What is the QTc interval and why does it matter?

The QT interval on an ECG measures the total time for the ventricles to depolarise and repolarise. Because it shortens at faster heart rates, a raw QT reading is meaningless without rate correction. The corrected value (QTc) allows comparison across different heart rates and patients. A prolonged QTc is associated with torsades de pointes and sudden cardiac death, making it a critical safety parameter in drug development and clinical practice.

What is the Bazett formula?

Bazett (1920) defines QTc = QT divided by the square root of RR, where both QT and RR are in seconds. It is the most widely used formula in clinical software and textbooks. Its limitation is over-correction at fast heart rates (above 90 bpm) and under-correction at slow rates, which can lead to false classifications at extremes.

How does Fridericia differ from Bazett?

Fridericia (1920) uses the cube root of RR instead of the square root, giving QTc = QT divided by the cube root of RR. This produces more accurate corrections across a wider range of heart rates and is the formula recommended by the International Council for Harmonisation (ICH E14) for drug safety studies. It tends to give slightly lower QTc values than Bazett at high heart rates.

When should I use the Framingham or Hodges formula?

The Framingham (1992) formula applies a linear correction and performs well within the typical clinical heart-rate range of 50–90 bpm. Hodges (1983) uses the simple linear equation QTc = QT + 1.75 multiplied by (HR minus 60) in milliseconds and is popular in European cardiology practice, particularly in Holter analysis. Both reduce the over-correction seen with Bazett at elevated rates.

What are the normal QTc ranges?

Using AHA/ACC 2013 guidance, a QTc below 440 ms is normal for males and below 450 ms for females. Values of 440–459 ms (male) or 450–469 ms (female) are borderline prolonged. At or above 460 ms (male) or 470 ms (female) is considered prolonged and warrants clinical review. Different societies and drug guidelines sometimes cite slightly different cut-offs.

Is this tool suitable for clinical use?

No. This calculator is strictly educational. It demonstrates the standard formulas and provides reference classifications. Diagnosing QT prolongation, assessing drug safety, or making any patient management decision requires a qualified healthcare professional interpreting the full clinical context and the original ECG tracing.

QTc Interval Calculator

The QTc interval is one of the most clinically important measurements derived from a standard 12-lead ECG. The raw QT interval — the time from the start of the QRS complex to the end of the T wave — shrinks at faster heart rates and lengthens at slower ones. Without correcting for heart rate, two patients with identical underlying repolarisation characteristics can appear to have very different QT intervals simply because their heart rates differ. Cardiologists and pharmacologists therefore work with a corrected QTc value that removes the rate dependence, allowing fair comparison across patients, time points, and drug studies.

Prolonged QTc is clinically significant because it indicates delayed ventricular repolarisation — a condition that can degenerate into torsades de pointes, a polymorphic ventricular tachycardia that may cause syncope or sudden cardiac death. Dozens of common medications (antiarrhythmics, antipsychotics, antibiotics, antihistamines) are known QT-prolonging agents, and regulatory agencies require QTc analysis in all new drug approvals under the ICH E14 guideline. Understanding the correction formula used is essential because different formulas can disagree, sometimes changing a borderline result into a prolonged one.

This calculator implements all four major correction formulas in your browser, compares them side by side, and classifies each result against the AHA/ACC 2013 sex-specific thresholds.

How it works

All four formulas share the same input: the measured QT interval in milliseconds and the heart rate in beats per minute. The RR interval (the time between successive heartbeats) is first derived:

RR (seconds) = 60 / HR

The four corrections then apply different mathematical relationships between QT and RR:

Bazett (1920): QTc = QT / sqrt(RR) — divides by the square root of RR, both in seconds. Produces the highest QTc at fast rates.

Fridericia (1920): QTc = QT / cbrt(RR) — divides by the cube root of RR. More accurate across a wide rate range; the ICH E14 preferred formula for drug trials.

Framingham (1992): QTc = QT + 0.154 * (1 - RR) in seconds, then converted to ms. A linear correction that performs well for heart rates between 50–90 bpm.

Hodges (1983): QTc = QT + 1.75 * (HR - 60) in milliseconds. Simple additive formula; widely used in Holter analysis and European practice.

Worked example

Consider a patient with a QT of 420 ms and a heart rate of 75 bpm:

RR = 60 / 75 = 0.800 s
Bazett: 420 / sqrt(0.800) = 420 / 0.894 = 469.7 ms — Prolonged (female) / Borderline (male)
Fridericia: 420 / cbrt(0.800) = 420 / 0.928 = 452.5 ms — Borderline (female) / Normal (male)
Framingham: (0.420 + 0.154 * (1 - 0.800)) * 1000 = (0.420 + 0.031) * 1000 = 451.0 ms
Hodges: 420 + 1.75 * (75 - 60) = 420 + 26.25 = 446.3 ms

This example illustrates why formula choice matters: Bazett flags the same patient as borderline-prolonged (male) or prolonged (female), while Fridericia and Hodges place the male result solidly in the normal range. This discordance is particularly common at heart rates above 70 bpm.

Measured QT (ms)	HR (bpm)	Bazett QTc	Fridericia QTc	Classification (male)
400	60	400.0	400.0	Normal
420	75	469.7	452.5	Borderline / Normal
440	90	481.4	459.0	Prolonged / Borderline
380	55	370.6	374.5	Normal

All calculations run entirely in your browser — no ECG data, patient demographics, or results are ever transmitted to a server.

Educational use only. This tool is designed to illustrate the mathematics of QT correction and to support learning. It is not a medical device and must not be used to guide clinical decisions. Always consult a qualified healthcare professional and interpret QTc values in full clinical context.