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QTc Calculator
Correct the QT interval for heart rate with the Bazett and Fridericia formulas, and see whether the result falls in the normal, borderline or prolonged range.
Enter the QT interval in milliseconds and the heart rate in beats per minute, exactly as measured on the ECG.
Advanced options
The Fridericia, Framingham and Hodges corrections are already calculated in the result with their standard formulas. Open Advanced options to add a second QT reading or a manual RR interval.
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Key takeaways
- At a heart rate of 60 bpm, the RR interval is exactly 1 second, so QTc equals the measured QT: a 400 ms QT gives a 400 ms QTc by both Bazett and Fridericia.
- At 120 bpm, the same 400 ms QT corrects to 566 ms by Bazett and 504 ms by Fridericia, which shows why Bazett overcorrects at fast heart rates.
- Bazett is the most widely quoted formula, but Fridericia is more stable when the heart rate is far from 60 bpm.
- A QTc above 500 ms is widely regarded as markedly prolonged and carries a higher risk of torsades de pointes.
Contents
- What the QTc is
- Bazett, Fridericia, Framingham and Hodges
- Worked examples
- Normal limits and risk bands
- Common mistakes
Related tools
What the QTc is
The QT interval is the time from the start of the Q wave to the end of the T wave on an ECG, and it represents ventricular depolarization and repolarization. Because the QT shortens as the heart rate rises, a raw QT number is hard to compare between patients. The corrected QT, or QTc, adjusts the measured QT to what it would be at a heart rate of 60 beats per minute, which makes comparisons meaningful.
This calculator is an educational tool. It does not diagnose, and any prolonged QTc should be reviewed by a clinician with the full ECG, the medication list and the electrolyte results in hand.
Bazett, Fridericia, Framingham and Hodges
Bazett: QTc = QT divided by the square root of RR, with RR in seconds. Fridericia: QTc = QT divided by the cube root of RR. Framingham: QTc = QT plus 0.154 times (1 minus RR) in seconds, converted to milliseconds. Hodges: QTc = QT plus 1.75 times (heart rate minus 60).
| Formula | Best for | Known weakness |
|---|---|---|
| Bazett | Routine reporting, heart rates near 60 | Overcorrects at fast rates, undercorrects at slow rates |
| Fridericia | Fast or slow heart rates, drug studies | Less familiar to some readers |
| Framingham | Population studies | Linear approximation |
| Hodges | Bedside estimate | Linear approximation |
Worked examples
Example 1: QT 400 ms, heart rate 60 bpm. RR is 1.00 second, so Bazett gives 400 ms and Fridericia gives 400 ms.
Example 2: QT 400 ms, heart rate 120 bpm. RR is 0.50 seconds. Bazett gives 400 divided by 0.7071 = 565.7 ms, while Fridericia gives 400 divided by 0.7937 = 504.0 ms. The gap between the two formulas is the reason drug-safety work usually prefers Fridericia at fast rates.
Example 3: QT 360 ms, heart rate 75 bpm. RR is 0.80 seconds. Bazett gives 402.5 ms and Fridericia gives 387.8 ms, both in the normal range for most adults.
Normal limits and risk bands
Commonly quoted upper limits are about 450 ms for adult men and 470 ms for adult women, with 440 to 460 ms often called borderline. Values above 500 ms are widely treated as markedly prolonged. These bands are population guides, not personal diagnoses: a QTc must be read with the clinical picture, including potassium, magnesium and calcium, thyroid status, congenital long-QT history and every QT-prolonging drug on the list.
Common mistakes
The most common errors are entering the QT in seconds instead of milliseconds, using the heart rate instead of the RR interval in the formula, and measuring the QT in a single lead with a prominent U wave. Measure in leads II or V5 where the T wave is clearest, average several beats in atrial fibrillation, and never correct a QT from a paced or very wide-QRS rhythm without specialist input.
How it works
Use milliseconds, exactly as measured on the ECG tracing.
