QTc Calculator (Corrected QT Interval)
Correct a measured QT interval for heart rate using the Bazett and Fridericia formulas. The QT interval on an ECG shortens as the heart speeds up, so it is corrected (QTc) before it can be compared against normal limits. Enter the QT interval in milliseconds and the heart rate in beats per minute.
The QTc formulas
With the RR interval in seconds (RR = 60 ÷ heart rate): Bazett QTc = QT ÷ √RR; Fridericia QTc = QT ÷ RR1/3. QT and QTc are in milliseconds. Both equal the raw QT when the heart rate is exactly 60 bpm (RR = 1 s).
Frequently asked questions
What is the QTc formula?
The corrected QT interval adjusts the measured QT for heart rate using the RR interval (RR in seconds = 60 ÷ heart rate). Bazett: QTc = QT ÷ √RR. Fridericia: QTc = QT ÷ ∛RR. For a QT of 400 ms at a heart rate of 60 bpm, RR = 1 s and both give QTc = 400 ms. At faster rates the two formulas diverge — Bazett tends to over-correct at high heart rates and under-correct at low ones, which is why Fridericia is often preferred.
What is a normal QTc?
A QTc up to about 450 ms in men and 460–470 ms in women is generally considered normal. A QTc above 450 ms (men) or 470 ms (women) is prolonged, and a value at or above 500 ms carries a markedly increased risk of the dangerous arrhythmia torsades de pointes. A short QTc below about 350 ms can also be abnormal.
Bazett or Fridericia — which should I use?
Bazett is the historical standard and is what most ECG machines print, but it corrects poorly at heart rates far from 60 bpm. Fridericia (and other formulas such as Framingham and Hodges) correct more reliably across a range of rates and are preferred in many drug-safety and research settings. This calculator shows both so you can compare.
How is the QT interval measured?
QT is measured on the ECG from the start of the Q wave to the end of the T wave, ideally in lead II or V5–V6 and averaged over several beats. Measurement is easiest at steadier heart rates; atrial fibrillation, wide QRS complexes and prominent U waves all make it less reliable. The usual convention for the end of the T wave is the tangent method: draw a tangent to the steepest part of the T wave downslope and take where it crosses the baseline, excluding any following U wave. Read the heart rate from the same beats you measured, because the correction is only as good as the rate paired with it — this calculator takes the QT in milliseconds and the heart rate in beats per minute and reports Bazett and Fridericia side by side.
Is this QTc estimate a diagnosis?
A QTc value on its own is not a diagnosis. Its interpretation depends on the quality of the ECG, the patient’s medications, electrolytes and clinical picture, and on which correction formula is used. This calculator is an educational reference — an abnormal or borderline value should always be reviewed by a qualified clinician.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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