QTc Calculator – Bazett, Fridericia, Framingham & Hodges
Compare four fixed heart-rate correction formulas from a measured QT interval and representative heart rate, with ECG measurement and interpretation limits.
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About
This QTc calculator compares Bazett, Fridericia, Framingham, and Hodges corrected QT estimates from one clinician-reviewed measured QT interval and representative heart rate. QTc is calculated rather than directly measured, and the page does not automatically select a formula. [1, 2, 3, 4]
It does not analyze the ECG tracing, identify the T-wave endpoint, diagnose long QT syndrome, predict an individual torsades event, or determine medication, monitoring, or treatment. [8, 9, 13]
Formula
Interpretation
How this QTc calculator works
Enter one clinician-reviewed measured QT interval and a representative heart rate from the same ECG. The calculator derives RR as 60 divided by heart rate, then sends the same QT and RR context through four fixed corrections. It reports all four results without silently choosing a preferred formula or applying a diagnostic category. [7, 11]
QTc correction formulas
| Correction | Evidence and limitation |
|---|---|
| Bazett | The 1920 square-root correction remains widely reported, including in historical studies and reference landmarks, but it retains substantial residual heart-rate dependence. [1, 5, 12] |
| Fridericia | The 1920 cube-root correction is frequently used in drug-development QT assessment. That regulatory use does not make it a universal patient-level diagnostic standard. [2, 9, 12] |
| Framingham | Sagie and colleagues derived the linear RR correction in 5,018 Framingham participants aged 28–62 years, with RR intervals from 0.50 to 1.47 seconds. It is not a universal validation range for every patient. [4] |
| Hodges | Hodges, Salerno, and Erlien reported a linear heart-rate correction in 1983. A 2018 formal correction to the Vandenberk article changed its printed Hodges coefficient from 0.00175 to 1.75; this calculator uses 1.75. [3, 5, 6] |
- Correction
- Bazett
- Correction
- Fridericia
- Correction
- Framingham
- Evidence and limitation
- Sagie and colleagues derived the linear RR correction in 5,018 Framingham participants aged 28–62 years, with RR intervals from 0.50 to 1.47 seconds. It is not a universal validation range for every patient. [4]
- Correction
- Hodges
These are distinct fixed models. Formula performance varies with population, heart-rate distribution, ECG acquisition, endpoint measurement, and study purpose; the page does not rank them from an individual result. [5, 11]
Bazett vs Fridericia: QTcB vs QTcF
Bazett divides QT by the square root of RR, whereas Fridericia divides QT by the cube root of RR. The same measured QT and heart rate can therefore produce different QTcB and QTcF values. At 60 bpm, RR is exactly one second and both equal the measured QT; farther from 60 bpm, the difference between the correction models may become more visible. This is a model difference, not evidence that the tracing changed twice. [1, 2, 5]
Which QTc correction formula should be used?
Bazett remains widely reported and underlies many historical studies and thresholds, but its residual heart-rate dependence is a recognized limitation. In the adult clinical-study context, FDA/ICH guidance states that Bazett has performed poorly for correcting heart-rate differences within and between subjects; routine Bazett submission is no longer warranted in every application unless historical comparison provides a compelling reason. Fridericia may be appropriate in most situations in that context, while other methods may be more appropriate in some cases, and the guidance explicitly identifies no single recommended alternative. [9]
Comparative studies likewise do not erase population and purpose differences. Vandenberk and colleagues found materially different residual heart-rate behavior among formulas. In a 2025 cohort of 22,063 medically assessed healthy phase-1 participants, Fridericia had the lowest residual QTc–heart-rate association, Bazett the highest, and Hodges and Framingham intermediate. That healthy research cohort does not establish one universally best formula for every patient. The relevant study, protocol, or clinical workflow determines what should be reported; this page only compares the four calculations. [5, 14]
What is a normal QTc? Source-specific reference landmarks
There is no single context-free QTc cutoff that can be safely applied across every correction formula, age, sex, rhythm, QRS duration, measurement method, and diagnostic purpose.
