Audited —·Last updated 27 Jul 2026·6 citations·Tier 1·0 uses

QTc Calculator

Calculate corrected QT interval with Bazett, Fridericia, Framingham, and Hodges formulas from QT and heart rate.

QTc Calculator

Correction formula
Sex (display only)
QTc (Bazett)
402.4922
RR interval
0.8
QTc (Fridericia)
387.7982
QTc (Framingham)
390.8
QTc (Hodges)
386.25
Selected primary QTc
402.4922

Background.

A QTc calculator estimates the corrected QT interval from a measured ECG QT interval and heart rate or RR interval. The canonical use case is an ECG with measured QT of 360 milliseconds and heart rate of 75 beats per minute. The RR interval is 0.8 seconds. The same QT and heart rate produce different corrected values depending on formula: Bazett 402.492235949962 ms, Fridericia 387.798244205739 ms, Framingham 390.8 ms, and Hodges 386.25 ms. That spread is the reason the calculator should always show the formula name.

People search for QTc calculators because raw QT changes with heart rate. When heart rate rises, the QT interval usually shortens; when heart rate falls, it lengthens. A corrected QT attempts to express the QT interval in a way that is more comparable across heart rates. QTc appears in cardiology, emergency medicine, psychiatry, pharmacology, congenital long QT evaluation, electrolyte disturbance review, and drug-safety monitoring. It is also reported automatically by ECG machines, but the machine's formula and measurement method may not always be obvious.

The history is unusually formula-heavy. Bazett and Fridericia corrections date from early twentieth-century work and remain widely recognized. The Framingham correction was published by Sagie and colleagues using Framingham Heart Study data. Hodges uses a heart-rate linear correction. Luo and colleagues compared commonly used correction formulas in more than 10,000 normal ECGs and found that the formulas behaved differently with heart rate. More recent clinical guidance continues to warn that formula choice matters and that Bazett may overcorrect or undercorrect at heart-rate extremes.

The calculator's math is straightforward but clinically sensitive. First, RR interval is 60 divided by heart rate. Then the QT interval is adjusted. Bazett divides by the square root of RR. Fridericia divides by the cube root of RR. Framingham adds 154 times one minus RR to QT, with QT in milliseconds. Hodges adds 1.75 times heart rate minus 60 to QT. Each formula returns milliseconds when QT is entered in milliseconds and RR is entered in seconds.

This calculator must not diagnose long QT syndrome, torsades risk, medication safety, or electrolyte disease from arithmetic alone. QT measurement can be difficult, especially with U waves, bundle branch block, wide QRS, atrial fibrillation, tachycardia, noisy tracings, paced rhythms, and automated ECG errors. Clinical interpretation depends on the tracing, symptoms, sex, age, medications, potassium, magnesium, calcium, congenital history, and professional review.

The product should also preserve the raw QT and heart-rate inputs beside the corrected results. A clinician or reviewer needs to know whether a high QTc came from a long measured QT, a high heart rate, or a formula that behaves strongly at that rate. Showing the RR interval makes that audit trail clearer.

For Quanta, the safest product behavior is to compute all formulas and let users choose one primary display. It should label any rounded display value, preserve full precision for tests, and include warnings that QTc is a clinical ECG calculation. It should not present a single threshold as universal. A future interpretation module could add context-specific cutoffs, but the base calculator should be formula-first.

What is qtc calculator?

QTc is the QT interval corrected for heart rate. The QT interval is measured on an electrocardiogram from the start of ventricular depolarization to the end of repolarization, as represented by the Q wave through T wave. QTc tries to adjust that measured interval because QT normally changes with heart rate.

The key terms are QT interval, QTc, ECG, heart rate, RR interval, Bazett, Fridericia, Framingham, Hodges, repolarization, milliseconds, and formula mode. RR interval is the time between consecutive R waves and equals 60 divided by heart rate when rhythm is regular. Bazett and Fridericia use power functions of RR. Framingham and Hodges use linear adjustments.

