QTc Calculator
Convert a measured QT interval into a heart-rate-corrected QTc. Enter the QT interval and either a heart rate or an RR interval, choose a correction formula, and the calculator returns your QTc plus all five common corrections side by side with a sex-specific interpretation.
Formula
Worked example
With QT = 400 ms and a heart rate of 75 bpm, RR = 60 / 75 = 0.8 s. Bazett QTc = 400 / sqrt(0.8) = 447 ms (borderline for a male), while Fridericia gives 400 / cbrt(0.8) = 431 ms.
What the QTc tells you
The QT interval on an electrocardiogram measures the time from the start of the Q wave to the end of the T wave, the total span of ventricular depolarisation and repolarisation. Because this interval naturally shortens as the heart beats faster and lengthens when it slows, a raw QT value cannot be interpreted without accounting for heart rate. The corrected QT, or QTc, normalises the measurement to a reference rate of 60 beats per minute so that values measured at different pulses can be compared on a single scale. A prolonged QTc reflects delayed repolarisation, which can predispose the heart to a dangerous arrhythmia called torsades de pointes.
The five correction formulas
This calculator computes every common correction at once so you can compare them. Bazett (QTc = QT / sqrt(RR)) is the oldest and most widely used, but it over-corrects above roughly 100 bpm and under-corrects below 60 bpm. Fridericia (QTc = QT / cbrt(RR)) uses a cube root and stays more stable across heart rates, which is why drug-safety studies often prefer it. Framingham (QTc = QT + 154 x (1 - RR)) and Hodges (QTc = QT + 1.75 x (HR - 60)) are linear corrections that behave well at moderate rates. Rautaharju (QTc = QT x (120 + HR) / 180) was derived from a large population and predicts drug-induced torsades better than Bazett. When the formulas disagree, the slow or fast heart rate is usually the reason, and the more rate-stable corrections deserve the most weight.
Entering QT and heart rate
You can supply the QT interval directly in milliseconds or count small boxes on the ECG grid; at the standard 25 mm/s paper speed each 1 mm box is worth 40 ms, and at 50 mm/s it is worth 20 ms. The heart rate can be entered as beats per minute or as a measured RR interval in milliseconds, and the calculator converts between them using RR = 60000 / heart rate. The American Heart Association considers a QTc of 450 ms or more in men and 460 ms or more in women to be prolonged, with 500 ms marking a clearly elevated arrhythmia risk regardless of sex.
Using this result responsibly
The figures here are estimates generated from the numbers you enter and are not a substitute for a clinical reading. Accurate QTc depends on correctly measuring the QT interval on a properly recorded twelve-lead ECG, which can be affected by lead choice, U waves, wide QRS complexes, and atrial fibrillation. Many common medications and electrolyte disturbances, including low potassium, magnesium, or calcium, lengthen the QT interval. Any borderline or prolonged result, or a value that surprises you, should be reviewed by a qualified healthcare professional who can confirm the measurement and decide whether it is clinically significant.
QTc interpretation thresholds
| Category | Male QTc (ms) | Female QTc (ms) | Risk |
|---|---|---|---|
| Normal | < 430 | < 440 | Low |
| Borderline | 430-449 | 440-459 | Borderline |
| Prolonged | 450-499 | 460-499 | High |
| Markedly prolonged | >= 500 | >= 500 | High |
Bazett-corrected QTc bands from AHA/ACCF/HRS guidance. Thresholds differ slightly by sex; >=500 ms is high-risk for either sex.
Frequently asked questions
What is a normal QTc value?
A QTc up to about 450 ms in men and 460 ms in women is generally considered normal. Values from there to 500 ms are prolonged and merit review, while a QTc of 500 ms or more carries a markedly higher risk of dangerous arrhythmias in either sex.
Which QTc formula should I use?
Bazett is the traditional default and is what most thresholds are quoted against. Fridericia and Rautaharju stay more accurate at fast or slow heart rates and are often preferred in drug-safety work. This calculator shows all five so you can compare; when they disagree, trust the more rate-stable corrections.
Can I enter QT by counting boxes on the ECG?
Yes. Switch the QT entry method to small boxes and set the paper speed. At the standard 25 mm/s each 1 mm box equals 40 ms, so a QT spanning 10 boxes is 400 ms. At 50 mm/s each box equals 20 ms.
Why does heart rate need to be corrected for?
The raw QT interval shortens at faster heart rates and lengthens at slower ones, so the same heart can show very different QT values minute to minute. Correcting to a standard rate of 60 bpm produces the QTc, a single comparable number that reflects repolarisation independent of pulse.