Free QTc Calculator

Corrected QT Interval (QTc): Overview and Clinical Relevance

The QT interval measured from a standard ECG reflects the total duration of ventricular depolarization and repolarization. Because this interval is strongly influenced by the prevailing heart rate—normal shortening occurs with tachycardia and lengthening with bradycardia—clinicians must apply a rate correction to obtain a heart‑rate‑independent value known as the corrected QT (QTc). This corrected QT interval calculator automates the correction using four widely validated formulas (Bazett, Fridericia, Framingham, and Hodges), allowing rapid assessment of QT prolongation risk from routine ECG measurements. Proper identification and management of QT prolongation are critical, as it is a well‑established risk factor for potentially fatal arrhythmias such as torsades de pointes and ventricular fibrillation.

The QT Interval

On a surface ECG, the QT interval is measured from the onset of the QRS complex (the start of ventricular depolarization) to the end of the T wave (the completion of ventricular repolarization). The normal uncorrected QT varies with age, sex, and heart rate. Because of this rate dependence, comparing raw QT values across different heart rates is unreliable. Rate correction standardizes the measurement and makes it possible to define universally applicable thresholds for normality and prolongation.

Correction Formulas

Several mathematical approaches have been proposed to correct the QT interval for heart rate. The four most common are described below.

Bazett Formula

$$QTc = \frac{QT}{\sqrt{RR}}$$

• Developed by: Henry Bazett (1920).
• Comments: The Bazett formula remains the most widely used correction method in clinical practice, primarily because it is embedded in most automated ECG interpretation algorithms. Despite its popularity, it is recognized to overcorrect the QT when the heart rate is high (producing falsely long QTc in tachycardia) and to undercorrect at low heart rates. This systematic error has led many clinicians to request alternative formulas in borderline or high‑risk cases.

Fridericia Formula

$$QTc = \frac{QT}{\sqrt[3]{RR}}$$

• Developed by: Louis Sigurd Fridericia.
• Comments: By using the cube root of the RR interval, this equation provides a less aggressive correction than Bazett, and it is considered more accurate at elevated heart rates. It is often the preferred method when Bazett gives unexpectedly long QTc values in tachycardic patients.

Framingham Correction

$$QTc = QT + 0.154\,(1 - RR)$$

• Source: Framingham Heart Study.
• Comments: This linear formula adjusts the QT by an addition term based on the RR interval (expressed in seconds). It has been repeatedly shown to offer better rate correction across a wide range of heart rates and to correlate more closely with clinical outcomes than Bazett. Some authorities now recommend Framingham as the primary correction method.

Hodges Formula

$$QTc = QT + 1.75\,(HR - 60)$$

• Developed by: M. Hodges.
• Comments: The Hodges formula uses heart rate directly (beats per minute) rather than the RR interval. It is mathematically equivalent to a linear adjustment and yields corrected values very similar to those of Framingham. Because many clinicians measure heart rate routinely, this formula is convenient for quick manual estimates.

A comparative analysis published in the Journal of the American Heart Association (2018) evaluated these four formulas in a large cohort and concluded that the Framingham and Fridericia methods provided the most accurate rate correction and were superior in predicting all‑cause mortality. Nonetheless, Bazett persists as the default in many devices, highlighting the importance of verifying QTc using a second method when the result is borderline or unexpected.

How to Measure QT and Compute QTc

To derive a reliable QTc from an ECG, precise measurement of the QT interval and knowledge of heart rate (or RR interval) are required. The following steps assume a standard recording speed of 25 mm/s:

1. Identify the QRS onset – The start of the Q wave (or the initial deflection of the QRS complex in the absence of a Q wave) marks the beginning of the QT interval.
2. Determine the T‑wave end – This is the most subjective step. The end of the T wave is defined as the return of the T wave to the isoelectric baseline. When the T wave has a clear, steep downslope, the tangent method (drawing a line along the steepest part of the descent and intersecting the baseline) can improve consistency. If a distinct U wave follows the T wave, care must be taken not to include it in the QT measurement.
3. Measure the distance – Count the number of small (1‑mm) and large (5‑mm) boxes between the two points. At 25 mm/s, 1 mm = 40 ms and 5 mm = 200 ms.
4. Convert to milliseconds – Multiply the total number of small boxes by 40 ms, or use a combination of large and small boxes.
5. Obtain the RR interval or heart rate – The RR interval is measured from the peak of one R wave to the peak of the next; at the same recording speed. Alternatively, you can record the heart rate in beats per minute.
6. Input into the calculator – Enter the QT interval (in milliseconds) and the heart rate (bpm) or RR interval (ms). The tool will display QTc values from all four formulas, making it easy to compare results and identify any clinically important prolongation.

