QTc Calculator – Bazett, Fridericia, Framingham & Hodges

It applies four fixed heart-rate corrections—Bazett, Fridericia, Framingham (Sagie), and Hodges—to one clinician-reviewed QT interval and representative heart rate from the same ECG. It displays all four mathematical estimates without choosing a formula or assigning a diagnosis.

Sources: [1, 2, 3, 4]

QT is the measured ECG interval from the beginning of the QRS complex to the end of the T wave. QTc is a formula-adjusted estimate intended to reduce the mathematical effect of heart rate; it is not a directly measured biological constant.

Sources: [7, 10, 11]

The formulas pair a measured QT with the RR interval represented by that heart rate. Combining values from different times or tracings can create a correction that does not describe either ECG.

Sources: [7, 10]

Bazett divides QT by the square root of RR, Fridericia divides QT by the cube root of RR, Framingham adds 154 × (1 − RR) milliseconds, and Hodges adds 1.75 × (heart rate − 60) milliseconds. RR is 60 divided by heart rate.

Sources: [1, 2, 3, 4, 6]

Each equation models the relationship between QT and heart rate differently. The estimates converge when heart rate is exactly 60 bpm because RR equals one second, but their residual heart-rate dependence and cohort performance differ away from that point.

Sources: [5, 11, 14]

No. Formula performance depends on heart-rate distribution, population, ECG acquisition, endpoint measurement, protocol, and purpose. Drug-safety guidance is a specific regulatory context and is not a universal patient-level diagnostic rule.

Sources: [9, 11, 14]

A submitted number cannot be classified safely without knowing the correction formula, age, sex, rhythm, QRS duration, tracing quality, clinical context, and applicable source or protocol. The page therefore keeps adult threshold examples as static evidence context and does not compare the result with them.

Sources: [7, 8, 13]

The 2010 AHA/ACCF hospital-settings statement described approximate 99th-percentile QTc values of 470 ms for otherwise healthy postpubertal males and 480 ms for otherwise healthy postpubertal females, and placed above 500 ms in its acquired or drug-induced QT prolongation and torsades-prevention context. The 2022 ESC guideline uses at least 480 ms on repeated ECGs or an LQTS score above 3, and conditionally at least 460 ms with arrhythmic syncope or cardiac arrest, in specific long-QT diagnostic pathways. These are different source contexts answering different clinical questions, not universal labels or a unified grading scale applied by this calculator.

Sources: [8, 13]

QT begins at QRS onset and ends at the end of the T wave. Lead choice, the method used to identify the T-wave endpoint, low-amplitude or notched T waves, fused U waves, noise, and automated annotation can materially change the measurement, so a clinician should review the tracing.

Sources: [7, 10]

Bundle branch block, ventricular pacing, or another wide-QRS state adds depolarization time to the measured QT. A simple QTc can then misrepresent repolarization, and a clinician may need a different approach such as protocol-specific JT assessment or another validated adjustment.

Sources: [7, 10]

Not from one arbitrary QT and one heart rate when RR intervals vary substantially. Atrial fibrillation, frequent ectopy, pauses, pacing, or rapidly changing rate can require beat selection, averaging, and specialist ECG review that this page does not perform.

Sources: [8, 11]

Yes. Medicines, drug interactions, potassium, magnesium, calcium, bradycardia, organ function, and acute illness can alter repolarization or clinical risk. The four calculations do not identify a cause, review a medication list, or select monitoring or treatment.

Sources: [8, 9, 12]

No. Diagnosis can involve repeated ECGs, symptoms, family history, genotype, measurement quality, acquired causes, and syndrome-specific criteria. A single formula result neither confirms nor excludes congenital long QT syndrome.

Sources: [13]

Not automatically. Age and physiologic context affect interpretation, and pediatric or pregnancy-specific evaluation should use the relevant reference and clinical pathway. This page does not apply an age- or pregnancy-specific classification.

Sources: [7, 13]

Record the measured QT, heart rate, rhythm, lead and endpoint method, correction formula, and acquisition context. Changing any of these can create an apparent trend that is not solely a biological QT change.

Sources: [7, 10, 11]

No. The comparison is educational arithmetic. It does not diagnose or exclude an arrhythmia syndrome, predict an individual torsades event, decide whether a medicine is safe, or choose testing, monitoring, dose changes, admission, or treatment.

Sources: [8, 9, 13]