| Source | Source context | Calculator boundary |
|---|---|---|
| AHA/ACCF/HRS 2009 ECG standardization | Practical adult limits described adjusted QT ≥460 ms in women and ≥450 ms in men as prolonged, and QT ≤390 ms as short. The statement also recommends considering rate, sex, and age in QT adjustment. [7] | Not applied automatically to any of the four cards |
| AHA/ACCF 2010 hospital TdP statement | Approximate 99th-percentile QTc values were 470 ms for otherwise healthy postpubertal males and 480 ms for otherwise healthy postpubertal females; >500 ms was highly abnormal in this hospital acquired/drug-induced TdP-prevention context. [8] | No monitoring, admission, medication, or treatment instruction |
| ESC 2022 LQTS pathway | LQTS diagnosis is recommended with repeated 12-lead ECG QTc ≥480 ms or a diagnostic score >3. ECG diagnosis of LQTS should be considered with QTc ≥460 ms on repeated 12-lead ECGs in a patient with an unexplained syncopal episode and in the absence of secondary causes of QT prolongation. [13] | Symptoms, repeated ECGs, scores, and secondary causes are not assessed |
- Source
- AHA/ACCF/HRS 2009 ECG standardization
- Source context
- Practical adult limits described adjusted QT ≥460 ms in women and ≥450 ms in men as prolonged, and QT ≤390 ms as short. The statement also recommends considering rate, sex, and age in QT adjustment. [7]
- Calculator boundary
- Not applied automatically to any of the four cards
- Source
- AHA/ACCF 2010 hospital TdP statement
- Source context
- Approximate 99th-percentile QTc values were 470 ms for otherwise healthy postpubertal males and 480 ms for otherwise healthy postpubertal females; >500 ms was highly abnormal in this hospital acquired/drug-induced TdP-prevention context. [8]
- Calculator boundary
- No monitoring, admission, medication, or treatment instruction
- Source
- ESC 2022 LQTS pathway
- Source context
- LQTS diagnosis is recommended with repeated 12-lead ECG QTc ≥480 ms or a diagnostic score >3. ECG diagnosis of LQTS should be considered with QTc ≥460 ms on repeated 12-lead ECGs in a patient with an unexplained syncopal episode and in the absence of secondary causes of QT prolongation. [13]
- Calculator boundary
- Symptoms, repeated ECGs, scores, and secondary causes are not assessed
These numbers come from different documents and answer different clinical questions; they are not a QuickMedCalc grading scale. The calculator does not compare a submitted value with those landmarks, label any formula normal, borderline, prolonged, dangerous, or high risk, diagnose or exclude congenital long QT syndrome, or generate medication, monitoring, admission, or treatment advice.
How to measure the QT interval on ECG
QT extends from QRS onset to the T-wave end, and manual review is important when morphology makes automated annotation uncertain. Tangent and threshold methods are both recognized approaches to locating the endpoint; the tangent method generally yields a shorter value, and there is no gold-standard consensus between the two. Biphasic T waves require careful handling, while U waves should not simply be included as T-wave termination. The ESC educational article describes lead II or V5 as preferred measurement approaches, not as mandatory inputs to this calculator. Use one documented method consistently for serial comparison. [7, 10, 15]
When this simple QTc calculation is unreliable
Atrial fibrillation and variable rhythm
With atrial fibrillation, substantial beat-to-beat RR variability, frequent ectopy, long–short cycles, or rapidly changing rate, one arbitrary QT and one heart rate may not represent the rhythm. Beat selection, averaging, or specialist ECG review may be required; this page does not implement an AF averaging method. [8, 11]
Wide QRS, LBBB and ventricular pacing
A wide QRS includes additional depolarization time, so simple QTc may over-represent repolarization duration in left bundle branch block or ventricular pacing. The 2009 AHA/ACCF/HRS statement discusses QRS-duration adjustment or JT interval with JT-specific standards. This calculator does not collect QRS, calculate JTc, or apply an LBBB or pacing correction. [7]
Clinical boundaries
The four outputs are arithmetic comparisons, not direct ECG interpretation. Age, sex, symptoms, syncope, family history, genotype, QRS duration, medicines, electrolytes, organ function, tracing quality, and serial ECG change are not collected. A QTc result from this page cannot diagnose or exclude long QT syndrome, estimate one person's torsades probability, determine whether a medicine is safe, or select testing, monitoring, admission, dose changes, or treatment. [8, 9, 13]
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370. Reprinted in Ann Noninvasive Electrocardiol. 1997;2(2):177–194. DOI 10.1111/j.1542-474X.1997.tb00325.x.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53(1):469–486. DOI 10.1111/j.0954-6820.1920.tb18266.x.
- Hodges M, Salerno D, Erlien D. Bazett’s QT correction reviewed. Evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol. 1983;1(2):694. Abstract.
- Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797–801. PMID 1519533. DOI 10.1016/0002-9149(92)90562-D.
- Vandenberk B, Vandael E, Robyns T, et al. Which QT Correction Formulae to Use for QT Monitoring? J Am Heart Assoc. 2016;5(6):e003264. PMID 27317349. PMCID PMC4937268. DOI 10.1161/JAHA.116.003264.
