QTc Calculator

QTc Calculator for Cardiology. Report QTc against sex-specific thresholds. Normal is roughly up to 450 ms in men and 460 ms in women; 450-470 (men) / 460-480 (women) is borderline. QTc at or above 480 ms is prolonged and supports a long QT syndrome diagnosis, and any QTc at or above 500 ms marks a high absolute risk of torsades de pointes and should trigger review of offending drugs, correction of potassium and magnesium, and closer monitoring. A drug-related increase of 60 ms or more from baseline is itself a warning sign even if the absolute value is not extreme. At the low end, QTc under about 340-350 ms raises the possibility of short QT syndrome.

How this calculator works

The tool corrects the measured QT interval for heart rate so repolarization can be compared across different rates. It accepts the QT (ms) and either the RR interval or heart rate, converts to RR in seconds (RR = 60/HR), and applies the chosen correction. Bazett divides QT by the square root of RR; Fridericia divides by the cube root of RR; Framingham (Sagie) adds a linear term, QTc = QT + 154 x (1 - RR); and Hodges adds 1.75 x (HR - 60) ms. All express QTc as the QT expected at a heart rate of 60 bpm.

When to use this calculator

Use it whenever QT must be judged against a rate, before and during QT-prolonging drugs (e.g. sotalol, dofetilide, methadone, antipsychotics, ondansetron, azithromycin), in suspected congenital long or short QT syndrome, in electrolyte disturbance, and in unexplained syncope. Bazett remains the reporting default but overcorrects at fast rates and undercorrects when bradycardic; Fridericia or Framingham are preferred outside roughly 60-100 bpm. All rate-correction formulas are unreliable in atrial fibrillation, marked tachycardia, and wide QRS (bundle branch block, paced or ventricular rhythms), where JT-based or QRS-adjusted approaches are needed instead.

Inputs used

  • Measured QT interval
  • Heart rate or RR interval
  • Correction formula

Clinical interpretation

Report QTc against sex-specific thresholds. Normal is roughly up to 450 ms in men and 460 ms in women; 450-470 (men) / 460-480 (women) is borderline. QTc at or above 480 ms is prolonged and supports a long QT syndrome diagnosis, and any QTc at or above 500 ms marks a high absolute risk of torsades de pointes and should trigger review of offending drugs, correction of potassium and magnesium, and closer monitoring. A drug-related increase of 60 ms or more from baseline is itself a warning sign even if the absolute value is not extreme. At the low end, QTc under about 340-350 ms raises the possibility of short QT syndrome.

Worked example

A patient has a measured QT of 440 ms at a heart rate of 100 bpm (RR = 0.60 s). Bazett: 440 / sqrt(0.60) = 568 ms. Fridericia: 440 / (0.60)^(1/3) = 522 ms. Framingham: 440 + 154 x (1 - 0.60) = 502 ms. The 66 ms spread between Bazett and Framingham illustrates Bazett's overcorrection at tachycardia: Bazett flags critical prolongation (>500 ms) whereas Fridericia and Framingham place the value in a lower risk band, which can change whether a QT-prolonging drug is stopped.

Limitations and safety notes

Bazett systematically overestimates QTc at heart rates above 100 bpm (producing false positives) and underestimates it during bradycardia, so a single Bazett value near a threshold should be re-checked with Fridericia or Framingham. None of these formulas correct reliably in atrial fibrillation, where beat-to-beat RR variation demands averaging over multiple cycles, nor with wide QRS, where the prolonged depolarization inflates QT independent of repolarization. Automated QT measurements can misplace the T-wave end (worsened by prominent U waves or low-amplitude T waves), so borderline or high values should be confirmed by manual measurement using the tangent method in lead II or V5.

Frequently asked questions

Which formula should I use?

Bazett is the historical default and what most ECG machines print, but it is least accurate away from 60 bpm. For rates outside roughly 60-100 bpm, Fridericia or Framingham track true rate-independence better and are preferred in drug-safety and research settings. Report which formula was used, since the same QT can cross a threshold under Bazett but not under Fridericia.

How do I measure QT correctly before entering it?

Measure from the start of the QRS to the end of the T wave, ideally in lead II or V5-V6, using the tangent method where a line along the steepest downslope of the T wave meets the baseline. Exclude U waves. Average over several beats, especially in atrial fibrillation.

What QTc counts as dangerous?

A QTc at or above 500 ms, or a rise of 60 ms or more from the patient's baseline, marks substantially increased torsades risk and warrants stopping culprit drugs, correcting potassium and magnesium, and monitoring. Absolute risk also depends on bradycardia, female sex, structural heart disease, and concurrent QT-prolonging agents.

Why does my Bazett QTc look alarmingly high at fast heart rates?

Bazett overcorrects at tachycardia, so a tachycardic patient can show a spuriously prolonged Bazett QTc. Recompute with Fridericia or Framingham; if those are normal and the QT normalizes once the rate slows, the Bazett value was a rate artifact rather than true repolarization prolongation.

References

  • Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992. PMID: 1519533.
  • Rautaharju PM, Surawicz B, Gettes LS, 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: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. Circulation. 2009. PMID: 19228821.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 13, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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