QTc Calculator — Corrected QT Interval with Bazett, Fridericia, Framingham and Hodges
Calculator
QTc Calculator (corrected QT interval)
From the QT measured on the ECG and the heart rate, the corrected QT with the Bazett, Fridericia, Framingham and Hodges formulas, compared with the thresholds for adult women and men
The tool accepts QT between 200 and 700 ms, heart rate between 30 and 250 bpm, RR between 240 and 2000 ms: these are the calculator's plausibility limits, not clinical values. Comma or point both work as the decimal separator.
The calculator does not make a diagnosis. The QT must be measured on the ECG and interpreted by a doctor, who assesses the whole tracing, the measurement technique, medications taken, electrolytes and clinical history: the same number can mean different things in different people. The thresholds are those for adults from the 2009 AHA/ACCF/HRS recommendations; the 500 ms limit comes from the 2010 AHA/ACCF scientific statement. Do not use this result to start, change or stop a treatment.
More Details
How the QTc calculator works
The QT interval is the segment of the electrocardiogram running from the start of the QRS complex to the end of the T wave: it includes the depolarization and repolarization of the ventricles. It changes with heart rate: when the heart speeds up it shortens, when it slows down it lengthens. To compare ECGs recorded at different rates, the QTc is used: the QT corrected and adjusted to an RR interval of one second, that is, to 60 beats per minute.
Enter the measured QT, sex, and either the heart rate or the RR interval: the calculator applies four published formulas and compares each result with the thresholds for adults. The calculation takes place entirely in the browser: no data is sent or saved.
The four correction formulas
In the formulas QT is in milliseconds, RR in seconds and heart rate (HR) in beats per minute, with RR = 60 / HR: 75 bpm corresponds to 0.8 seconds, 100 bpm to 0.6.
| Formula | QTc calculation | Origin |
|---|---|---|
| Bazett | QT / √RR | Bazett, 1920 |
| Fridericia | QT / ∛RR | Fridericia, 1920 |
| Framingham | QT + 154 × (1 − RR) | Sagie et al., 1992 (in seconds: QT + 0.154 × (1 − RR)) |
| Hodges | QT + 1.75 × (HR − 60) | Hodges et al., 1983 |
At 60 bpm the RR is one second and the correction is zero: a QT of 400 ms gives 400 ms with any formula. The further you move from that rate, the more the formulas diverge. Bazett derived his from measurements of 39 young subjects; the Framingham formula is a linear regression on the ECGs of 5,018 participants in the Framingham Heart Study, and its authors note that Bazett overcorrects the QT at high heart rates and undercorrects it at low heart rates.
The thresholds used to interpret the result
QT is on average longer in young and middle-aged women than in men: this is why the limit changes with sex. The calculator uses the practical clinical limits for adults from the 2009 AHA/ACCF/HRS recommendations on ECG interpretation:
| Outcome | Woman | Man | Source |
|---|---|---|---|
| Prolonged corrected QT | 460 ms or more | 450 ms or more | AHA/ACCF/HRS 2009 |
| Short corrected QT | 390 ms or less | 390 ms or less | AHA/ACCF/HRS 2009 |
| Highly abnormal value | over 500 ms | over 500 ms | AHA/ACCF 2010 |
The 500 ms threshold comes from the 2010 AHA/ACCF scientific statement on the prevention of torsade de pointes in hospital settings: a QTc over 500 ms is considered highly abnormal in both sexes and, in data on congenital long QT syndrome, is associated with a 2 to 3 times higher risk of torsade de pointes. The same document also gives the upper limit of normal (99th percentile) in healthy post-pubertal adults: about 470 ms in men and 480 ms in women.
Calculation example
In the first case a woman has a QT of 360 ms at 100 bpm: the RR is 60 / 100 = 0.6 seconds, so Bazett gives 360 / √0.6 = 360 / 0.7746 = 464.8 ms, Fridericia 360 / ∛0.6 = 360 / 0.8434 = 426.8 ms, Framingham 360 + 154 × 0.4 = 421.6 ms and Hodges 360 + 1.75 × 40 = 430 ms. In the second a man has a QT of 480 ms at 50 bpm, with an RR of 1.2 seconds. Here are the calculator's values:
| Formula | Woman, QT 360 ms at 100 bpm | Man, QT 480 ms at 50 bpm |
|---|---|---|
| Bazett | 465 ms — prolonged | 438 ms — within limits |
| Fridericia | 427 ms — within limits | 452 ms — prolonged |
| Framingham | 422 ms — within limits | 449 ms — within limits |
| Hodges | 430 ms — within limits | 463 ms — prolonged |
At a high heart rate Bazett is the only formula above the 460 ms threshold; at a low heart rate it gives the shortest value, and the other three disagree, with Framingham one millisecond from the 450 ms threshold.
Which formula to choose and when the calculation does not apply
Bazett remains the most widely used formula, and when a report does not specify the method, the QTc given is usually Bazett's. The 2009 recommendations point out, however, that it leaves a strong residual correlation with heart rate and can produce false QT prolongation: they recommend linear regression functions and stating the method used in the report. The 2010 statement specifies that Bazett gives values that are too long especially above 85 beats per minute. In a study of 6,609 patients at the university hospitals of Leuven, Fridericia and Framingham corrected best, Bazett worst.
Correction should not be attempted when RR intervals vary greatly from one beat to the next, as often happens in atrial fibrillation, nor when the end of the T wave cannot be identified reliably. The thresholds apply to adults: different references are used in children. A prolonged QT can be caused by medications, low potassium or calcium, or congenital forms: only a doctor can assess it, on the whole ECG and together with the clinical history. This tool helps you understand a number; it does not make a diagnosis.
Frequently asked questions
How is QTc calculated?
With the Bazett formula, the most widely used, you divide the QT in milliseconds by the square root of the RR interval in seconds, which is obtained by dividing 60 by the heart rate. With a QT of 360 ms at 100 bpm the RR is 0.6 seconds and the Bazett QTc is 360 / 0.7746 = 465 ms. Fridericia uses the cube root of RR, Framingham and Hodges a linear correction: for the same example they give 427, 422 and 430 ms.
What is the normal QTc value?
The 2009 AHA/ACCF/HRS recommendations consider a corrected QT in adults prolonged at 460 ms or more in women and 450 ms or more in men, and short at 390 ms or less in both sexes. A QTc over 500 ms is considered highly abnormal for everyone. These are practical limits: other documents use slightly different thresholds and interpretation is always up to the doctor.
Why does Bazett give a different QTc from the other formulas?
Because it does not fully remove the effect of heart rate: at high rates it overcorrects and the QTc comes out too long, at low rates it undercorrects and comes out shorter. The 2010 AHA/ACCF statement flags the problem especially above 85 beats per minute.
How do you convert heart rate to the RR interval?
The RR interval is the duration of one cardiac cycle, the time between one beat and the next: it is obtained by dividing 60,000 by the rate in beats per minute, and the result is in milliseconds. At 60 bpm the RR is 1,000 ms, at 75 bpm 800 ms, at 100 bpm 600 ms.
Related tools: the article on heart rate, the glomerular filtration rate (GFR) calculator and the guide to health and fitness calculators.