The result is normally reported in milliseconds, but it is not a direct measurement. It is a formula-adjusted value derived from the measured QT and rate. Because formulas differ, the product should treat formula selection as part of the result rather than a hidden implementation detail.

The calculator is valid for arithmetic from a measured QT and a heart rate or RR interval. It is not valid for diagnosing long QT syndrome, managing drug therapy, interpreting wide-QRS rhythms, handling atrial fibrillation without expert judgement, or replacing manual ECG review. It should always display the formula used because QTc values from different formulas are not interchangeable.

How to use this calculator.

  1. Measure QT interval on the ECG in milliseconds.
  2. Enter heart rate in beats per minute or RR interval in seconds.
  3. Select a formula or choose all formulas.
  4. Review RR interval if heart rate was entered.
  5. Compare Bazett, Fridericia, Framingham, and Hodges values when formula choice matters.
  6. Interpret QTc only with ECG quality, rhythm, QRS duration, medications, electrolytes, and clinical context.
  7. Use professional clinical review for abnormal values or symptoms.

The formula.

QTc = QT ⁄ √RR
QTc · corrected QT (ms)QT · measured QT interval (ms)RR · 60 ⁄ HR (seconds)Bazett shown · Fridericia uses ∛RR

The calculator first finds RR interval. In a regular rhythm, RR in seconds equals 60 divided by heart rate. For a heart rate of 75 beats per minute, RR is 60 divided by 75, or 0.8 seconds. If the user enters RR directly, the calculator can skip this conversion, but it should still validate that RR is positive and plausible.

Bazett's formula divides QT by the square root of RR. With QT of 360 ms and RR of 0.8 seconds, the denominator is sqrt(0.8), or 0.894427190999916. The QTc is 360 divided by that value, or 402.492235949962 ms. Bazett is historically common and appears on many ECG systems, but it is known to behave poorly at some heart-rate extremes.

Fridericia's formula divides QT by the cube root of RR. The cube root of 0.8 is 0.928317766722556, so 360 divided by that denominator is 387.798244205739 ms. Fridericia is often preferred in drug-safety and research contexts because it may be less distorted than Bazett in some heart-rate ranges, but it is still a fixed correction formula.

Framingham uses a linear RR correction. With milliseconds, the formula is QT plus 154 times one minus RR. For RR of 0.8, one minus RR is 0.2, and 154 times 0.2 is 30.8. Adding that to 360 gives 390.8 ms. Hodges uses heart rate directly: QT plus 1.75 times heart rate minus 60. At 75 bpm, the heart-rate difference is 15, and 1.75 times 15 is 26.25. Adding that to 360 gives 386.25 ms.

All formulas are estimates. The calculator should not average them or imply that one is universally correct. It should show the selected formula and allow all-formula output when comparing values. Automated ECG systems, clinical protocols, and drug labels may specify a particular formula. The calculation should follow that context.

A worked example.

Example

The example ECG has a measured QT interval of 360 milliseconds and a heart rate of 75 beats per minute. The calculator first computes RR interval as 60 divided by 75, which equals 0.8 seconds. Bazett's formula divides the QT by the square root of RR. The square root of 0.8 is 0.894427190999916, so Bazett QTc is 402.492235949962 ms. Fridericia uses the cube root of RR. The cube root of 0.8 is 0.928317766722556, so Fridericia QTc is 387.798244205739 ms. Framingham adds 154 times one minus RR: 154 times 0.2 is 30.8, and 360 plus 30.8 is 390.8 ms. Hodges adds 1.75 times heart rate minus 60: 1.75 times 15 is 26.25, so Hodges QTc is 386.25 ms. The result shows why formula labels matter; the same ECG inputs produce different QTc values. None of these outputs should be detached from the ECG context.

heart Rate Bpm75
qt Ms360

Frequently asked questions.

