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QTc Calculator

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Enter the QT interval and heart rate from an ECG to calculate the corrected QT interval (QTc) using three validated formulas.

QTc Calculator

Live
Bazett's formula460.1 ms
Fridericia's formula446.3 ms
Framingham's formula445.7 ms
Bazett tends to overcorrect above 90 bpm and undercorrect below 60 bpm - Fridericia and Framingham are generally considered more reliable outside the normal heart rate range. This is a calculation aid, not a diagnosis - clinical interpretation requires a qualified professional.

The formulas

RR interval (seconds) = 60 / Heart rate (bpm) Bazett: QTc = QT / sqrt(RR) Fridericia: QTc = QT / cube-root(RR) Framingham: QTc = QT + 154 x (1 - RR)
Example

QT of 420 ms at a heart rate of 72 bpm (RR = 0.833 sec): Bazett gives 460.1 ms, Fridericia gives 446.3 ms, Framingham gives 445.7 ms - all within the normal range for a heart rate near 72 bpm, where the three formulas tend to agree closely.

Step-by-step guide

  1. Measure the QT interval from the ECG - most standard paper runs at 25 mm/sec, so each small box represents 40 ms; count boxes from QRS onset to the end of the T wave and multiply by 40.
  2. Enter the heart rate shown on the monitor, or calculate it from the RR interval.
  3. Compare the three corrected values. If they diverge significantly, the heart rate is likely outside Bazett's reliable range (60-100 bpm), and Fridericia or Framingham should be weighted more heavily.

Why Bazett, the most-used formula, isn't always the most reliable one

Bazett's formula has been the clinical standard for nearly a century simply because it's the simplest to calculate, but it has a well-documented mathematical weakness: it systematically overcorrects at higher heart rates and undercorrects at lower ones. This means a patient with tachycardia (fast heart rate) can show a falsely elevated QTc under Bazett even when their true corrected interval is normal, while a patient with bradycardia (slow heart rate) can show a falsely reassuring QTc even when there's genuine prolongation. Fridericia's cube-root formula and the linear Framingham formula both correct more evenly across a wider range of heart rates, which is why many cardiology guidelines now recommend cross-checking Bazett against one of these, especially outside the 60-100 bpm range.

Common mistakes

Trusting Bazett alone at extreme heart rates - relying only on Bazett's formula for a patient with significant tachycardia or bradycardia can produce a meaningfully misleading QTc value.
Mixing up seconds and milliseconds when entering the QT interval - a QT interval is typically 350-450 ms (or 0.35-0.45 seconds); entering the wrong scale produces a meaningless result.

Frequently asked questions

What's considered a normal QTc value?

Commonly cited thresholds are under about 440-450 ms for men and 450-460 ms for women, with values over 500 ms considered significantly abnormal and associated with increased arrhythmia risk. Exact cutoffs vary somewhat by guideline and clinical context.

Why do the three formulas sometimes give noticeably different results?

They diverge most at heart rates far from 60 bpm, since Bazett's square-root correction behaves differently from Fridericia's cube-root and Framingham's linear approach. Near a resting heart rate of 60-80 bpm, all three tend to agree closely.

What can cause a short QTc instead of a prolonged one?

Values below about 340-350 ms can indicate short QT syndrome, a less common but genuine arrhythmia risk in its own right, unrelated to any structural heart problem, and can appear at any age.

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