QTc Interval Calculator (Bazett & Fridericia)
Overview
The QT interval is the time from the start of ventricular depolarization to the end of ventricular repolarization on an ECG—classically measured from the beginning of the QRS complex (often the Q wave) to the end of the T wave. Because QT duration changes with heart rate, clinicians often use a corrected QT (QTc) to compare QT values across different heart rates.
This calculator estimates QTc using two widely used correction methods:
- Bazett (most commonly reported, but biased at very fast/slow heart rates)
- Fridericia (often more reliable than Bazett at higher heart rates)
Before you calculate: what to enter
QT interval (ms)
Enter the measured QT interval in milliseconds. On a standard ECG, QT is typically measured from the beginning of QRS to the end of the T wave (avoid including U waves unless clinically indicated). Measurement approaches vary by rhythm, lead selection, and guideline.
RR interval (ms)
Enter the RR interval in milliseconds (time between two consecutive R peaks). If your ECG gives heart rate (HR) rather than RR, you can convert:
- RR (seconds) = 60 / HR
- RR (ms) = 60000 / HR
Formulas (with units)
Most QT correction formulas assume RR is in seconds. This tool accepts RR in milliseconds and converts it internally to seconds.
Bazett
Where QT is in milliseconds and RR is in seconds (after conversion). Conceptually, this is:
QTcBazett = QT / √RR
Fridericia
Conceptually:
QTcFridericia = QT / RR1/3
How to interpret QTc results (general guidance)
QTc cutoffs vary by guideline, age, sex, clinical context, ECG method, and medication status. Still, clinicians often use approximate ranges like the following for adults (educational overview only):
| QTc category | Typical adult thresholds (approx.) | What it may mean |
|---|---|---|
| Normal | Men: < 450 ms Women: < 460 ms |
Usually low concern in isolation; interpret with symptoms and clinical context. |
| Borderline | Men: ~450–470 ms Women: ~460–480 ms |
May warrant review of medications, electrolytes, and repeat ECG depending on context. |
| Prolonged | ≥ 470 ms (men) or ≥ 480 ms (women) (varies) | Higher concern; consider causes (drugs, electrolytes, congenital, ischemia, etc.). |
| Markedly prolonged | ≥ 500 ms (commonly used high-risk flag) | Associated with increased risk of torsades de pointes, especially with triggers. |
Bazett vs Fridericia: Bazett is widely reported but can overestimate QTc at high heart rates and underestimate at low heart rates. Fridericia may perform better at higher heart rates, but no correction is perfect—especially in irregular rhythms.
Worked example
Scenario: An ECG shows QT = 420 ms and RR = 800 ms (which corresponds to HR ≈ 75 bpm).
- Convert RR to seconds: RR = 800 ms = 0.8 s
- Bazett: QTc = 420 / √0.8 = 420 / 0.894 ≈ 470 ms
- Fridericia: QTc = 420 / (0.8)^(1/3) = 420 / 0.928 ≈ 453 ms
This example shows why reporting multiple correction methods can matter: the two QTc estimates can differ meaningfully at the same QT and RR.
Practical tips for measurement
- Lead selection matters: QT can differ slightly by lead; many clinicians measure in leads where the T-wave end is clearest.
- Use a representative beat: Avoid ectopic beats and measure in a stable segment.
- Confirm rhythm regularity: QT correction is less reliable with irregular RR intervals (e.g., atrial fibrillation).
- Consider clinical context: Symptoms (syncope, palpitations), family history, and meds/electrolytes are critical.
Assumptions & limitations
- Educational use: This calculator provides estimates and is not a diagnosis or treatment tool.
- RR unit conversion: The formulas assume RR in seconds; the calculator converts from ms. Entering RR in the wrong unit will produce incorrect results.
- Irregular rhythms: In atrial fibrillation or frequent ectopy, a single RR interval may not represent typical cycle length; corrections can be unreliable.
- Heart-rate extremes: Bazett is notably biased at very fast or very slow heart rates; Fridericia may be preferable at higher HR, but neither is perfect.
- Measurement variability: Manual vs automated QT, U waves, T-wave morphology, and baseline artifact can change QT by tens of milliseconds.
- Population differences: Pediatrics, pregnancy, bundle branch block, pacing, and certain cardiomyopathies may require specialized interpretation.
- Threshold variability: “Normal” and “prolonged” cutoffs vary among guidelines and clinical settings.
When to seek medical advice
If an ECG (or this estimate) suggests a markedly prolonged QTc (often cited as ≥ 500 ms), or if you have symptoms such as fainting, seizures, or sustained palpitations—especially while taking QT-prolonging medications—seek urgent medical evaluation. Always discuss QT/QTc results with a qualified clinician.
References (high-level)
General QT/QTc concepts and clinical thresholds are discussed across major cardiology guidelines and ECG reference standards. Thresholds and preferred correction formulas can vary by organization and clinical use case.