Common sources of error include failing to identify the true T‑wave end (especially with flat or notched T waves) and inaccurate conversion from paper‑grid distance to time. Averaging measurements across three consecutive beats in the same lead (typically lead II or V₅) can enhance precision.

Normal QTc Values and Sex Differences

After rate correction, the expected QTc ranges differ by sex. The following thresholds are widely adopted:

• Adult male: QTc < 430 ms
• Adult female: QTc < 450 ms

QT prolongation is considered present when:

• Adult male: QTc > 450 ms
• Adult female: QTc > 470 ms

Sex‑related differences are partly attributed to the influence of sex hormones on cardiac ion channels. Children also have different normal ranges, with upper limits of 440–460 ms depending on age and sex. These thresholds are based on expert consensus and should be interpreted in the clinical context.

Pathophysiology and Risks of QT Prolongation

QT prolongation reflects delayed repolarization of the ventricular myocardium. At the cellular level, this delay is usually caused by reduced outward potassium currents or enhanced inward sodium or calcium currents, prolonging the action potential duration. When the action potential is lengthened, a premature depolarization (often an early afterdepolarization) can trigger torsades de pointes, a polymorphic ventricular tachycardia characterized by gradually twisting QRS complexes around the isoelectric line. Torsades may degenerate into ventricular fibrillation, leading to sudden cardiac death if not terminated.

The risk of torsades increases non‑linearly with QTc prolongation, especially when QTc exceeds 500 ms. Additional risk factors—such as female sex, hypokalemia, hypomagnesemia, bradycardia, heart disease, and concurrent use of multiple QT‑prolonging drugs—synergistically elevate the hazard.

Causes of Acquired QT Prolongation

Most cases of QT prolongation encountered in clinical practice are acquired rather than congenital. Major contributors include:

• Electrolyte abnormalities: low potassium, low magnesium, low calcium (including their corrected values).
• Medications: a diverse array of drugs, as listed below.
• Cardiac conditions: myocardial ischemia, heart failure, left ventricular hypertrophy, myocarditis.
• Endocrine and metabolic disorders: hypothyroidism, hypothermia, anorexia nervosa.
• Neurological conditions: subarachnoid hemorrhage, stroke, traumatic brain injury.
• Other: bradycardia, recent conversion from atrial fibrillation, liquid protein diets, sarcoidosis.

In evaluating a patient with prolonged QTc, the first step is to correct any reversible electrolyte disturbances. For this purpose, corrected calcium and corrected magnesium calculators can be useful, because total calcium and magnesium levels do not always reflect the ionized fraction.

Drug‑Induced QT Prolongation

Over 150 drugs have been associated with QT prolongation, some with a robust mechanism (blockade of the hERG potassium channel) and others based on case reports. Major categories include:

Drug Class	Examples	Risk Level
Antiarrhythmics	Amiodarone, sotalol, dofetilide, ibutilide	High
Macrolide antibiotics	Erythromycin, clarithromycin, azithromycin	Moderate‑high
Fluoroquinolones	Levofloxacin, moxifloxacin, ciprofloxacin	Moderate
Antipsychotics	Haloperidol, thioridazine, ziprasidone, quetiapine	Variable
Antidepressants	Citalopram (> 40 mg/d), escitalopram	Moderate
Antiemetics	Domperidone, ondansetron	Moderate
Methadone	—	High
Antimalarials	Chloroquine, hydroxychloroquine	Moderate

Before initiating any QTc‑prolonging agent, a baseline ECG and QTc calculation are prudent. If the baseline QTc is already prolonged (e.g., > 450 ms in men or > 470 ms in women), an alternative non‑QT‑prolonging drug should be chosen. When the drug is deemed necessary, monitoring QTc after reaching steady state is recommended. Polypharmacy that involves two or more QTc‑prolonging drugs should be avoided unless the benefit clearly outweighs the cumulative risk.

Practical Use of the QTc Calculator

This tool provides a streamlined way to compute the corrected QT interval using the four most validated formulas simultaneously. By offering Bazett’s QTc alongside those from Fridericia, Framingham, and Hodges, the calculator helps clinicians identify inconsistencies that might suggest over‑ or undercorrection. It is designed for use by healthcare professionals familiar with ECG interpretation. The output must be integrated with the patient’s history, other ECG findings (e.g., QRS duration, U‑wave presence), and laboratory results before making clinical decisions. The calculator does not replace formal medical consultation; any significant QTc prolongation should prompt further evaluation and, when appropriate, referral to a cardiologist.