Cardiology

QTc Interval Calculator

Calculate corrected QT interval using all 4 validated formulas simultaneously: Bazett, Fridericia, Framingham, and Hodges. Instant drug-induced QT prolongation risk classification, sex-specific normal ranges, and TdP risk assessment.

4 formulas simultaneously Sex-specific thresholds PDF export
QTc Calculator
ms
Measure from start of QRS to end of T-wave. Use the longest QT (usually V5-V6). Enter a valid QT interval (200-700 ms).
bpm
Enter a valid heart rate (20-300 bpm).
QTc Results
-
ms - QTc (Bazett, primary clinical reference)

All four formula results:

Bazett Most Used
-
ms
QTc = QT ÷ √RR
Fridericia EMA Preferred
-
ms
QTc = QT ÷ ∛RR
Framingham
-
ms
QTc = QT + 154×(1-RR)
Hodges
-
ms
QTc = QT + 1.75×(HR-60)

Normal QTc ranges (Bazett):

Male - Normal
< 440 ms
Female - Normal
< 460 ms
Borderline (both)
440-500 ms
High TdP risk
> 500 ms

QTc interval correction

The QT interval shortens at faster heart rates and lengthens when the heart slows. QTc corrects for that, letting you compare intervals across different rates. Four formulas are widely used. None is perfect, and they diverge most at extreme heart rates. Because QTc is also sensitive to serum electrolytes, converting lab values reported in different units (our Lab Values Converter handles potassium and magnesium in mmol/L vs mEq/L) matters just as much as picking the right formula.

Which formula to use

Bazett is everywhere. Drug labels quote it, hospital systems use it, and most clinicians know the thresholds. Its weakness is overcorrection at fast rates and undercorrection below 60 bpm. At normal heart rates (60-100 bpm) the difference between formulas is clinically negligible.

The European Medicines Agency's ICH E14 guideline recommends Fridericia for formal drug safety studies. If you're evaluating drug-induced QT prolongation in a tachycardic patient, Fridericia gives a more reliable number.

Common QT-prolonging drugs

Antipsychotics (haloperidol, quetiapine), antidepressants (citalopram, tricyclics), antibiotics (azithromycin, moxifloxacin), antiemetics (ondansetron, domperidone), and antiarrhythmics (amiodarone, sotalol) all carry QT risk. Check CredibleMeds for drug-specific risk classification. Hypokalaemia and hypomagnesaemia compound the risk significantly.

Renal function and QT-prolonging drugs

Several QT-prolonging drugs (sotalol, dofetilide, and some macrolides) are renally cleared and accumulate in kidney impairment, increasing QTc risk independent of the drug's baseline QT effect. Checking Creatinine Clearance before starting or renewing one of these agents is a reasonable safeguard alongside a baseline ECG.

Frequently Asked Questions

Why do the four formulas give different QTc values?+
Each formula corrects for heart rate differently. Bazett divides by the square root of the RR interval and overcorrects at fast heart rates; Fridericia uses a cube root and is more stable across a wider range; Framingham and Hodges use linear adjustments instead of a division. At a normal resting heart rate (60-100 bpm) the four values are usually close together. They diverge most at heart rates below 60 or above 100 bpm.
What should I do if the QTc is prolonged?+
A QTc of 440-500 ms (men) or above the female threshold up to 500 ms is borderline: review QT-prolonging medications, correct any electrolyte abnormalities, and repeat the ECG. A QTc above 500 ms carries a substantially higher risk of torsades de pointes; consider stopping the offending drug, correcting electrolytes urgently, arranging continuous cardiac monitoring, and getting an urgent specialist review. This calculator is a reference tool, not a substitute for clinical judgement.
Do low potassium and magnesium really affect QTc?+
Yes, significantly. Hypokalaemia and hypomagnesaemia both prolong the QT interval and increase the risk of torsades de pointes independent of any QT-prolonging drug the person may be taking. Electrolyte correction is one of the first steps recommended for any borderline or prolonged QTc, alongside reviewing the medication list.

Related tools

For 10-year CVD risk, see Framingham Risk Score. For ACS risk in chest pain, see HEART Score.

References

  • Bazett HC. "An analysis of the time-relations of electrocardiograms." Heart. 1920;7:353-370.
  • Fridericia LS. "Die Systolendauer im Elektrokardiogramm." Acta Med Scand. 1920;53:469-486.
  • European Medicines Agency. ICH E14 Guideline: QT/QTc interval prolongation. EMA, 2005.
Clinical note: QTc calculation is a reference tool. Always interpret in full clinical context, including medications, electrolytes, and clinical status.