liminfoCorrected QT Calculator
Free web tool: Corrected QT Calculator
RR interval: 857 ms | Prolonged threshold (male): >450 ms
Bazett
Borderline432 ms
QTc = QT / sqrt(RR). Most commonly used.
Fridericia
Normal421 ms
QTc = QT / cbrt(RR). Better at extreme HRs.
Framingham
Normal422 ms
QTc = QT + 154(1-RR). Linear correction.
Hodges
Normal418 ms
QTc = QT + 1.75(HR-60). Linear HR-based.
Normal QTc Ranges
QTc Calculator. For clinical reference only.
About Corrected QT Calculator
The Corrected QT Calculator (QTc calculator) computes the heart-rate-corrected QT interval from a 12-lead ECG using four widely accepted formulas: Bazett (QTc = QT / √RR), Fridericia (QTc = QT / ∛RR), Framingham (QTc = QT + 154 × (1 − RR)), and Hodges (QTc = QT + 1.75 × (HR − 60)). You enter the measured QT interval in milliseconds and the heart rate in beats per minute, then select the patient sex. The tool displays the QTc result from all four formulas simultaneously and classifies each as Normal, Borderline, or Prolonged based on sex-specific thresholds.
QT interval correction is essential in clinical cardiology because the QT interval naturally shortens at higher heart rates and lengthens at lower rates. Without rate correction, a long QT interval in a patient with bradycardia might be misinterpreted as normal, and a normal QT in a tachycardic patient might appear falsely prolonged. Prolonged QTc is a risk factor for torsades de pointes, a potentially fatal ventricular arrhythmia. Clinicians use QTc calculation when reviewing ECGs, before prescribing QT-prolonging medications, during drug safety monitoring, and in the evaluation of inherited channelopathies like Long QT Syndrome.
This tool runs entirely in your browser — no patient data is sent to any server. The calculator shows the RR interval derived from heart rate and the sex-specific prolongation thresholds (>450 ms for males, >460 ms for females) alongside each result. All four formulas are computed simultaneously so clinicians can compare results and choose the most appropriate formula for their clinical context. For clinical reference only; not a substitute for professional medical judgment.
Key Features
- Calculates QTc using all four standard formulas: Bazett, Fridericia, Framingham, and Hodges
- Classifies each result as Normal, Borderline, or Prolonged with color-coded indicators
- Sex-specific prolongation thresholds: >450 ms (male), >460 ms (female)
- Displays RR interval derived from heart rate for formula transparency
- Shows formula description and equation for each method
- Includes a reference table of normal QTc ranges for male and female patients
- 100% client-side processing — no patient data leaves the browser
- Instant real-time results as QT interval, heart rate, or sex selection changes
Frequently Asked Questions
What is the QTc interval and why does it need correction?
The QT interval on an ECG represents ventricular depolarization and repolarization. Its duration is inversely related to heart rate — it shortens at higher rates and lengthens at lower rates. QTc (corrected QT) adjusts for heart rate, making comparisons meaningful regardless of the patient's HR. Without correction, interpreting QT prolongation would require knowing whether the rate is normal.
Which QTc formula is most accurate?
No single formula is universally superior. Bazett is the most widely used and cited in guidelines, but overcorrects at high heart rates and undercorrects at low rates. Fridericia performs better at extremes of heart rate. Framingham and Hodges use linear correction, which some studies find more robust across the normal heart rate range. Many cardiologists compare multiple formulas when borderline results occur.
What QTc values are considered prolonged?
The widely used thresholds are: males — normal <430 ms, borderline 430–450 ms, prolonged >450 ms; females — normal <440 ms, borderline 440–460 ms, prolonged >460 ms. Values above 500 ms (regardless of sex) significantly increase the risk of torsades de pointes. These thresholds are used in this calculator.
What is the Bazett formula?
The Bazett formula is QTc = QT / √RR, where QT is in seconds and RR is the RR interval in seconds (RR = 60 / heart rate). It was published by Henry Bazett in 1920 and remains the most commonly used correction formula in clinical practice, though it is known to overcorrect at heart rates above 100 bpm.
What is the Fridericia formula and when is it preferred?
The Fridericia formula is QTc = QT / ∛RR, using the cube root of RR rather than the square root. It tends to perform better at heart rates that deviate significantly from 60 bpm, particularly in patients with bradycardia or tachycardia. Some drug regulatory agencies and trial protocols specify the Fridericia correction.
What drugs prolong the QTc interval?
Many drugs can prolong the QTc: antiarrhythmics (amiodarone, sotalol, quinidine), antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine), antidepressants (tricyclics), antihistamines (terfenadine), and antifungals (fluconazole). Before prescribing QT-prolonging drugs, clinicians should calculate baseline QTc and monitor during therapy.
What is torsades de pointes?
Torsades de pointes (TdP) is a form of polymorphic ventricular tachycardia associated with a prolonged QT interval. On ECG, the QRS complexes appear to twist around the isoelectric baseline, giving the condition its name (French for "twisting of the points"). TdP can degenerate into ventricular fibrillation and cardiac arrest. QTc >500 ms substantially increases TdP risk.
Can this calculator be used for pediatric patients?
The normal QTc ranges displayed in this calculator are for adults. Pediatric normal QTc values differ — neonates and infants have higher acceptable upper limits that decrease with age. For pediatric patients, consult age-specific reference ranges and consider specialist review. This tool is intended for clinical reference only and should not replace formal clinical assessment.