Introduction
The QTc Calculator helps you find the corrected QT interval from an electrocardiogram (ECG). The QT interval measures how long it takes your heart's ventricles to contract and then recover. Because the QT interval changes with heart rate, doctors need to correct it to get an accurate reading. This is called the corrected QT interval, or QTc. Clinical practice uses cut-offs of 450 ms for men and 470 ms for women.1 This calculator treats 350 ms as the lower edge of normal. A QTc that is too long or too short can point to serious heart rhythm problems, including a risk of sudden cardiac arrest. This tool uses common correction formulas like Bazett, Fridericia, Framingham, and Hodges to calculate your QTc quickly. Simply enter the QT interval and heart rate, and the calculator computes the result. It is a useful resource for medical students, nurses, and healthcare providers who need fast, reliable QTc results at the bedside or in the clinic.
How to Use Our QTc Calculator
Enter your QT interval and heart rate below to calculate your corrected QT interval (QTc). The calculator uses common correction formulas to adjust the QT interval for heart rate, helping you assess whether the QTc falls within a normal range.
QT Interval (ms): Enter the measured QT interval from the ECG reading in milliseconds. This is the time from the start of the Q wave to the end of the T wave. A typical uncorrected QT interval falls between 300 and 500 ms.
Heart Rate (bpm): Enter the patient's heart rate in beats per minute. You can find this on the ECG strip or by taking the pulse manually. The heart rate is needed because the QT interval naturally gets shorter as the heart beats faster and longer as it beats slower.
Correction Formula: Choose the formula you want to use for the QTc calculation. Bazett's formula is the most widely used in clinical practice, but it can overcorrect at high heart rates and undercorrect at low heart rates. Fridericia's formula is often preferred for patients with faster or slower heart rates. Other options, such as Framingham and Hodges, may also be available depending on your clinical needs.
Once you have entered all the values, the calculator will display your corrected QT interval. Clinical standards put the cut-off at 450 ms for men and 470 ms for women.1 The StatPearls long-QT chapter calls a QTc prolonged above 440 ms in men and 460 ms in women.2 A prolonged QTc may increase the risk of dangerous heart rhythms such as Torsades de Pointes. Always discuss your results with a qualified healthcare provider.
What Is a Corrected QT Interval (QTc)?
The QT interval is a measurement taken from an electrocardiogram (ECG or EKG). It tells doctors how long the heart's lower chambers (ventricles) take to electrically reset after each heartbeat. This time span runs from the start of the Q wave to the end of the T wave on the ECG tracing. The QT interval naturally gets shorter when your heart beats faster and longer when your heart beats slower. Because of this, doctors need to adjust, or "correct," the raw QT measurement for heart rate. The adjusted value is called the corrected QT interval, or QTc.
Why Does QTc Matter?
A QTc that is too long (prolonged QTc) means the heart takes longer than normal to recharge between beats. This can set the stage for a dangerous heart rhythm called Torsades de Pointes, a type of ventricular tachycardia that can cause fainting, cardiac arrest, or sudden death. A QTc greater than 500 ms is considered especially risky.2 On the other hand, a QTc that is too short can also signal a rare but serious condition called short QT syndrome, which carries its own risk of abnormal heart rhythms.
Common causes of a prolonged QTc include certain medications (such as some antibiotics, antipsychotics, and antiarrhythmic drugs), low potassium or magnesium levels in the blood, and inherited genetic conditions like long QT syndrome. Monitoring kidney function with a tool like our GFR Calculator is also important in this context, since impaired renal function can affect electrolyte balance and drug clearance, both of which influence the QT interval.
QTc Correction Formulas
There is no single perfect way to correct the QT interval for heart rate, so several formulas exist:
- Bazett's formula is the most widely used in everyday clinical practice. It divides the QT interval by the square root of the RR interval (the time between heartbeats in seconds). However, it tends to overcorrect at fast heart rates and undercorrect at slow heart rates.
- Fridericia's formula uses a cube-root correction instead. It performs better when the heart rate falls outside the normal 60–100 bpm range and is increasingly preferred in drug safety research.
- Framingham and Hodges formulas use linear math, making them more stable across different heart rates.
- Rautaharju's formula includes an adjustment for biological sex and is often used in large population studies.
Normal QTc Ranges
Normal values differ slightly between males and females. This calculator treats a QTc of 360–450 ms as normal for males, 451–470 ms as borderline and above 470 ms as prolonged; for females, 370–460 ms normal, 461–480 ms borderline and above 480 ms prolonged. In a healthy subset of 6,609 patients the 99% upper limit of normal for Bazett was higher still — 472 ms in men and 482 ms in women.1 Women naturally have slightly longer QTc values than men, which is why sex-specific thresholds are important.
How Is the QT Interval Measured?
To measure the QT interval on an ECG strip, you count the number of small boxes from the very start of the Q wave (or the R wave if no Q wave is visible) to the point where the T wave returns to the baseline. At a standard paper speed of 25 mm/sec, each small box equals 40 milliseconds.3 At 50 mm/sec, each small box equals 20 milliseconds. Multiply the number of small boxes by the appropriate value to get your QT interval in milliseconds.
Clinical Considerations
QTc values should always be interpreted alongside the full clinical picture. Doctors look at the patient's medication list, electrolyte levels (especially potassium, magnesium, and calcium), family history, and overall heart health. If a QTc is found to be prolonged, steps may include stopping or switching a medication, correcting electrolyte imbalances, and setting up continuous heart monitoring. This calculator is a helpful educational tool, but it does not replace professional medical judgment. Always consult a healthcare provider for decisions about diagnosis and treatment.