QTc Calculator

Introduction: Correcting the QT Interval for Heart Rate

This calculator estimates the corrected QT interval (QTc) from a measured QT interval and heart rate, using the four most common correction formulas — Bazett, Fridericia, Framingham, and Hodges. It reports each result and an interpretation against established thresholds.

The QT interval is the time, in milliseconds, from the start of the QRS complex to the end of the T wave on an electrocardiogram — roughly the time it takes the heart's ventricles to depolarize and repolarize. Because the QT interval naturally shortens as heart rate rises and lengthens as it falls, a raw QT reading at a fast or slow heart rate is hard to interpret. The correction normalizes the QT to what it would be at a standard heart rate, giving a single comparable number: the QTc[aha-2009].

A prolonged QTc is clinically important because it is associated with an increased risk of a dangerous heart rhythm called torsades de pointes, a form of ventricular arrhythmia that can lead to fainting or, rarely, sudden cardiac death. Recognizing a prolonged QTc helps clinicians identify patients who may need closer monitoring or adjustment of medications that prolong the interval[statpearls-lqts][mayo-lqts].

This is an educational tool, not a diagnosis. QTc interpretation requires a clinician's judgment in the context of the full ECG, symptoms, and the patient's medications. If you are concerned about a QT result, discuss it with a healthcare professional.

How to Use: Finding the QTc

Enter the heart rate and QT interval, choose sex, and the calculator returns all four corrected values plus an interpretation.

1. Heart rate (bpm) — from the ECG or your pulse. The RR interval is derived as 60 ÷ heart rate.
2. QT interval (ms) — from the ECG. This is the measured QT, usually read by the ECG machine or a clinician.
3. Sex — used to apply the correct threshold (women have a slightly longer normal QTc than men).

Example 1 — normal rate, normal QT. Heart rate 60 bpm, QT 400 ms. All four formulas give QTc close to 400 ms (RR = 1.0 s, so the corrections are minimal). Interpretation: within the typical threshold for both men (≤450 ms) and women (≤460 ms).

Example 2 — fast rate, borderline QT. Heart rate 100 bpm, QT 350 ms (the worked example below). Bazett gives 451.8 ms, Fridericia 415.0 ms, Framingham 411.6 ms, Hodges 420.0 ms. Note how Bazett crosses the 450 ms threshold for men while the other three formulas stay below — a live illustration of Bazett's over-correction at high heart rate[sagie-1992].

Example 3 — clearly prolonged. Heart rate 60 bpm, QT 520 ms. All four formulas give QTc above 500 ms. Interpretation: markedly prolonged, consistent with increased torsades risk — this warrants urgent clinical review[statpearls-lqts].

The Formulas: Four Ways to Correct the QT

All four formulas correct the measured QT (in ms) using the RR interval (in seconds), which equals 60 ÷ heart rate:

Bazett (1920) — the most widely used; divides QT by the square root of the RR interval:

Fridericia (1920) — divides by the cube root of RR; preferred at higher heart rates:

Framingham (Sagie 1992) — a linear correction:

Hodges (1983) — a linear correction in heart rate:

The formulas do not always agree, especially at extremes of heart rate. Bazett is the historical standard and the most reported, but it is known to over-correct at heart rates above ~100 bpm (falsely prolonging the QTc) and under-correct below ~60 bpm. For this reason, Fridericia is often preferred at high heart rates, and many modern ECG systems automatically report Fridericia alongside Bazett[pmc-hospital-qtc][sagie-1992]. The calculator shows all four so you can see how much the choice of formula matters for a given heart rate.

A Worked Example: HR 100 bpm, QT 350 ms

Let's compute the QTc with each formula. First, the RR interval: RR = 60 ÷ 100 = 0.600 s.

Bazett: QTc = 350 / √0.600 = 350 / 0.7746 = 451.8 ms.

Fridericia: QTc = 350 / 0.600^(1/3) = 350 / 0.8434 = 415.0 ms.

Framingham: QTc = 350 + 154 × (1 − 0.600) = 350 + 61.6 = 411.6 ms.

Hodges: QTc = 350 + 1.75 × (100 − 60) = 350 + 70 = 420.0 ms.

At this heart rate, only Bazett crosses the 450 ms threshold for men. This is the classic demonstration of why relying on Bazett alone at a high heart rate can overestimate the QTc — the other three formulas place the value comfortably within normal range[sagie-1992].

Reference Table: QTc at Different Heart Rates

The table shows how the four formulas diverge with heart rate, using a fixed QT of 350 ms:

The table makes the disagreement visible. At 60 bpm all four formulas agree (350 ms). At 100 bpm they spread across ~40 ms, and at 120 bpm the spread reaches ~72 ms. The choice of formula matters most when heart rate is fast — exactly the situation where a false Bazett prolongation could lead to unnecessary concern[pmc-formula-comparison].

