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QTc Calculator (Corrected QT Interval)

Correct a measured QT interval for heart rate using Bazett's and Fridericia's formulas, and see where the result falls against commonly cited ECG reference ranges.

QTc (Bazett)
447 ms
QTc (Fridericia)
431 ms
RR interval
0.8 s
Where this falls (Bazett)
Borderline

Where this falls

447.2 falls in the "Borderline" range.

How it works

The QT interval on an ECG — from the start of the Q wave to the end of the T wave — shortens as heart rate rises, purely as a mechanical side effect of faster beats, so it has to be rate-corrected before comparing it against a reference range. Both corrections below work from the RR interval in seconds, RR = 60 ÷ heart rate (the time between consecutive heartbeats).

Bazett's formula, QTc = QT ÷ √RR, is the version most widely used in clinical practice and most ECG machines, despite a well-documented tendency to over-correct (push the result too high) at fast heart rates and under-correct at slow ones. Fridericia's formula, QTc = QT ÷ RR^(1/3), is the commonly cited refinement that corrects for Bazett's rate-dependence more accurately across a wider range of heart rates — this calculator shows both side by side so the gap between them is visible, especially at heart rates far from 60 bpm.

The banding in the result (Normal below 440ms, Borderline 440–460ms, Prolonged 460–500ms, Significantly prolonged above 500ms) reflects widely taught general ECG reference points, evaluated against the Bazett figure since that's the number most commonly quoted in practice — not a patient-specific diagnostic threshold, since actual clinical cutoffs vary slightly by sex, lab, and guideline.

FAQ

Why do Bazett's and Fridericia's formulas give different answers?

Both divide the measured QT by a power of the RR interval, but Bazett uses the square root (RR^0.5) while Fridericia uses the cube root (RR^(1/3)) — a smaller exponent that corrects less aggressively. At exactly 60 bpm (RR = 1 second) the two formulas agree exactly, since any power of 1 is 1; the gap between them grows the further the heart rate is from 60 bpm.

Is this a diagnostic tool?

No — it's the two standard textbook formulas applied to whatever QT interval and heart rate you enter, useful for understanding how rate correction works. Reading an actual ECG, picking the correct QT interval off a real tracing, and interpreting a prolonged result clinically all require a trained clinician looking at the full tracing and the patient's history, not a standalone calculator.

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