qtc fridericia calculator

What is QTc Fridericia?

The QT interval on an ECG changes with heart rate. At faster rates, QT shortens; at slower rates, it lengthens. To compare QT values fairly, clinicians use a heart-rate correction, producing a corrected QT (QTc). The Fridericia correction is one commonly used method, especially in clinical pharmacology and drug safety work.

Fridericia Formula

QTc_F = QT / RR^1/3

• QT: measured QT interval in seconds
• RR: interval between heartbeats in seconds
• If heart rate (HR) is known, then RR = 60 / HR

This calculator handles milliseconds for convenience and converts units automatically.

How to use this calculator

1. Enter the measured QT interval in milliseconds.
2. Enter either heart rate (bpm) or RR interval (ms).
3. Select interpretation reference (male, female, or unspecified).
4. Click Calculate QTcF.

The result box shows QTc Fridericia (ms), estimated RR used in the calculation, and a basic interpretation band.

Quick interpretation guide

Reference	Likely Normal	Borderline	Prolonged
Male	≤ 430 ms	431-450 ms	> 450 ms
Female	≤ 450 ms	451-470 ms	> 470 ms
Unspecified	< 440 ms	440-469 ms	≥ 470 ms

A QTc around or above 500 ms is generally considered markedly prolonged and may require urgent clinical attention depending on context.

Why Fridericia instead of Bazett?

Bazett correction (QTcB = QT / √RR) is very common but can over-correct at high heart rates and under-correct at low heart rates. Fridericia (cube-root correction) often performs better across a wider heart-rate range, which is why it is widely used in QT safety evaluations.

Common pitfalls

• Using poor-quality ECG tracing or incorrect lead selection.
• Confusing milliseconds and seconds.
• Applying a hard threshold without considering symptoms, meds, electrolytes, and clinical history.
• Ignoring rhythm issues (e.g., atrial fibrillation) that can make QT assessment less reliable.

Example calculation

If QT = 400 ms and HR = 60 bpm:

• RR = 60 / 60 = 1.0 s
• QT = 0.400 s
• QTcF = 0.400 / (1.0)^1/3 = 0.400 s = 400 ms

If the same QT is measured at HR 100 bpm (RR = 0.6 s), QTcF becomes higher after correction because the observed QT is being adjusted for the faster heart rate.

Clinical reminder

This tool is educational and should not replace professional interpretation. QT/QTc assessment is context-dependent and should be reviewed by qualified clinicians, particularly if there are symptoms (syncope, palpitations), medication risks, congenital long QT concerns, or electrolyte abnormalities.