QTc Calculator — Corrected QT Interval

About the QTc Calculator — Corrected QT Interval

The corrected QT interval (QTc) is one of the most clinically important ECG measurements. It represents the duration of ventricular depolarization and repolarization, corrected for heart rate. Prolonged QTc is a significant risk factor for Torsades de Pointes (TdP), a potentially fatal polymorphic ventricular tachycardia.

This calculator supports four validated correction formulas — Bazett (most widely used), Fridericia (recommended by the FDA for drug studies), Framingham (linear), and Hodges (linear). Each formula handles the relationship between heart rate and QT interval differently, with clinical significance at extreme heart rates where Bazett's square-root correction may over- or under-correct.

QT prolongation can be congenital (Long QT Syndrome) or acquired from medications, electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), structural heart disease, or metabolic conditions. Identifying and managing QT prolongation is a critical patient safety concern, particularly in hospitalized patients receiving multiple QT-prolonging medications. The calculator keeps the QT measurement, heart rate, and correction method together so the same ECG can be reviewed consistently across formulas.

When This Page Helps

QTc is most helpful when the raw QT interval is placed in the context of heart rate and the correction formula being used. This calculator keeps the four common correction methods side by side so a single ECG measurement can be reviewed consistently, compared across formulas, and interpreted with the same clinical framing used in routine ECG review.

How to Use the Inputs

1. Measure the QT interval from the ECG — from the beginning of the QRS complex to the end of the T wave — and enter it in milliseconds.
2. Enter the heart rate (bpm) or RR interval (ms) directly. The RR interval overrides heart rate if both are provided.
3. Select the correction formula — Bazett is standard, Fridericia is preferred for very high or low heart rates.
4. Select the patient sex (normal QTc ranges differ by sex) and enter age.
5. Optionally enter serum potassium, as hypokalemia prolongs the QT interval.
6. Review QTc, formula comparison, risk interpretation, and QT-prolonging drug reference.

Example Calculation

Tips & Best Practices

• Always check serum potassium and magnesium when QTc is prolonged — correction of electrolytes alone may normalize QTc.
• Bazett's formula is reliable between 60–100 bpm. Outside this range, use Fridericia for closer estimates.
• Check CredibleMeds.org for an up-to-date list of QT-prolonging medications before prescribing.
• For patients with QTc > 500 ms, avoid adding any additional QT-prolonging medications and consider cardiac monitoring.
• A QTc increase > 60 ms from baseline is clinically significant even if the absolute value remains below 500 ms.

Formula Choice Matters

Different correction methods are useful in different heart-rate ranges. Bazett remains familiar in routine ECG reporting, while Fridericia, Framingham, and Hodges provide alternative views when the rate is very fast or very slow. Looking at more than one formula helps show whether a QTc result is stable across methods or sensitive to the correction chosen.

Reading QTc in Context

QTc is best interpreted with the rest of the ECG and the clinical setting in mind. Rate, rhythm, medication exposure, electrolyte status, and the quality of the QT measurement all influence how much weight to place on a single value. This makes the calculator useful as a structured review aid rather than as an isolated number.

Serial Comparisons

When QTc is followed over time, the main value of the calculation is consistency. Using the same measurement method and formula allows comparison from one ECG to the next without changing the meaning of the underlying tracing. That is especially helpful when reviewing medication changes, acute illness, or inpatient ECG trends.

Frequently Asked Questions

• Why is Bazett's formula most commonly used?

  Bazett's formula (1920) was the first rate-correction formula and remains the most referenced in clinical practice and drug labeling. However, it overcorrects at high heart rates (>100 bpm) and undercorrects at low heart rates (<60 bpm). For these situations, Fridericia is more accurate.

• When is QTc prolongation dangerous?

  QTc > 500 ms is associated with significantly increased risk of Torsades de Pointes (TdP). QTc increases of > 60 ms from baseline are also concerning regardless of absolute value. The combination of QTc > 500 ms with hypokalemia or multiple QT-prolonging drugs creates the highest risk.

• What causes acquired QT prolongation?

  Common causes include medications such as antiarrhythmics, antibiotics, antipsychotics, and antiemetics; electrolyte derangements including low potassium, magnesium, or calcium; bradycardia; hypothermia; hypothyroidism; structural heart disease; and intracranial pathology. Those possibilities are usually reviewed together when a new QTc prolongation appears on ECG.

• How do I measure the QT interval correctly?

  Measure from the onset of the QRS complex to the end of the T wave in lead II or V5/V6. Use the tangent method if the T wave termination is ambiguous. Average at least 3–5 beats. Avoid measuring in U-wave-prominent leads.

• Is short QT dangerous?

  Yes. Short QT syndrome (QTc < 330–340 ms) is a rare genetic condition associated with atrial fibrillation and sudden cardiac death. Acquired short QT can occur with hypercalcemia or digitalis effect.

• Which formula does the FDA recommend?

  The FDA recommends Fridericia correction for thorough QT studies (drug development). Fridericia provides more consistent correction across a wider range of heart rates compared to Bazett, reducing false-positive and false-negative QT prolongation signals.