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 depolarisation and repolarisation. The problem is that the QT shortens as the heart rate rises, so a raw QT number cannot be compared between two heart rates — a QT of 400 ms means something different at 60 beats a minute than it does at 120. The correction is the QTc: the QT interval corrected for heart rate. Two formulas do the correcting, Bazett’s and Fridericia’s, and they agree at 60 beats a minute and diverge everywhere else. This guide shows both, works the numbers at every heart rate, and says when to use each.

What the QTc is

The QT interval is measured from the beginning of the QRS complex to the end of the T wave, ideally in lead II or V5, where the T wave is clearest. It is the time the ventricles take to depolarise and repolarise — to contract and then to reset for the next beat. Because the reset is faster when the heart beats faster, the QT shortens as the heart rate rises: the same heart has a shorter QT at 120 beats a minute than at 60.

The QTc removes that rate dependence. It answers the question: what would this QT be if the heart were beating at 60 beats a minute? The correction divides the QT by a function of the RR interval — the time between two beats, in seconds — and the two formulas differ in which function they use. The QTc is educational information, not a diagnosis: a QTc value must be interpreted by a qualified clinician using the full ECG and the clinical context.

Bazett: divide by the square root of RR

Bazett’s formula is the older and the most widely reported: QTc = QT ÷ √RR, with QT in milliseconds and RR in seconds. At 60 beats a minute the RR interval is exactly 1 second, the square root of 1 is 1, and the QTc equals the QT — a QT of 400 ms is a QTc of 400 ms. That is the reference point the whole correction is built around.

Move away from 60 and the correction bites. At 120 beats a minute the RR interval is 0.5 seconds, the square root of 0.5 is 0.707, and a QT of 400 ms gives a QTc of 400 ÷ 0.707, which is 566 ms. At 50 beats a minute the RR interval is 1.2 seconds, the square root of 1.2 is 1.095, and a QT of 440 ms gives a QTc of 440 ÷ 1.095, which is 402 ms. Bazett’s weakness is that it overcorrects: at fast rates it pushes the QTc too high, and at slow rates it pushes it too low.

Fridericia: divide by the cube root of RR

Fridericia’s formula uses the cube root instead of the square root: QTc = QT ÷ ∛RR, with the same units. The cube root changes more slowly than the square root, so the correction is gentler away from 60 beats a minute — and the formula holds up better at fast and slow rates, which is why it is preferred in drug-safety work, where the heart rate is often far from 60.

The same two examples show the difference. At 120 beats a minute, a QT of 400 ms gives a QTc of 400 ÷ ∛0.5, which is 400 ÷ 0.794, or 504 ms — 62 ms lower than Bazett’s 566 ms. At 50 beats a minute, a QT of 440 ms gives 440 ÷ ∛1.2, which is 440 ÷ 1.063, or 414 ms — 12 ms higher than Bazett’s 402 ms. The gap between the formulas is the story of this guide, and the table below shows it at every heart rate.

Side by side: QT 400 ms at every heart rate

Hold the QT fixed at 400 ms and vary the heart rate, and the two formulas trace two different curves. They meet at 60 beats a minute, where both give 400 ms, and they separate in both directions — Bazett rising faster above 60 and falling faster below it.

QTc for a QT of 400 ms at each heart rate. RR = 60 ÷ heart rate, in seconds.

The clinical consequence is in the last two rows. At 100 beats a minute, Bazett calls a 400 ms QT prolonged at 516 ms while Fridericia calls it 474 ms; at 120 beats a minute, Bazett says 566 ms and Fridericia says 504 ms. The same tracing, the same QT, and a 62 ms difference in the corrected value — enough to cross a limit. That is why the choice of formula matters, and why reporting both, when in doubt, is the safer course.

Framingham and Hodges: the other two

Two more formulas are in common use, and both avoid the root entirely. Framingham’s formula adds a linear correction: QTc = QT + 154 × (1 − RR), with QT in milliseconds and RR in seconds. Hodges’ formula corrects by heart rate directly: QTc = QT + 1.75 × (HR − 60), with QT in milliseconds and HR in beats a minute. At 60 beats a minute both give the QT unchanged, as they should.

