Calculate the heart-rate corrected QT interval (QTc) from a measured QT interval and heart rate.
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The QT interval, measured on an electrocardiogram (ECG), represents the time it takes the heart's ventricles to electrically depolarize and then repolarize between heartbeats. Because the raw QT interval naturally shortens at faster heart rates and lengthens at slower ones, it's typically adjusted, or "corrected," for heart rate before being interpreted — the result is called QTc. This calculator computes QTc using two widely used formulas: Bazett's formula, the most commonly cited in clinical practice, and Fridericia's formula, which is generally considered more accurate at heart rates well above or below a normal resting rate, since Bazett's formula is known to overcorrect at fast heart rates and undercorrect at slow ones. Reference ranges for a normal QTc are commonly cited as roughly up to 430 ms for men and up to 450 ms for women, with higher values considered borderline or prolonged, and values above about 500 ms associated with a meaningfully increased risk of dangerous arrhythmias. This calculator is intended purely as an educational reference; a prolonged QTc always warrants evaluation by a qualified physician.
Every heartbeat captured on an electrocardiogram traces out a sequence of waves, and the QT interval — measured from the start of the QRS complex to the end of the T wave — represents the total time the heart's ventricles spend electrically depolarizing and then repolarizing before the next beat. This interval isn't fixed: it naturally shortens when the heart beats faster and lengthens when it beats slower, purely as a function of the shorter or longer time available between beats. Because of this relationship, comparing a raw QT interval measured at one heart rate to a reference range, or to a previous ECG taken at a different heart rate, can be misleading unless the value is first adjusted, or corrected, for heart rate — producing the QTc, or corrected QT interval, that clinicians actually use for interpretation.
The most widely used correction, Bazett's formula, divides the QT interval by the square root of the RR interval (the time between consecutive heartbeats, in seconds). Introduced in 1920, it remains the default formula built into most ECG machines and used in most clinical settings, largely because of its simplicity and long history. However, Bazett's formula has a well-documented weakness: it tends to overcorrect, producing an artificially high QTc, at heart rates faster than about 60 to 100 beats per minute, and undercorrect, producing an artificially low QTc, at heart rates slower than that range. Fridericia's formula, which divides QT by the cube root of the RR interval rather than the square root, produces a gentler correction that holds up better across a wider range of heart rates, which is why many cardiologists prefer it, particularly when heart rate is unusually fast or slow. This calculator reports both values so they can be compared directly.
Reference ranges for QTc differ slightly by sex, reflecting a well-established physiological difference in baseline QT duration. Values up to roughly 430 ms in men and 450 ms in women are generally considered within the normal range; QTc values a little above that, up to about 450 ms in men or 470 ms in women, are often described as borderline; and QTc values beyond those thresholds are generally considered prolonged. A QTc above roughly 500 ms, in either sex, is a level clinicians pay particularly close attention to, since it's associated with a meaningfully increased risk of a dangerous ventricular arrhythmia called torsades de pointes.
A prolonged QTc has a wide range of possible causes, and identifying which one applies to a specific person is a task for a physician working from a full clinical picture, not a single calculated number. Certain medications — including some antiarrhythmics, antibiotics, antipsychotics, and antihistamines — are well known to prolong the QT interval as a side effect, which is why QTc monitoring is a routine part of starting or adjusting many of these drugs. Electrolyte imbalances, particularly low potassium, low magnesium, and low calcium, are another common and often reversible cause. Less commonly, an inherited condition called Long QT Syndrome can cause a prolonged QTc from birth, sometimes discovered only after a family member experiences an unexplained fainting episode or cardiac event.
Because so many different factors can prolong or shorten a QTc, and because accurate QT measurement itself requires a properly recorded and correctly interpreted ECG, this calculator is best used as an educational tool for understanding the correction math involved, not as a substitute for an actual ECG reading. Manual QT measurement from an ECG tracing also carries its own margin of error, particularly when the T wave is difficult to distinguish from a following U wave, which is one more reason a single QTc number should never be interpreted in isolation. Any concern about a prolonged QTc, a family history of sudden cardiac events, or symptoms like unexplained fainting should always be discussed directly with a qualified physician.
QTc is the QT interval on an ECG adjusted for heart rate, since the raw QT interval naturally shortens at faster heart rates and lengthens at slower ones; correcting for heart rate allows QT intervals measured at different heart rates to be compared on the same scale.
Commonly cited reference ranges are roughly up to 430 ms for men and up to 450 ms for women, with values above about 450 ms (men) or 470 ms (women) generally considered prolonged; a QTc above about 500 ms is associated with a meaningfully increased arrhythmia risk regardless of sex.
Bazett's formula is the most widely used in clinical practice, but it's known to overcorrect (overestimate QTc) at faster heart rates and undercorrect (underestimate QTc) at slower heart rates; Fridericia's formula uses a cube root instead of a square root and is generally considered more accurate outside a normal resting heart rate range.
A prolonged QTc can result from certain medications, electrolyte imbalances (such as low potassium, magnesium, or calcium), inherited long QT syndromes, and various cardiac or systemic conditions — the specific cause requires clinical evaluation by a physician.
No. This calculator only performs the mathematical QTc correction from the numbers you enter; diagnosing Long QT Syndrome or assessing arrhythmia risk requires a full ECG reading and clinical evaluation by a qualified physician.