QTc Calculator

Compute corrected QT interval (QTc) using four validated formulas — Bazett, Fridericia, Framingham, and Hodges.Instantly interpret results, assess arrhythmia risk, and visualize your patient's QTc against clinical thresholds.

Measured from QRS onset to T-wave end. Typical range 300–460 ms.
Resting heart rate from ECG. Typical range 50–100 bpm.
Used for refined interpretation thresholds.
? Normal sinus: QT 400, HR 60
⚡ Tachycardia: QT 320, HR 110
? Bradycardia: QT 480, HR 45
⚠ Long QT: QT 520, HR 70
? Drug-induced: QT 480, HR 85
? Athlete: QT 440, HR 52
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What Is the Corrected QT Interval (QTc)?

The QT interval on the electrocardiogram (ECG) represents the total duration of ventricular depolarization and repolarization. Because the QT interval varies inversely with heart rate — shorter at faster rates and longer at slower rates — the corrected QT interval (QTc) adjusts the measured QT to a standard heart rate (usually 60 bpm), enabling meaningful comparison across patients and clinical contexts.

Prolonged QTc is a well-established risk marker for torsade de pointes (TdP), a potentially fatal polymorphic ventricular tachycardia. Accurate QTc calculation is therefore essential in drug safety monitoring, electrolyte management, genetic arrhythmia syndromes (e.g., Long QT Syndrome, Brugada), and preoperative risk assessment.

QTc = f( QT, RR )   where   RR = 60 / HR

Four validated correction formulas are implemented in this calculator.

Why Use a Multi‑Formula QTc Calculator?

  • Clinical accuracy: No single formula performs optimally across all heart rates. Bazett over-corrects at high rates and under-corrects at low rates; Fridericia is more rate‑robust. Comparing multiple formulas gives a more complete picture.
  • Drug safety: Regulatory agencies (FDA, EMA) require QTc assessment for new drugs. Our tool helps clinicians and researchers quickly evaluate QT prolongation risk.
  • Education & training: Medical students and residents can explore how different formulas behave across heart rates and understand the rationale behind guideline recommendations.
  • Research: Epidemiologists and clinical investigators can use the tool for rapid QTc calculation in cohort studies or chart reviews.

The Four Correction Formulas Explained

Each formula was derived from a different population and has distinct strengths and limitations. Understanding their origins helps clinicians choose the most appropriate correction for a given patient.

Bazett (1920)
QTc = QT / √(RR)
The most widely used formula historically, but known to over-correct at heart rates > 80 bpm and under-correct at rates < 60 bpm. Derived from a small cohort of healthy young men.
Fridericia (1920)
QTc = QT / (RR)1/3
Uses cube‑root correction, which is more rate‑independent than Bazett across a wider HR range. Recommended by the European Society of Cardiology (ESC) and preferred in many electrophysiology labs.
Framingham (1992)
QTc = QT + 154 × (1 − RR)
Derived from the Framingham Heart Study cohort (n ≈ 5,000). Uses linear regression on the logarithm of QT. Performs well in general populations and is less affected by extreme heart rates.
Hodges (1983)
QTc = QT + 1.75 × (HR − 60)
A simple linear correction that uses heart rate directly rather than RR interval. Validated in emergency department settings and useful for rapid bedside calculation.

Clinical Interpretation & Thresholds

The following thresholds are widely used in clinical practice, based on the 2009 AHA/ACCF/HRS Recommendations and the 2015 ESC Guidelines for the management of ventricular arrhythmias. Gender‑specific cutoffs are applied when gender is provided.

Category Male (ms) Female (ms) Unspecified (ms) Clinical implication
Normal < 430 < 450 < 440 No increased risk; routine follow‑up.
Borderline 430 – 450 450 – 470 440 – 460 Consider further evaluation; review medications and electrolytes.
Prolonged > 450 > 470 > 460 Increased risk of TdP; urgent review of QT‑prolonging drugs, serum K⁺/Mg²⁺, and ECG monitoring.
Case Study: Drug‑Induced QT Prolongation

A 68‑year‑old female with atrial fibrillation is started on sotalol (80 mg twice daily). Her baseline ECG shows QT = 420 ms, HR = 75 bpm (RR = 0.80 s). Her QTc by Fridericia is 452 ms — borderline. After 3 days, she develops bradycardia (HR = 52 bpm) and a prolonged QT of 500 ms. Her QTc Fridericia is now 491 ms (prolonged), indicating increased torsade risk. Sotalol is discontinued, potassium is repleted, and the QTc normalizes to 440 ms. This case highlights the importance of serial QTc monitoring during drug initiation, especially in patients with bradycardia or electrolyte disturbances.

Reference: Roden DM. Drug‑induced prolongation of the QT interval. N Engl J Med 2004;350:1013‑1022.