At 120 beats a minute with a QT of 400 ms, Framingham gives 400 + 154 × 0.5, which is 477 ms, and Hodges gives 400 + 1.75 × 60, which is 505 ms — close to Fridericia’s 504 ms, and well below Bazett’s 566 ms. Both perform better than Bazett at high heart rates, which is why the calculator offers all four: the spread between them is itself information about how much the correction can be trusted at that rate.

When to use each

Near 60 beats a minute, it hardly matters: all four formulas agree within a few milliseconds, and Bazett is the right choice because it is the most widely reported and the one other clinicians will expect. Between 60 and 100, Bazett is still defensible, but Fridericia is the safer default. Above 100 or below 50, Fridericia is preferred, because Bazett overcorrects at both extremes — and in drug-safety work, where the question is whether a drug prolongs the QT, Fridericia is the standard, because a formula that overcorrects at fast rates would cry wolf.

When in doubt, report both, and let the clinician judge. A QTc that is prolonged by both formulas is prolonged; a QTc that is prolonged by Bazett alone, at a fast heart rate, is a reason to re-measure and to check the Fridericia value before acting on it. The normal QTc range, and the special cases where the standard reading needs adjusting, are covered in the companion guide.

Mistakes that change the answer

First, the units: RR must be in seconds, not milliseconds. A QT of 400 ms with an RR of 500 ms gives a Bazett QTc of 400 ÷ √500, which is nonsense; with RR in seconds, 0.5, it gives 566 ms. Second, the wrong root: dividing by the square root when you meant the cube root, or the reverse, moves the answer by tens of milliseconds at fast rates. Third, a single beat in atrial fibrillation, where the RR varies beat to beat — average several beats instead. Fourth, including the U wave in the QT, which lengthens it falsely: the QT ends where the T wave ends, and the U wave, when present, is excluded unless the T and U waves are fused.

Fifth, trusting the machine’s number without looking. Automated QT measurements are a starting point, and the end of the T wave is the hardest part of the tracing for an algorithm to find. Measure by hand — the tangent method, in lead II or V5 — and compare with the machine before believing either one.

Frequently asked questions

How do you calculate QTc with Bazett’s formula?

Divide the QT interval by the square root of the RR interval: QTc = QT ÷ √RR, with QT in milliseconds and RR in seconds. At 120 beats a minute, RR is 0.5 seconds and √0.5 is 0.707, so a QT of 400 ms gives a QTc of 566 ms. At 60 beats a minute RR is 1 second, so the QTc equals the QT.

What is the QTc calculation formula?

There are four in common use. Bazett: QTc = QT ÷ √RR. Fridericia: QTc = QT ÷ ∛RR. Framingham: QTc = QT + 154 × (1 − RR). Hodges: QTc = QT + 1.75 × (HR − 60). In all of them QT is in milliseconds, RR is in seconds, and HR is in beats per minute. They agree at 60 beats a minute and diverge away from it.

Is the QT interval the same as the QTc?

No. The QT interval is the raw measurement from the ECG — the time from the start of the Q wave to the end of the T wave, in milliseconds. The QTc is the QT corrected for heart rate, answering what the QT would be at 60 beats a minute. The QT shortens as the heart rate rises, so the QTc is the number that can be compared between heart rates.

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. The cube root changes more slowly than the square root, so Fridericia overcorrects less at fast and slow rates.

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. The calculator shows all four formulas so the spread between them is visible.

How do I calculate QTc manually on an ECG?

Measure the QT interval by hand — from the start of the QRS complex to the end of the T wave, using the tangent method in lead II or V5 — and measure the RR interval between two beats in seconds. Then apply the formula: divide the QT by the square root of RR for Bazett, or by the cube root for Fridericia. Average three to five beats, and exclude the U wave unless it is fused with the T wave.

Formulas as in the AHA/ACCF/HRS ECG standardisation statement (Circulation 2009) and Vandenberk et al. (Journal of the American Heart Association 2016). Educational information only, not medical advice.