How do you calculate the QTc?
Work out the RR interval from the heart rate, then apply a correction formula. The QT shortens as the heart speeds up, so a raw QT cannot be compared between ECGs recorded at different rates. Each formula estimates the QT at 60 beats/min, where the RR interval is exactly 1 second.
RR (s) = 60 ÷ heart rate
The calculator then applies four formulas, with QT in milliseconds:
Bazett: QTc = QT ÷ √RR
Fridericia: QTc = QT ÷ ∛RR
Framingham: QTc = QT + 154 × (1 − RR)
Hodges: QTc = QT + 1.75 × (heart rate − 60)
At exactly 60 beats/min all four give back the measured QT unchanged. For a closer look at how they differ, read Bazett vs Fridericia: QTc formulas compared.
How to use the calculator
- Enter the Measured QT in milliseconds, from the start of the QRS complex to the end of the T wave.
- Enter the Heart rate in beats per minute, ideally from the same beats you used for the QT.
- Select the Sex. It changes only the warning threshold, not the numbers.
- Choose the Main formula (Fridericia by default). Its result is the main figure; the other three are listed underneath, so you can see how far they agree.
- Check the working: the RR interval (up to three decimals), then the chosen formula. All QTc values are rounded to whole milliseconds.
Worked examples
Example 1 – heart rate 60. QT 400 ms, heart rate 60 beats/min. RR = 60 ÷ 60 = 1 s. Every formula returns 400 ms: at RR = 1 there is nothing to correct.
Example 2 – a fast heart rate. QT 360 ms, heart rate 110 beats/min. RR = 60 ÷ 110 = 0.545 s. Fridericia: 360 ÷ ∛0.545 = 441 ms. The others give Bazett 487 ms, Framingham 430 ms and Hodges 448 ms. The rate is above 100/min, so the calculator warns that Bazett is least reliable here. With Fridericia as the main formula there is no prolonged-QTc warning. Switch the main formula to Bazett and the same ECG crosses both the 450 and 460 ms thresholds.
Example 3 – a slow heart rate. QT 480 ms, heart rate 50 beats/min, male. RR = 60 ÷ 50 = 1.2 s. Fridericia: 480 ÷ ∛1.2 = 452 ms, just over the 450 ms threshold for males, so the prolonged-QTc warning appears. Bazett gives 438 ms, Framingham 449 ms and Hodges 463 ms: at slow rates Bazett makes the QTc look shorter. Select female and the threshold becomes 460 ms, so the Fridericia result is no longer flagged.
How the formulas diverge
The same measured QT of 380 ms corrected at different heart rates (values in ms, RR as the calculator shows it).
| Heart rate | RR (s) | Bazett | Fridericia | Framingham | Hodges |
|---|---|---|---|---|---|
| 50 | 1.2 | 347 | 358 | 349 | 363 |
| 60 | 1 | 380 | 380 | 380 | 380 |
| 75 | 0.8 | 425 | 409 | 411 | 406 |
| 90 | 0.667 | 465 | 435 | 431 | 433 |
| 100 | 0.6 | 491 | 451 | 442 | 450 |
| 120 | 0.5 | 537 | 479 | 457 | 485 |
What QTc is too long?
| QTc of the main formula | What the calculator shows |
|---|---|
| Below 450 ms (male) or 460 ms (female) | No warning |
| From 450 ms (male) or 460 ms (female) up to 500 ms | At or above the usual threshold for a prolonged QTc |
| Above 500 ms | Markedly higher risk of torsades de pointes |
The AHA/ACCF/HRS recommendations also describe a QTc of 390 ms or less as short; the calculator does not flag short values.
Common mistakes
- Including the U wave. The QT ends where the T wave returns to the baseline. A separate U wave is not part of it; measuring to its end makes the QT far too long.
- Using a heart rate from a different moment. It’s best to use the rate from the beats you measured, not the monitor ten minutes later.
- Entering the QT in seconds. The field expects milliseconds: 0.40 s is 400 ms.
- Comparing results from different formulas. A QTc of 470 ms with Bazett and 440 ms with Fridericia can come from the same ECG. It’s advisable to note the formula each time.
- Relying on Bazett at extreme rates. Above about 100/min it tends to raise false alarms; below about 60/min it can hide a long QT.
Limits
All four formulas are population averages, and none is accurate for every patient or every heart rate. A wide QRS (bundle branch block, paced rhythm) lengthens the QT without any change in repolarisation, so the QTc is overestimated. Irregular rhythms, flat or notched T waves and noisy tracings make the end of the T wave hard to place, and the result is only as good as that measurement.
A borderline QTc is best read together with the ECG, the medicines and the electrolytes: low potassium, magnesium or calcium all lengthen the QT. For calcium, the corrected calcium calculator adjusts a total calcium for albumin.
Frequently asked questions
Which QTc formula should I use?
Fridericia is a sound default, which is why the calculator starts with it. Bazett is the oldest and still the most widely printed, but it over-corrects at fast heart rates and under-corrects at slow ones. Fridericia and Framingham behave better away from 60 beats/min. Whatever your unit prefers, it’s best to use the same formula each time so serial ECGs are comparable.
What QTc counts as prolonged?
A QTc of about 450 ms or more in men and 460 ms or more in women, according to the AHA/ACCF/HRS recommendations. Values above 500 ms carry a clearly higher risk of torsades de pointes. The calculator flags the main result when it reaches the threshold for the selected sex and, separately, when it goes above 500 ms.
Why does Bazett give a much higher number when the heart is fast?
Because it divides the QT by the square root of the RR interval. As the RR shortens, the square root falls quickly and the corrected value climbs, often more than repolarisation really changes. Fridericia uses the cube root, a gentler correction, so it stays closer to the truth at fast rates.
Why does the calculator ask for sex?
Only to choose the warning threshold: 450 ms for males and 460 ms for females. The QTc values themselves are identical whichever sex you select. Women have a slightly longer QTc on average, which is why the guidelines set their limit 10 ms higher. The default selection is female.
Can I use the QTc calculator in atrial fibrillation?
Yes, but read the result as a rough estimate. With an irregular rhythm both the QT and the RR change from beat to beat. A common approach is to average the QT and the heart rate over several beats and correct the average. Even then, the result remains an approximation, so it’s worth noting the rhythm next to the number and comparing like with like on later ECGs.
Should I trust the QTc printed by the ECG machine?
It’s best used as a starting point, not the final word. The machine may use a different formula, often Bazett, and its automatic end-of-T detection can be wrong, especially with flat T waves or U waves. Measuring the QT by hand and recalculating tells you exactly what the number means.
Sources
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353–370
- Fridericia LS. The duration of systole in an electrocardiogram in normal humans and in patients with heart disease (1920, English translation). Ann Noninvasive Electrocardiol 2003;8:343–351
- Sagie A et al. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol 1992;70:797–801
- Hodges M, Salerno D, Erlien D. Bazett’s QT correction reviewed: evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol 1983;1:694 (abstract)
- Rautaharju PM et al. 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:e241–e250