Calculate QTcF, measured QT, or the implied heart rate and RR interval using Fridericia’s formula. Choose the value to solve for, then enter the required intervals with their units.
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QTcF Formula
The Fridericia formula corrects the QT interval for heart rate using the cube root of the RR interval:
Formula source: LITFL ECG Library – “QT Interval” (2024)
Variables:
- QTcF = Fridericia-corrected QT interval (ms)
- QT = measured QT interval (ms), from QRS onset to the end of the T wave
- RR = RR interval (seconds) = 60 / heart rate in bpm
QTc Interpretation Limits
The AHA/ACCF describes approximate adult 99th-percentile QTc values of 470 ms for men and 480 ms for women. These are method-dependent guides, not universal Fridericia-specific diagnostic limits. A single QTc result cannot diagnose long or short QT syndrome or establish individual safety.
| Classification | Men | Women |
|---|---|---|
| Approximate adult 99th-percentile QTc guide | 470 ms | 480 ms |
| Above the example guide | >470 ms | >480 ms |
| Marked QTc prolongation associated with increased TdP risk | >500 ms | >500 ms |
| At or below a guide | Does not establish safety | Does not establish safety |
| Very short QTc | Clinical assessment required | Clinical assessment required |
QTc above 500 ms is associated with increased torsades de pointes (TdP) risk, but no QTc threshold makes TdP certain. ICH E14’s 10 ms criterion concerns a confidence bound on the largest time-matched mean drug–placebo QTc effect in a thorough QT study; it is not a mandatory-reporting rule triggered by one person’s change.
Why Fridericia over Bazett?
Four commonly used correction formulas are shown below. All are population models, and their performance depends on heart rate, rhythm and the setting. Use QT and QTc in milliseconds and RR in seconds in every equation shown here.
| Formula | Equation | Known Bias | Primary Use |
|---|---|---|---|
| Bazett (QTcB) | QT / √RR | Overcorrects at HR >100 bpm; undercorrects at HR <60 bpm | Historical clinical default |
| Fridericia (QTcF) | QT / RR^(1/3) | May remain rate-dependent at extremes | FDA TQT studies; oncology trials |
| Framingham (QTcFra) | QT + 154(1-RR) | Linear population correction; context dependent | Drug-effect studies |
| Hodges (QTcH) | QT + 1.75(HR-60) | Linear population correction; context dependent | Clinical research |
ICH E14 discusses choosing and evaluating a suitable correction; its 2022 questions and answers describe Fridericia as appropriate in most situations while allowing other methods when justified. There is no universal FDA mandate to use QTcF or a universal 414 ms QTcF equivalent to 460 ms Bazett. Compare results only with their correction method and clinical or study context. Inverse solving uses QT = QTcF × RR^(1/3), RR = (QT/QTcF)^3 and HR = 60/RR; it does not measure a patient’s heart rate.
Drug-Induced QT Prolongation
Many medicines can prolong QT. One important mechanism is inhibition of the hERG-mediated rapid delayed-rectifier potassium current (I_Kr), which can delay ventricular repolarization. Mechanisms and torsades risk vary by medicine and clinical context.
| Drug Class | Examples | Risk interpretation |
|---|---|---|
| Class Ia antiarrhythmics | Quinidine, disopyramide, procainamide | Agent, exposure and patient factors matter |
| Class III antiarrhythmics | Sotalol, dofetilide, ibutilide | Agent, exposure and patient factors matter |
| Fluoroquinolones | Moxifloxacin, levofloxacin | Review drug-specific labeling and risk factors |
| Macrolides | Erythromycin, azithromycin | Review drug-specific labeling and risk factors |
| Antipsychotics | Haloperidol, ziprasidone, thioridazine | Review drug-specific labeling and risk factors |
| Antiemetics | Ondansetron, domperidone | Review drug-specific labeling and risk factors |
| Opioids | Methadone | Review drug-specific labeling and risk factors |
Factors associated with TdP include low potassium or magnesium, bradycardia, female sex, older age, heart disease, congenital long QT syndrome and exposure to interacting medicines. Drug-specific metabolic or renal-elimination interactions can increase exposure; consult current labeling and a clinician or pharmacist. Do not change medication based on this calculator.
