QTc Calculator
Use this QT calculator to estimate corrected QT (QTc) from a measured QT interval and heart rate. It reports Bazett, Fridericia, Framingham, and Hodges formulas and highlights the selected method.
Educational tool only. Always interpret ECG findings in full clinical context.
What is a QT calculator?
A QT calculator helps estimate the corrected QT interval (QTc), which adjusts the raw QT interval for heart rate. Because the QT interval naturally shortens at faster heart rates and lengthens at slower rates, using QTc gives clinicians a better way to compare repolarization timing across different pulse rates.
If you have ever searched for a “qt calculator,” you were likely trying to answer one practical question: is this QT interval normal for this heart rate? That is exactly what this page is designed to help with.
Why QTc matters clinically
QT prolongation can increase risk of ventricular arrhythmias, including torsades de pointes, especially when QTc is markedly prolonged. QTc interpretation becomes especially important when:
- Starting or monitoring QT-prolonging medications
- Reviewing electrolyte disorders (low potassium, magnesium, or calcium)
- Assessing syncope, palpitations, or inherited channelopathy risk
- Comparing serial ECGs in hospitalized or high-risk patients
How this QT calculator works
Inputs
- Measured QT (ms): The QT interval from ECG.
- Heart rate (bpm): Used to estimate RR interval in seconds (RR = 60 / HR).
- Sex: Used only for broad reference interpretation thresholds.
- Formula selection: Chooses the primary QTc result shown at the top.
Formulas included
- Bazett: QTc = QT / √RR
- Fridericia: QTc = QT / RR^(1/3)
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
QT values are in milliseconds, RR in seconds, and final QTc values are reported in milliseconds.
Interpreting your result
This qt calculator gives a simple interpretation based on common adult cutoffs. Different institutions and patient populations can use slightly different ranges, so think of this as a quick screening aid rather than a final diagnosis.
- Markedly prolonged: QTc ≥ 500 ms (higher risk zone)
- Prolonged: usually > 450 ms in men, > 460 ms in women
- Short QT range: roughly < 350 ms (context-dependent)
- Normal range: between lower and upper thresholds above
Example
Case scenario
Suppose QT = 440 ms and heart rate = 90 bpm. RR becomes 0.67 s. Depending on formula, QTc will vary. Bazett typically gives a higher value at faster heart rates, while Fridericia often behaves more consistently in tachycardia. That is why many clinicians review more than one correction method.
Common mistakes to avoid
- Using a poorly measured QT interval from a low-quality tracing
- Ignoring U waves that may falsely lengthen or confuse endpoint selection
- Treating one formula as perfect in every heart-rate range
- Ignoring medications, electrolytes, and symptoms
Best practices for QT evaluation
Before you calculate
- Confirm ECG quality and lead selection.
- Measure QT carefully and consistently.
- Check current medication list for QT-risk agents.
After you calculate
- Review potassium, magnesium, and calcium if QTc is prolonged.
- Compare with prior ECGs rather than relying on a single value.
- Escalate urgent findings (especially QTc ≥ 500 ms with symptoms).
Final note
This online qt calculator is intended for education and quick bedside estimation. It does not replace formal ECG interpretation or professional medical judgment. If you are managing a patient with concerning ECG findings, syncope, or medication-related risk, use local protocols and specialist consultation when needed.