Beats per minute from the same ECG. The RR interval is calculated automatically.
Read Bazett first, then check Fridericia. If they disagree by a wide margin, the heart rate is far from 60 and Fridericia is the safer guide.
Measuring the QT interval by hand: the tangent method
The calculator does the arithmetic, but the QT itself has to be measured off the ECG, and the measurement is where most errors creep in. The standard method is the tangent method, used in the studies that produced the normal limits. Pick lead II or V5, whichever shows the T wave most clearly, and use the same lead every time you measure, because the QT varies between leads and a change of lead can look like a change in the patient.
Find the start of the QRS complex and the end of the T wave. The end is not where the T wave fades to nothing — it is where the tangent to the steepest downslope of the T wave crosses the baseline, which is the flat line between beats. Draw the steepest straight line you can down the falling side of the T wave, and read the QT where that line meets the baseline. If a U wave follows the T wave, ignore it unless it is fused with the T wave, in which case the measurement is unreliable and a different beat should be chosen. Measure three to five beats and average them; in atrial fibrillation, where every beat is different, ten beats are the usual recommendation, and the shortest and longest R-R intervals should both be covered.
Two adjustments complete the method. If the QRS complex is wider than 120 ms, as in bundle branch block or a paced rhythm, the wide QRS inflates the QT without any change in repolarisation, so subtract the excess over 120 ms from the QT before correcting — or use the JT interval, which is the QT minus the QRS width, with its own normal limits. And measure at a stable heart rate: a QT taken during a sudden rate change, or averaged across beats at very different rates, is not a measurement the correction formulas were designed for.
Frequently asked questions
What is a normal QTc?
For most adults, up to about 450 ms in men and 470 ms in women is considered normal, with 440 to 460 ms often called borderline. Above 500 ms is widely regarded as markedly prolonged and warrants prompt clinical review.
Why do Bazett and Fridericia disagree?
Both formulas correct for heart rate, but Bazett divides by the square root of RR while Fridericia divides by the cube root. At heart rates far from 60 bpm the two diverge: at 120 bpm a 400 ms QT gives 566 ms by Bazett but 504 ms by Fridericia.
Which formula should I use?
Bazett is the most widely reported and is fine near 60 bpm. Fridericia is preferred in drug-safety work and at fast or slow heart rates because it overcorrects less. When in doubt, report both and let the clinician judge.
Can I use this calculator for a child?
Pediatric QTc interpretation uses age-specific ranges and often requires specialist input. This calculator shows the arithmetic for any input, but its adult bands do not apply to children.
Does a long QTc always mean danger?
No. Risk depends on how long the QTc is, whether it is congenital or acquired, electrolytes, and drugs. A single borderline reading on a noisy tracing is very different from a persistent QTc above 500 ms, which is why the full clinical picture matters.
What drugs prolong the QT interval?
Many antiarrhythmics, some antibiotics, antipsychotics and antidepressants, methadone and several others can prolong the QT. Always review the full medication list and check potassium and magnesium before judging a QTc value.
How do you measure the QT interval on an ECG?
With the tangent method: in lead II or V5, draw a tangent to the steepest downslope of the T wave and read the QT where it crosses the baseline, from the start of the QRS complex. Exclude the U wave unless it is fused with the T wave, average three to five beats, and if the QRS is wider than 120 ms, subtract the excess over 120 ms before correcting.
Is the QT interval the same as the QTc?
No. The QT is the raw measurement off the ECG in milliseconds; the QTc is that measurement corrected for heart rate, so that a QT taken at 120 beats per minute can be compared with one taken at 60. The same raw QT can be normal at a slow rate and prolonged at a fast one, which is why the correction exists.
What is a high QTc level?
A QTc over 500 ms is the level most often called highly abnormal in both sexes, and it is the threshold most associated with a higher risk of torsade de pointes. A QTc between the prolonged limit (450 ms in men, 460 ms in women) and 500 ms is prolonged and worth a clinical review, but it is the 500 ms line that most often prompts action.