How this instrument works
The QT interval is measured on an ECG from the start of the Q wave to the end of the T wave, spanning the time the ventricles spend depolarizing and then recovering before the next beat. Because that window naturally compresses as the heart beats faster, comparing raw QT values between two people — or the same person at rest versus mid-exercise — says more about pulse than about the heart's electrical recovery. QTc removes heart rate from the comparison by dividing the measured interval by the square root of the RR interval, the gap between consecutive beats in seconds.
The correction traces back to a 1920 paper by Henry Bazett, built from measurements taken in just 39 subjects — a sample size that would draw scrutiny under modern statistical standards. Even so, the square-root formula became the default correction wired into ECG machines and clinical guidelines, and it has stayed there for a century largely because nothing simpler ever displaced it once manufacturers built it into their software.
Bazett's formula has a well-documented blind spot: it over-corrects at fast heart rates and under-corrects at slow ones, stretching QTc upward for a tachycardic patient and compressing it for a bradycardic one in ways that don't match reality as closely as they should. Lars Fridericia proposed a cube-root alternative that same year, and it tracks measured values more evenly outside the roughly 60-to-100-bpm range where Bazett's method is least reliable — one reason clinicians reach for Fridericia's version when a heart rate sits well outside normal resting bounds.
- Enter QT interval in milliseconds, measured on the ECG from the start of the Q wave to the end of the T wave.
- Enter Heart rate in beats per minute, read from the same strip at the same moment as the QT measurement.
- Read RR interval in seconds — sixty divided by heart rate — the term Bazett's correction divides against.
- Read QTc in milliseconds, the corrected value used to judge whether ventricular recovery looks prolonged.
Worked example — 360 ms QT at 100 bpm
A measured QT of 360 ms at a heart rate of 100 bpm gives an RR interval of 60 / 100 = 0.6 seconds. Dividing the QT by the square root of that: 360 / √0.6 ≈ 360 / 0.7746 ≈ 464.8 ms — noticeably longer than the raw 360 ms, because Bazett's correction stretches hardest at faster heart rates like this one.
Slow the same heart to exactly 60 bpm and the RR interval becomes a clean 1.0 second. A 400 ms QT then needs no adjustment at all: 400 / √1 = 400 ms — the single heart rate at which Bazett's formula and the raw measurement always agree exactly.
Questions
Why does a QT interval need correcting for heart rate at all?
Because ventricular repolarization naturally speeds up along with the heart. Without correction, a fast pulse would make the electrical recovery time look artificially short and reassuring, while a slow pulse would make it look artificially prolonged — neither reflects a genuine change in cardiac electrical safety, only the pulse at the moment of measurement.
What counts as a normal QTc value?
Roughly under 440-450 ms, depending on sex and which guideline is used — women tend to run a touch longer than men on average. Values further above that raise the statistical odds of a dangerous rhythm called torsades de pointes, but a single elevated reading is informational, not a diagnosis, and belongs in front of a clinician with the full ECG and history.
Why do some clinicians use Fridericia's formula instead of Bazett's?
Because Bazett's square-root correction over-corrects above roughly 100 bpm and under-corrects below roughly 60 bpm, drifting from what's actually being measured. Fridericia's cube-root version, published the same year, tracks more evenly outside that band. Bazett's remains the default on most ECG hardware simply because it was there first.
Is a long QTc dangerous?
A prolonged QTc raises the statistical risk of torsades de pointes, a potentially life-threatening rhythm — but risk is not certainty, and this calculator only performs Bazett's arithmetic on the numbers given. It cannot see medications, electrolyte levels, or a family history that a clinician would weigh alongside the raw figure.
Does this tool diagnose long QT syndrome?
No. It applies one correction formula to a QT and heart rate you supply; it has no knowledge of the patient, their medications, potassium or calcium levels, or inherited channelopathies that also lengthen QT. Any concerning value deserves review by a clinician with the complete picture, not a standalone number.
Why did the original Bazett study use only 39 people?
Because it was published in 1920, decades before modern clinical trial standards existed. Henry Bazett fit a simple square-root relationship to the data he had on hand, and it stuck — partly on merit, partly because it was the first correction available and later got built directly into ECG machine firmware.
References
- Bazett HC 1920, reprinted Ann Noninvasive Electrocardiol 1997 (Wiley)
- Fridericia LS 1920 — translated classic article (PMC)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.