QTc Calculator

Free QTc calculator using Bazett, Fridericia, Framingham, Hodges & Rautaharju formulas. Enter heart rate and QT interval for instant corrected QT with gender-specific normal ranges, visual risk scale, and side-by-side formula comparison.

bpm
ms

Most widely used. Simple square-root correction. Over-corrects at high heart rates and under-corrects at low heart rates.

Corrected QT Interval (Bazett)

438.2ms
Borderline

Heart Rate

72 bpm

QT Interval

400 ms

RR Interval

833 ms

QTc range (male)438.2 ms
200340430450500700

Borderline

QTc is borderline prolonged for males (430–450 ms). Consider monitoring if the patient is on QT-prolonging medications.

Compare All Formulas

Side-by-side QTc results from all five correction formulas

BazettSelected

Borderline

438.2

ms

Fridericia

Normal

425.1

ms

Framingham

Normal

425.7

ms

Hodges

Normal

421.0

ms

Rautaharju

Normal

426.7

ms

At a normal resting heart rate (60–100 bpm), all formulas are comparable. Bazett is the most commonly reported in clinical practice.

This calculator provides estimates for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for interpretation of your ECG results.

What is QTc (Corrected QT Interval)?

Understanding QT correction and its clinical significance

The QT interval on an ECG measures the total time for ventricular depolarization and repolarization. Because it naturally varies with heart rate, a corrected QT (QTc) is needed to compare values across different heart rates.

Adjusts QT for heart rate
Essential for drug monitoring
Detects arrhythmia risk

Why does QTc matter?

A prolonged QTc increases the risk of torsades de pointes — a potentially fatal ventricular arrhythmia. QTc monitoring is essential when prescribing QT-prolonging medications, evaluating unexplained syncope, or screening for Long QT Syndrome.

QTc Correction Formulas

Five formulas with different strengths depending on heart rate

Bazett (1920)

Most Common
QTc = QT / √RR
Best for: HR 60–100 bpm

Over-corrects at high HR, under-corrects at low HR

Fridericia (1920)

FDA Preferred
QTc = QT / RR^(1/3)
Best for: All heart rates

Recommended for drug safety (TQT) studies

Framingham (1992)

Linear
QTc = QT + 0.154 × (1000 − RR_ms)
Best for: Wide HR range

Derived from the Framingham Heart Study

Hodges (1983)

HR-Based
QTc = QT + 1.75 × (HR − 60)
Best for: Tachycardia

Linear correction using heart rate directly

Rautaharju (2014)

Ratio
QTc = QT × (120 + HR) / 180
Best for: General clinical use

Simple ratio-based correction

At normal resting heart rates (60–100 bpm), all formulas produce similar results. Fridericia is preferred by the FDA for drug studies and performs best outside that range.

QTc Normal Ranges

Gender-specific thresholds based on ACC/AHA/HRS and ESC guidelines

Category
Males
Females
Normal
< 430
< 450
Borderline
430–450
450–470
Prolonged
450–500
470–500
Significantly Prolonged
> 500
> 500
Short QT
< 340
< 340

QTc > 500 ms carries an especially high risk of torsades de pointes. Per ESC 2022 guidelines, QTc ≥ 480 ms warrants evaluation for Long QT Syndrome.

Common Causes of QT Prolongation

Medications, electrolytes, and conditions that affect QTc

Medications

  • Antiarrhythmics — amiodarone, sotalol, dofetilide
  • Antibiotics — azithromycin, fluoroquinolones
  • Antipsychotics — haloperidol, ziprasidone
  • Antidepressants — citalopram (high doses)
  • Antiemetics — ondansetron, domperidone

Electrolytes

  • Hypokalemia (low potassium) — most common
  • Hypomagnesemia (low magnesium)
  • Hypocalcemia (low calcium)

Cardiac

  • Congenital Long QT Syndrome (LQTS)
  • Heart failure, myocardial infarction
  • Bradycardia (can unmask prolongation)

Other

  • Hypothyroidism
  • Intracranial pathology (SAH, stroke)
  • Multiple QT-prolonging drugs combined

Frequently Asked Questions

Common questions about QTc and ECG interpretation

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