 bpm | 463 ms | Prolonged |
| Man, QT 340 ms, 60 bpm | 340 ms | Short |
| Woman, QT 480 ms, 60 bpm | 480 ms | Prolonged |
| Man, QT 360 ms, 110 bpm, Bazett | 487 ms | Prolonged |
| Woman, QT 420 ms, 100 bpm, Framingham | 482 ms | Prolonged |
| Man, QT 460 ms, 85 bpm | 517 ms | Markedly prolonged |
What is the QTc interval?
The QT interval runs from the start of the QRS complex to the end of the T wave. It covers ventricular depolarisation and repolarisation — the time the ventricles take to contract and then electrically reset before the next beat.
Because the QT naturally shortens as the heart speeds up, a fixed upper limit only makes sense once the QT has been adjusted to a standard heart rate. That adjusted value is the QTc. A long QTc means repolarisation is delayed, which can set the stage for a dangerous ventricular arrhythmia called torsade de pointes.
What is a normal QTc for men and women?
| Interpretation | Men | Women |
|---|---|---|
| Short | 390 ms or less | 390 ms or less |
| Normal | 391–449 ms | 391–459 ms |
| Prolonged | 450–499 ms | 460–499 ms |
| Markedly prolonged | 500 ms and above | 500 ms and above |
The prolonged thresholds of 450 ms in men and 460 ms in women come from the 2009 AHA/ACCF/HRS recommendations for ECG interpretation. Women have a slightly longer QTc than men after puberty, which is why their threshold is higher. A short QTc is far less common and is mainly relevant when an inherited short QT syndrome is suspected.
The same thresholds are applied whichever formula is used, but the formulas can differ by 10–20 ms or more at heart rates well away from 60 bpm — so a borderline result should be read with the formula in mind.
Which QT correction formula should I use?
| Formula | Equation | Behaviour |
|---|---|---|
| Fridericia | QT / RR^(1/3) | Good rate correction across typical heart rates |
| Bazett | QT / √RR | Overcorrects at fast rates, undercorrects at slow rates |
| Framingham | QT + 154 × (1 − RR) | Linear; derived from 5,018 Framingham participants |
| Hodges | QT + 1.75 × (HR − 60) | Linear in heart rate |
At exactly 60 bpm all four formulas return the measured QT unchanged. The further the heart rate is from 60, the more they diverge. At a heart rate of 75 bpm, a QT of 400 ms is 431 ms by Fridericia but 447 ms by Bazett — close to the 450 ms limit for men.
The ICH E14 guideline on drug QT studies notes that Bazett overcorrects at elevated heart rates, and asks for both Bazett- and Fridericia-corrected values in drug applications. If you are tracking a patient over time, the most important thing is to use the same formula every time.
When is a prolonged QTc dangerous?
The risk of torsade de pointes rises as the QTc lengthens, but there is no single value at which it becomes certain. An AHA/ACCF scientific statement on preventing torsade de pointes in hospital reports that a QTc above 500 ms is associated with a 2- to 3-fold higher risk, and regards any QTc above 500 ms as highly abnormal in both sexes.
In drug trials, an increase in QTc to more than 500 ms, or of more than 60 ms over baseline, is commonly used as a threshold for considering stopping the study drug.
- QT-prolonging medicines — some antiarrhythmics, antibiotics, antipsychotics, antidepressants and antiemetics among them. CredibleMeds, run by the non-profit AZCERT, maintains a regularly updated list.
- Low potassium, low magnesium, or low calcium.
- Slow heart rate, heart block or pauses, and heart disease such as heart failure or a prior heart attack.
- Female sex, age over 65, diuretic treatment, and an inherited long QT syndrome.
When is a QTc calculation unreliable?
- Fast heart rates. Above roughly 100 bpm every formula becomes less reliable, and Bazett in particular overstates the QTc. Repeating the ECG at a slower rate is often more useful than choosing a different formula.
- Atrial fibrillation and other irregular rhythms. The RR interval changes beat to beat, so a single heart rate does not describe the tracing. Averaging over several beats is usual, and the result should be read with caution.
- Bundle branch block, ventricular pacing, or a wide QRS. These lengthen the QT through slower depolarisation rather than delayed repolarisation; the JT interval (QT minus QRS duration) is often used instead.
- Measurement error. The end of the T wave is hard to place when it is flat or merges with a U wave. Use a lead with a well-defined T-wave end, and the same lead on repeat ECGs, so that changes over time are real.
How do you measure the QT interval?
- 1.Choose a lead with a tall, clearly defined T wave — lead II or V5 is often used — and use the same lead on repeat ECGs.
- 2.Mark the start of the QRS complex.
- 3.Mark the end of the T wave. Where it is unclear, a common method is to draw a line along the steepest part of the T wave's downslope and take the point where it meets the baseline.
- 4.Measure across several beats and average them, especially if the rhythm is not perfectly regular.
- 5.Take the heart rate from the same beats, then apply the correction.
Frequently asked questions
What is a normal QTc?
Under the 2009 AHA/ACCF/HRS recommendations, a QTc is prolonged from 450 ms in men and from 460 ms in women, and values of 390 ms or less are considered short. Anything at or above 500 ms is markedly prolonged in either sex and is associated with a higher risk of torsade de pointes.
Why does my ECG report a different QTc?
Most ECG machines use Bazett's formula, which gives higher values than Fridericia at heart rates above 60 bpm and lower values below it. The machine's measurement of the QT itself can also differ from a manual read. Check which formula your report used and compare like with like.
Is Fridericia better than Bazett?
For most adults, yes. Bazett overcorrects at fast heart rates and undercorrects at slow ones. In a large hospital study, Fridericia and Framingham corrected for heart rate best and predicted mortality better, while Bazett flagged more patients as dangerously prolonged. The key is to use one formula consistently over time.
What QTc is too high to start a QT-prolonging drug?
There is no universal cut-off, and the decision depends on the drug, the dose and the patient's other risk factors. A QTc of 500 ms or more is widely treated as high risk, and electrolytes are usually corrected before starting. This is a decision for the prescriber, not the calculator.
Can I calculate QTc in atrial fibrillation?
You can, but the result is less reliable. Because the RR interval varies from beat to beat, a single heart rate does not describe the rhythm. Clinicians usually average QT and RR across several beats or a longer strip, and treat the corrected value as an approximation rather than a precise figure.
Does a bundle branch block affect the QTc?
Yes. A bundle branch block widens the QRS complex and lengthens the QT for reasons unrelated to repolarisation, so the QTc overstates the risk the threshold is meant to capture. The JT interval, which subtracts the QRS duration, is often used instead, and a specialist read is advisable.
Sources
- 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 — Rautaharju PM et al., Circulation (PubMed), 2009
- An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study) — Sagie A et al., American Journal of Cardiology (PubMed), 1992
- Which QT Correction Formulae to Use for QT Monitoring? — Vandenberk B et al., Journal of the American Heart Association (PubMed), 2016
- Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation — Drew BJ et al., Circulation (PubMed Central), 2010
- ICH E14 Clinical evaluation of QT/QTc interval prolongation and proarrhythmic potential for non-antiarrhythmic drugs — European Medicines Agency, 2005