Practical Tips: Reading the QTc

• All four formulas are estimates, not measurements. The QTc is a derived value. If the formulas disagree, the clinical context — not the formula — usually decides how it is interpreted. A clinician weighs the full ECG, symptoms, electrolytes, and medications[aha-2009].
• Prefer Fridericia at high heart rates. When heart rate is above ~100 bpm, Bazett tends to overcorrect and may falsely flag a prolonged QTc. Many modern ECG systems report both; use Fridericia as the check against Bazett's artifact[pmc-hospital-qtc].
• Sex matters for the threshold. Women have a slightly longer normal QTc than men. The calculator applies >450 ms (men) and >460 ms (women) per the AHA/ACCF/HRS 2009 guidance, and flags >500 ms as the severe/torsades threshold[aha-2009].
• A single borderline value is not a diagnosis. QTc varies with measurement, lead selection, and beat-to-beat changes. A repeated, consistently prolonged QTc with symptoms is what prompts clinical concern[statpearls-lqts].
• Know what prolongs the QT. More than a hundred medications can prolong the QT interval, as can low potassium, low magnesium, low calcium, hypothermia, and congenital long QT syndrome. The calculator estimates the value; it does not tell you why it is prolonged[mayo-lqts].
• Bundle branch block and wide QRS lengthen the QT independent of repolarization, so a wide-QRS ECG should be interpreted carefully — the correction assumes a normal QRS.
• Use the same formula when comparing repeated ECGs. Because the four formulas disagree, tracking a patient's QTc over time is only meaningful if the same formula is used consistently. Mixing Bazett one visit and Fridericia the next can create false apparent changes. Many hospital systems fix on one formula for this reason[pmc-hospital-qtc].
• Automated QTc readings are a starting point, not the final word. ECG machines compute QTc automatically, but automated measurement can be wrong at extremes of rate or with artifact. A clinician should verify the QT on the tracing before acting on the number[aha-2009].

Limitations: What This Calculator Does Not Do

• It is not a diagnostic tool. A prolonged QTc is a risk marker, not a diagnosis of long QT syndrome. Diagnosis requires clinical evaluation, family history, genetic testing in some cases, and a clinician's interpretation of the full ECG[statpearls-lqts].
• Bazett's formula is imperfect at extremes. It over-corrects at high heart rates and under-corrects at low ones, which is why the calculator shows all four formulas rather than relying on one[sagie-1992].
• The QT measurement itself is variable. Lead selection, automated vs manual measurement, and beat-to-beat variation affect the raw QT. The calculator assumes the QT you entered is accurate; it cannot correct for measurement error[aha-2009].
• Wide QRS and bundle branch block distort the correction. These prolong the QT for reasons unrelated to repolarization, so the formulas are less reliable when the QRS is wide.
• It does not replace an ECG or a clinician. The heart rate and QT inputs come from an ECG that must be interpreted by a professional. This tool is for education and estimation only.
• It does not assess risk factors (medications, electrolytes, family history). Those are essential to understanding what a QTc means for a specific person.
• Thresholds vary slightly between guidelines and laboratories. The calculator uses the AHA/ACCF/HRS 2009 convention (>450 ms for men, >460 ms for women, >500 ms for severe prolongation). Some older sources and some institutions use a stricter 440/460 ms convention. If your lab or guideline uses different cutoffs, interpret the value against those instead[statpearls-lqts].

Frequently Asked Questions

❓ What is QTc?

✅ QTc is the corrected QT interval — the QT interval adjusted for heart rate so it can be compared across different heart rates. It estimates what the QT interval would be at a standard heart rate, making it easier to assess whether the heart's repolarization is prolonged.

❓ What is a normal QTc?

✅ A commonly cited threshold is QTc of 450 ms or less for men and 460 ms or less for women, per the AHA/ACCF/HRS 2009 guidance. A QTc above 500 ms is considered markedly prolonged and associated with increased risk of torsades de pointes. Ranges can vary slightly by source.

❓ Why are there multiple QTc formulas?

✅ Because no single formula is accurate at all heart rates. Bazett is the most common but over-corrects at high heart rates and under-corrects at low ones. Fridericia, Framingham, and Hodges were developed to improve on this, and they can disagree with Bazett and with each other, especially at fast heart rates.

❓ Which QTc formula should I use?

✅ Bazett is the historical standard and most reported. However, at heart rates above about 100 bpm, Fridericia is often preferred because Bazett tends to overcorrect. Many modern ECG systems report both. The calculator shows all four so you can see the range of estimates.

❓ What does a prolonged QTc mean?

✅ A prolonged QTc is a marker of abnormal ventricular repolarization and is associated with an increased risk of torsades de pointes, a dangerous arrhythmia. It is not a diagnosis on its own — it warrants clinical evaluation, including review of medications, electrolytes, and symptoms.

❓ What causes a prolonged QT interval?

✅ Common causes include more than a hundred QT-prolonging medications, low potassium or magnesium, low calcium, hypothermia, and congenital long QT syndrome. Certain heart conditions and genetic mutations also contribute. The cause must be evaluated by a clinician.

❓ Can I calculate my QTc at home?

✅ The QTc requires a QT interval measured from an ECG, which is not available from a simple pulse measurement. The calculator helps interpret an ECG-derived QT and heart rate, but it is not a substitute for a professional ECG and clinical interpretation.

❓ Is a QTc calculator for medical use?

✅ This is an educational tool, not medical advice or a diagnostic device. QTc interpretation should be done by a qualified healthcare professional using the full clinical picture. If you have concerns about your heart rhythm, seek medical attention.