QTc Calculator – Corrected QT Interval for Educational Reference
The QT interval on an ECG represents the total time for ventricular depolarization and repolarization. Because the QT interval shortens at higher heart rates and lengthens at lower heart rates, raw QT values are difficult to compare across patients and situations. The corrected QT interval (QTc) adjusts the QT interval for heart rate to provide a standardized reference value.
This QTc Calculator helps you estimate heart-rate–corrected QT values using several widely referenced formulas, including Bazett, Fridericia, Framingham and Hodges. The tool is designed for students, trainees, and clinicians who want a quick educational reference. It is not intended to replace formal ECG interpretation, expert review, or clinical judgment.
How the QTc Calculator Works
You enter the measured QT interval in milliseconds and the current heart rate in beats per minute (bpm). The calculator converts heart rate to the RR interval in seconds and then applies one or more QT correction equations. The result is displayed as QTc in milliseconds, along with a broad reference category (normal, borderline, prolonged) based on sex-specific ranges.
Different formulas behave differently at slow and fast heart rates, so many users find it helpful to compare more than one QTc value, especially when the heart rate is far from 60 bpm.
QTc Formulas Used in This Calculator
1. Bazett Formula
RR is the RR interval in seconds and QT is expressed in seconds in the original formula. In this calculator, QT is entered in milliseconds and converted internally. Bazett’s correction is common in clinical reports but may overcorrect at high heart rates and undercorrect at low heart rates.
2. Fridericia Formula
Fridericia’s cube-root correction tends to be less extreme than Bazett’s at very high or low heart rates, and is often preferred when more stable behavior is desired across a wider heart rate range.
3. Framingham Formula
This linear correction uses the RR interval in seconds and adds an adjustment term to the measured QT interval. It is commonly used in research and provides a more modest correction compared with Bazett at extreme heart rates.
4. Hodges Formula
Hodges’ formula corrects QT based directly on heart rate rather than RR interval. It is sometimes used as an alternative when heart rate is available but RR is not easily calculated.
Heart Rate, RR Interval and Units
- QT is entered in milliseconds (ms).
- Heart rate is entered in beats per minute (bpm).
- The calculator converts heart rate to RR interval in seconds using RR = 60 ÷ HR.
- All QTc results are reported in milliseconds for easier comparison with ECG reports.
Approximate QTc Reference Categories
For educational display, the calculator assigns a broad reference category based on commonly cited QTc ranges:
- Normal: up to about 450 ms in men, 460 ms in women
- Borderline: around 450–470 ms in men, 460–480 ms in women
- Prolonged: above about 470 ms in men, 480 ms in women
These ranges are approximate, may vary by source, and are not intended as strict diagnostic cutoffs. Interpretation must always be individualized and performed by a qualified clinician in the full clinical context.
Practical Uses of a QTc Calculator
- Learning how different formulas respond to the same QT and heart rate
- Exploring how tachycardia or bradycardia affects QTc values
- Comparing Bazett with alternative formulas in borderline cases
- Teaching ECG interpretation concepts in academic settings
- Checking rough QTc estimates alongside formal ECG machine reports
Important Limitations and Safety Notice
This QTc Calculator is intended only for educational and informational use. QT interpretation depends on many factors including lead selection, measurement technique, presence of bundle branch block, medications, electrolyte status and comorbid conditions. Automated QTc calculations may differ from clinical readings.
Do not use this tool to diagnose, treat or manage any medical condition, and do not change medications or clinical plans based on this calculator alone. If you have concerns about QT prolongation or arrhythmia risk, consult a qualified healthcare professional or cardiologist.
How to Use the QTc Calculator Step by Step
- Measure the QT interval in milliseconds from a high-quality ECG tracing.
- Determine the heart rate in beats per minute using the ECG or monitor.
- Select one of the QTc formula tabs or the “Compare All Methods” tab.
- Enter QT interval, heart rate and sex (for broad reference ranges).
- Click “Calculate” to see QTc and the corresponding reference category.
- Use the “Compare All Methods” tab to see how different formulas behave for the same inputs.
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QTc Calculator FAQs
Frequently Asked Questions About QTc
Quick answers about corrected QT interval formulas, inputs and safe use of this tool.
You need the measured QT interval in milliseconds and the heart rate in beats per minute. The calculator converts heart rate to RR interval and applies the selected QTc formula internally.
Bazett is widely reported but may be less accurate at very high or low heart rates. Fridericia, Framingham and Hodges are alternatives that may behave better in some situations. Rather than relying on a single value, many users compare several formulas.
No. This tool is for education only. Formal ECG interpretation and decisions about medications, monitoring or further testing must always be made by a qualified clinician.
Yes, differences can occur due to measurement technique, rounding, lead selection, and the specific formula or algorithm used by the ECG device. Use machine values and this calculator only as rough references, not definitive answers.