Which QTc formula should I use?
The correct formula depends on the clinical context, institutional practice, ECG system, study protocol, medication guidance, and heart-rate range. Bazett is widely used and familiar, but it can be less reliable at heart-rate extremes. Fridericia, Framingham, and Hodges are often considered when heart rate is high or low. The calculator should compute all formulas and label them clearly rather than implying that one formula is universally correct. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
Why does Bazett give a higher value in the example?
Bazett divides by the square root of RR. At a heart rate of 75 bpm, RR is 0.8 seconds, and the square-root denominator is smaller than the cube-root denominator used by Fridericia. Dividing by a smaller denominator gives a larger corrected value. This behavior becomes more noticeable as heart rate moves away from 60 bpm. That is why formula selection can change whether a QTc appears borderline. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
Can QTc diagnose long QT syndrome?
No. QTc can be part of long QT syndrome evaluation, but diagnosis requires clinical context, ECG quality, repeated measurements, family history, symptoms, medications, electrolytes, genetics when appropriate, and expert review. Automated ECG QTc can be wrong. A calculator should not diagnose long QT syndrome or rule it out. It should compute transparent formula outputs from entered values and recommend clinical interpretation for concerning results. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
What if the rhythm is irregular?
Irregular rhythm makes QT and RR selection harder. A single heart rate may not represent the preceding RR interval that influenced repolarization. Atrial fibrillation, ectopy, sinus arrhythmia, and paced rhythms can complicate QTc interpretation. Some clinicians average beats or use specialized methods, but that is outside a simple calculator. The tool should warn that irregular rhythm requires professional ECG review rather than blind formula use. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
Should QT be entered in seconds or milliseconds?
This dossier uses milliseconds for QT and seconds for RR. If a formula is written in seconds, it can be converted consistently, but mixing units is a common error. For example, entering 0.360 as milliseconds would produce nonsense. The calculator should clearly label QT in milliseconds and RR in seconds, or provide a unit selector that converts before calculation. The output should be in milliseconds because that is common in ECG reporting. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
Does a wide QRS affect QTc?
Yes. Bundle branch block, ventricular pacing, and other wide-QRS rhythms can make ordinary QTc interpretation less straightforward because part of the QT interval reflects delayed depolarization rather than repolarization alone. Some contexts use JT interval or adjusted approaches. A simple QTc calculator should not attempt to solve wide-QRS interpretation unless it has a specific validated module. Users should seek clinical ECG review. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
Can medications affect QTc?
Many medications, electrolyte abnormalities, and clinical conditions can affect QT and torsades risk. Psychiatry and cardiology guidance documents discuss QTc in the context of medications, risk factors, and monitoring. A calculator can compute QTc, but medication decisions require drug-specific risk, dose, interactions, potassium, magnesium, renal function, symptoms, and alternatives. The calculator should not recommend starting, stopping, or changing medication. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.
When should I not use this calculator?
Do not use it as the only basis for emergency decisions, long QT diagnosis, drug safety clearance, or interpretation of poor-quality ECGs. Do not use it when QT is uncertain, rhythm is irregular, QRS is wide, or the patient has symptoms such as syncope, palpitations, seizures, or chest pain without clinical review. Use it to check formula arithmetic and compare correction methods, not to replace ECG expertise. The output should be treated as formula arithmetic and reviewed with the actual ECG tracing, patient risk factors, and clinical context before any decision.

References& sources.

  1. [1]Sagie, A., Larson, M.G., Goldberg, R.J., Bengtson, J.R., and Levy, D. (1992). "An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)." American Journal of Cardiology 70(7):797-801.
  2. [2]Luo, S., Michler, K., Johnston, P., and Macfarlane, P.W. (2004). "A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs." Journal of Electrocardiology 37 Suppl:81-90.
  3. [3]Malik, M. et al. (2015). "Nomenclature, categorization and usage of formulae to adjust QT interval for heart rate." Journal of Electrocardiology.
  4. [4]Cuthbertson, J. et al. (2020). "A Retrospective Analysis of Hospital Electrocardiogram Auto-Populated QT Interval Calculation." Cureus.
  5. [5]American Psychiatric Association (2018). "Resource Document on QTc Prolongation and Psychotropic Medications." APA.
  6. [6]Academy of Consultation-Liaison Psychiatry (2024). "How-To Guide: QTc Prolongation." ACLP.
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