Risk Factors for QTc Prolongation

QTc prolongation is multifactorial. The most common modifiable and non‑modifiable risk factors include:

  • Medications: Antiarrhythmics (amiodarone, sotalol, dofetilide), antibiotics (macrolides, fluoroquinolones), antipsychotics (haloperidol, risperidone), antidepressants (citalopram), and many others.
  • Electrolyte imbalances: Hypokalemia, hypomagnesemia, hypocalcemia.
  • Bradycardia: Prolonged RR interval increases the uncorrected QT and may unmask QT prolongation.
  • Structural heart disease: Ischemia, cardiomyopathy, myocarditis.
  • Genetic syndromes: Congenital long QT syndrome (LQTS types 1–3, etc.), Brugada syndrome, Andersen‑Tawil syndrome.
  • Autonomic influences: Acute stress, hypoglycemia, fever.

Common QT‑Prolonging Drugs

The CredibleMeds database (Arizona CERT) maintains an up‑to‑date list of drugs with known QT risk. Below are selected examples across therapeutic classes:

  • Amiodarone
  • Sotalol
  • Dofetilide
  • Ibutilide
  • Quinidine
  • Procainamide
  • Azithromycin
  • Clarithromycin
  • Erythromycin
  • Levofloxacin
  • Ciprofloxacin
  • Moxifloxacin
  • Haloperidol
  • Risperidone
  • Quetiapine
  • Citalopram
  • Escitalopram
  • Venlafaxine
  • Methadone
  • Ondansetron
  • Domperidone
  • Cisapride

This list is not exhaustive. Always consult a drug reference and the CredibleMeds database for comprehensive information.

Limitations & Clinical Considerations

  • Formula choice matters: At heart rates > 100 bpm, Bazett yields higher QTc values than Fridericia, potentially leading to false‑positive prolongation. The ESC recommends Fridericia for drug‑induced QT assessment.
  • ECG measurement accuracy: The QT interval should be measured in lead II or V5–V6, averaged over 3–5 beats, with clear T‑wave end definition. Automated algorithms may misread the T‑wave offset.
  • Population‑specific norms: Thresholds vary by age, sex, and ethnicity. The values in this tool are derived from adult guidelines; pediatric norms differ.
  • Not a substitute for clinical judgement: QTc is one piece of the puzzle. Always correlate with symptoms (syncope, palpitations), family history, and other ECG findings.

Guidelines & Recommendations

  • ACC/AHA/HRS (2009): Recommended QTc thresholds for drug‑induced QT prolongation: > 500 ms is a clear warning sign; an increase of > 60 ms from baseline is also significant.
  • ESC Guidelines (2015, 2022): Prefer Fridericia or Framingham correction over Bazett. Recommend QTc < 440 ms in men and < 460 ms in women as normal.
  • FDA/EMA regulatory guidance (ICH E14): The “thorough QT/QTc study” is mandatory for new drugs. A QTc effect of > 5 ms with upper confidence bound > 10 ms triggers further investigation.

Frequently Asked Questions

The European Society of Cardiology (ESC) recommends Fridericia for drug‑induced QT monitoring because it is more rate‑independent than Bazett. For general population screening, the Framingham formula has strong epidemiological validation. Many centers use multiple formulas and rely on the clinical context rather than a single number. This tool provides all four to support informed decision‑making.

Normal QTc is generally < 440 ms in men and < 460 ms in women. However, many experts use a stricter threshold of < 430 ms for men and < 450 ms for women. The tool uses gender‑specific cutoffs when available. Values above 500 ms are considered high risk for torsade de pointes.

Each formula uses a different mathematical relationship between QT and RR interval. Bazett assumes a square‑root dependency, Fridericia a cube‑root, Framingham uses linear regression, and Hodges uses a simple rate‑based linear correction. These differences reflect the populations and statistical methods used in their derivation. The disparity is most pronounced at extreme heart rates.

The thresholds in this tool are based on adult guidelines. Pediatric QTc norms are age‑dependent and generally higher in infants and young children. For pediatric use, consult specialized nomograms (e.g., Schwartz score for LQTS) and pediatric cardiology references.

The calculations use double‑precision floating point and are accurate to 0.1 ms. However, clinical accuracy depends on the quality of the ECG measurement. Automated QT measurements can be affected by U‑waves, T‑wave morphology, and baseline wander. Always manually verify the QT interval in the lead with the clearest T‑wave.

References: Bazett HC. J Physiol 1920;53:370‑377. Fridericia LS. Acta Med Scand 1920;53:469‑482. Sagie A et al. Am J Cardiol 1992;70:797‑801. Hodges M et al. J Electrocardiol 1983;16:189‑194. Drew BJ et al. Circulation 2009;120:1761‑1776. Priori SG et al. Eur Heart J 2015;36:2793‑2867. This tool has been developed in consultation with clinical cardiologists and regularly updated to reflect current guidelines. Last review: July 2026.