QTc Calculator — Corrected QT Interval by Multiple Formulas
Formula and evidence review: July 30, 2026. Your values stay in your browser and are not stored or transmitted by this calculator.
ECG values
QTc formula comparison
Difference across formulas
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Enter QT and rate information to compare four corrected values.
Interpretation limit
This calculator performs rate correction only. It does not determine whether QTc is normal, diagnose long QT syndrome, or predict an individual arrhythmia.
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Medical safety and limitations
Do not use this calculator alone to diagnose long QT syndrome, decide that an ECG is safe, or start, stop, or change a medicine. A QTc value depends on accurate QT and RR measurement, correction formula, rhythm, heart rate, age, sex, QRS duration, medicines, electrolytes, illness and the protocol being used.
Automated ECG measurements can require manual review. Irregular rhythms may require averaging multiple beats. A wide QRS from bundle branch block, ventricular pacing or another conduction delay can lengthen QT because depolarization is prolonged and may call for specialist assessment or a different metric. Seek urgent medical help for fainting, a seizure, sustained palpitations, chest pain or severe breathlessness—especially with a personal or family history of sudden cardiac death.
QTc formulas used
QT is entered in milliseconds. RR is converted to seconds before each formula is applied. When heart rate is entered, RR = 60 ÷ heart rate.
Bazett
QTcB = QT ÷ RR1/2
Tends to overcorrect at faster rates and undercorrect at slower rates. It remains widely reported and is used in some established clinical criteria.
Fridericia
QTcF = QT ÷ RR1/3
Less rate-sensitive than Bazett in many adult settings and commonly used for drug-related QT assessment.
Framingham
QTc = QT + 154 × (1 − RR)
A linear population-derived adjustment, with QT in milliseconds and RR in seconds.
Hodges
QTc = QT + 1.75 × (heart rate − 60)
A linear adjustment using heart rate in beats per minute.
At exactly 60 bpm (RR = 1 second), all four formulas return the measured QT unchanged. Away from 60 bpm, the estimates diverge because each formula models the QT–rate relationship differently.
Worked QTc example
QT 400 ms at 75 bpm
The equivalent RR interval is 60 ÷ 75 = 0.8 seconds.
Bazett: 400 ÷ √0.8 = 447.2 ms
Fridericia: 400 ÷ ∛0.8 = 430.9 ms
Framingham: 400 + 154 × (1 − 0.8) = 430.8 ms
Hodges: 400 + 1.75 × (75 − 60) = 426.3 ms
Why compare formulas?
In the same example, the highest and lowest estimates differ by about 21 ms. That spread is produced only by formula choice, not by a change in the underlying ECG.
Use the formula and decision threshold specified by the relevant clinician, ECG service, study, medicine protocol or guideline. Do not mix a threshold derived for one formula with another formula without validation.
Measurement assumptions
| Input or result | Assumption | Why it matters |
|---|---|---|
| QT interval | Measured from QRS onset to T-wave end in milliseconds. | Lead choice, T-wave shape, U waves and automated versus manual measurement can change the endpoint. |
| RR interval | Corresponds to the same beat or representative beats used for QT. | A displayed heart rate may be rounded; measured RR can preserve more precision. |
| Rhythm | A simple QT–RR pair adequately represents the tracing. | Irregular rhythms often require multi-beat measurement and averaging rather than one pair. |
| QTc output | A generic population formula is appropriate for the intended use. | Different formulas can materially disagree, particularly at high or low heart rates. |
How to use this QTc calculator
- Enter the measured QT interval in milliseconds. Do not enter an already corrected QTc value.
- Select heart rate or RR interval, then enter the value from the same ECG tracing.
- Calculate and compare all four QTc estimates and the displayed spread.
- Record which formula produced any value you copy or download.
- Use the interpretation method specified for the relevant patient and purpose. If the result affects treatment or risk assessment, have the ECG and measurement reviewed by a qualified clinician.
QTc calculator FAQ
What is QTc?
QTc is the QT interval adjusted mathematically for heart rate. It estimates what the QT might be at a standardized heart rate, but it is not a direct measurement and different formulas can produce different results.
Which QTc formulas does this calculator use?
It calculates Bazett, Fridericia, Framingham and Hodges QTc. Bazett and Fridericia use RR in seconds, Framingham applies a linear RR adjustment, and Hodges uses heart rate.
Which QTc formula is best?
There is no single best formula for every population and purpose. Bazett remains common but can overcorrect at faster rates and undercorrect at slower rates. Current FDA guidance says Fridericia is likely appropriate in most adult drug-safety situations, while also noting that other methods can be more appropriate. Use the method specified by the relevant protocol.
Can I calculate QTc from heart rate instead of RR interval?
Yes. The calculator derives RR in seconds as 60 divided by heart rate. If the ECG provides a measured RR interval, entering it directly avoids rounding introduced by a displayed heart rate.
What units should I use?
Enter QT in milliseconds. Enter heart rate in beats per minute or RR interval in milliseconds. The formulas internally use RR in seconds and return QTc in milliseconds.
Does a QTc result diagnose long QT syndrome?
No. Long QT assessment depends on ECG measurement quality, formula, age, sex, symptoms, medicines, electrolytes, family history and clinical context. This calculator cannot diagnose or exclude long QT syndrome or predict an individual's arrhythmia risk.
Can I use QTc when the rhythm is irregular or QRS is wide?
Use caution. Beat selection and averaging matter in irregular rhythms, and a wide QRS can lengthen QT because depolarization is prolonged. These situations may require specialist measurement or a different approach such as considering JT rather than routine use of a simple QTc result.
Sources and method review
- US Food and Drug Administration / ICH. E14 and S7B Questions and Answers: Clinical and Nonclinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential. Current guidance on correction methods and adult use of Bazett and Fridericia.
- Luo S, Michler K, Johnston P, Macfarlane PW. A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs. Journal of Electrocardiology. 2004. Comparison of Bazett, Fridericia, Framingham and Hodges in 10,303 ECGs.
- Tooley J, et al. Comparison of QT Interval Measurement Methods and Correction Formulas in Atrial Fibrillation. American Journal of Cardiology. 2019. Evidence on measurement and multi-beat averaging in irregular rhythm.
- Kligfield P, et al. Recommendations for the Standardization and Interpretation of the Electrocardiogram. AHA/ACCF/HRS scientific statement.
- Formula, interaction, and content review date: July 30, 2026.
Disclaimer: For general education and arithmetic checking only. This page does not diagnose, triage, or treat a medical condition. Discuss ECG findings, symptoms, medicine decisions and risk assessment with a qualified healthcare professional.