- Correction to: Which QT Correction Formulae to Use for QT Monitoring? J Am Heart Assoc. 2018;7(16):e004252. PMID 30369330. PMCID PMC6201387. DOI 10.1161/JAHA.117.004252.
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part IV: The ST Segment, T and U Waves, and the QT Interval. J Am Coll Cardiol. 2009;53(11):982–991. PMID 19281931. DOI 10.1016/j.jacc.2008.12.014.
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060. PMID 20142454. PMCID PMC3056123. DOI 10.1161/CIRCULATIONAHA.109.192704.
- U.S. Food and Drug Administration. E14 and S7B Clinical and Nonclinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential — Questions and Answers Guidance for Industry. August 2022.
- Goldenberg I, Moss AJ, Zareba W. QT interval: how to measure it and what is normal. J Cardiovasc Electrophysiol. 2006;17(3):333–336. PMID 16643414. DOI 10.1111/j.1540-8167.2006.00408.x.
- Malik M, Färbom P, Batchvarov V, Hnatkova K, Camm AJ. Nomenclature, categorization and usage of formulae to adjust QT interval for heart rate. World J Cardiol. 2015;7(6):315–325. PMID 26131336. PMCID PMC4478566. DOI 10.4330/wjc.v7.i6.315.
- Indik JH, Pearson EC, Fried K, Woosley RL. Bazett and Fridericia QT correction formulas interfere with measurement of drug-induced changes in QT interval. Heart Rhythm. 2006;3(9):1003–1007. PMID 16945790. DOI 10.1016/j.hrthm.2006.05.023.
- Zeppenfeld K, Tfelt-Hansen J, de Riva M, et al. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Eur Heart J. 2022;43(40):3997–4126. PMID 36017572. DOI 10.1093/eurheartj/ehac262.
- Hoek LJ, Voors AA, Maass AH, Riesebos M, Brouwer JL. A comparison of the four most commonly used formulae to adjust the QT-interval for heart rate in 22,000 healthy subjects. J Electrocardiol. 2025;92:154091. PMID 40829441. DOI 10.1016/j.jelectrocard.2025.154091.
- Robyns T. How to: measure the QT interval? ESC Council on Cardiovascular Genomics, CardioGenomics Insights, Volume 9. March 12, 2024.
FAQ
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.
Bazett divides QT by the square root of RR, while Fridericia divides QT by the cube root of RR. They therefore respond differently as heart rate moves away from 60 bpm. At exactly 60 bpm, RR is one second and both equal the measured QT. The difference reflects correction models, not two changes in the tracing.
There is no context-free universal choice. Bazett remains widely reported but retains substantial heart-rate dependence. In adult drug-development studies, FDA/ICH 2022 describes Bazett as inferior for correcting within- and between-subject heart-rate differences, says Fridericia is likely appropriate in most situations, allows that other methods may be more appropriate, and identifies no single recommended alternative. Population, heart-rate distribution, ECG method, protocol, and purpose still matter; this calculator compares rather than chooses.
There is no single cutoff that can be applied safely across every formula, age, sex, rhythm, QRS duration, measurement method, and diagnostic purpose. AHA/ACCF/HRS 2009 practical adult limits, the 2010 hospital torsades-prevention statement, and the 2022 ESC LQTS pathway answer different questions. This page presents them only as source-specific static context and never labels a submitted result normal, prolonged, or dangerous.
QT begins at QRS onset and ends at the T-wave endpoint. Manual review is important. Tangent and threshold endpoint methods are both recognized, with no gold-standard consensus between them; the tangent method usually yields a shorter interval. Biphasic T waves require careful handling and U waves should be excluded. Keep the lead, endpoint method, rhythm context, and correction formula consistent for serial comparison.
One QT and one heart rate may not represent atrial fibrillation, substantial RR variability, frequent ectopy, long–short cycles, or a rapidly changing rate. An uncertain T-wave endpoint also requires a different measurement workflow. Wide QRS, LBBB, and ventricular pacing add depolarization time and may require QRS adjustment, JT assessment, or another specialist method that this page does not calculate.
No. Long-QT diagnosis can require repeated ECGs, an unexplained syncopal episode, family history, genotype, exclusion of secondary causes, and syndrome-specific criteria. Medicines and electrolytes can affect QT and clinical risk, but this calculator does not review them, predict an individual torsades event, determine whether a medicine is safe, or choose testing, monitoring, dose changes, admission, or treatment.
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Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.