Question 1

Why does QT need to be corrected for heart rate?

Accepted Answer

The QT interval naturally shortens as heart rate increases and lengthens as it slows, so a raw QT reading can’t be compared across different heart rates. QTc estimates what the QT interval would be at a standardized heart rate of 60 bpm, making it comparable between readings and patients.

Question 2

Which correction formula should I use?

Accepted Answer

Bazett is the most widely used in clinical practice and most ECG machines report it by default, but it’s known to over-correct at fast heart rates and under-correct at slow ones. Fridericia is generally considered more accurate across a wider range of heart rates and is preferred in research and drug-safety studies. Framingham and Hodges are linear alternatives sometimes used for the same reason.

Question 3

What QTc is considered prolonged?

Accepted Answer

Commonly cited thresholds are roughly QTc > 450 ms in men and > 460 ms in women as prolonged, with values ≥ 500 ms considered high risk for the dangerous arrhythmia torsades de pointes. Exact cutoffs vary slightly between guidelines and clinical contexts.

Question 4

Can I use this instead of an ECG interpretation?

Accepted Answer

No. This calculator only applies a formula to numbers you type in. QT measurement itself is technically difficult (choosing where the T wave ends, handling U waves, picking the right lead) and best done by a clinician or validated ECG software directly from the tracing.

Question 5

What commonly causes QTc prolongation?

Accepted Answer

Many medications (including certain antiarrhythmics, antibiotics, antipsychotics, and antiemetics), electrolyte abnormalities (low potassium, magnesium, or calcium), and inherited long QT syndromes are common causes. If you’re concerned about a specific medication or symptom, talk to a healthcare provider rather than relying on a calculator.

QTc	Interpretation
≤ 450 ms (men) / ≤ 460 ms (women)	Typically normal
Above normal, under 500 ms	Prolonged
≥ 500 ms	Markedly prolonged — higher torsades de pointes risk

QTc Calculator

Correction formulas

Typical QTc ranges

Common questions