Sex affects interpretation thresholds — women have a naturally longer QTc than men
Measure from the start of the Q wave to the end of the T wave on ECG. Range: 200–700 ms typical.
Beats per minute (bpm). Range 10–300 bpm. Bazett is most accurate at 60–100 bpm.
Enter QT Interval and Heart Rate
Input your measured QT interval in milliseconds and heart rate in bpm (or RR interval) to see all five corrected QTc values, gender-specific interpretation, and the formula comparison chart.
How to Use the QTc Calculator
Select Biological Sex
Choose Male or Female. This determines which gender-specific classification thresholds are applied to each formula result. Women have a naturally longer QTc than men, so normal and borderline upper limits differ by approximately 20 ms between sexes. The ACC/HRS guidelines define normal QTc as less than 440 ms for men and less than 460 ms for women.
Enter the Measured QT Interval
Type the raw QT interval in milliseconds, measured from the beginning of the Q wave to the end of the T wave on the ECG. Use lead II or V5 for the most consistent measurement. If the T wave is difficult to identify, use the tangent method: extend a line along the steepest downslope of the T wave until it crosses the isoelectric baseline. A typical normal QT is 350–450 ms.
Enter Heart Rate or RR Interval
Select whether to input a Heart Rate (bpm) or a direct RR Interval (ms). If entering heart rate, the calculator automatically computes RR = 60,000 / HR. If you measured the RR interval directly from the ECG paper (distance between consecutive R waves), switch to RR interval mode. At 25 mm/sec paper speed, each large box is 200 ms and each small box is 40 ms.
Review All Five Formula Results
Results appear instantly for all five formulas — Bazett, Fridericia, Framingham, Hodges, and Rautaharju. Each result includes a classification badge (Normal, Borderline, Prolonged, Severely Prolonged, or Short QT). The bar chart compares all values visually. Review the Formula Accuracy Guidance section to determine which formula is most appropriate for your patient's heart rate. Export to CSV or print results for documentation.
Frequently Asked Questions
Why do the five QTc formulas give different results?
Each formula uses a different mathematical approach to adjust for heart rate. Bazett uses a square root correction, Fridericia uses a cube root, Framingham applies a linear correction based on RR interval, Hodges adds a linear term based directly on heart rate, and Rautaharju uses a multiplicative correction derived from a large population dataset. At heart rates near 60 bpm, all formulas produce similar results because corrections are minimal. The differences become most pronounced at very fast (above 100 bpm) or very slow (below 60 bpm) heart rates, where Bazett's nonlinear correction diverges significantly from the more linear formulas. In practice, the choice of formula matters most when the heart rate is outside the normal resting range of 60–100 bpm.
What QTc value requires urgent clinical action?
A QTc exceeding 500 ms is considered a critical threshold across all major guidelines — ACC/HRS, ESC 2022, and CredibleMeds — and is associated with a substantially elevated risk of Torsades de Pointes (TdP) ventricular arrhythmia. In this situation, QT-prolonging medications should be reviewed and withheld where possible, electrolytes (potassium, magnesium, calcium) should be corrected aggressively, continuous cardiac monitoring should be initiated, and specialist cardiology input should be obtained. The 2022 ESC Guidelines also define QTc at or above 480 ms as a criterion for diagnosing Long QT Syndrome in appropriate clinical context, even without symptoms. Short QTc at or below 340 ms likewise warrants evaluation for Short QT Syndrome.
Which formula should I use for a patient with tachycardia?
For patients with heart rates above 100 bpm, the Bazett formula is known to overcorrect, producing falsely elevated QTc values that may not reflect true repolarization prolongation. In tachycardia, Fridericia (cube-root correction) and Framingham (linear correction) consistently outperform Bazett in population studies and are preferred by the FDA for drug safety assessments. Rautaharju's formula, derived from over 57,000 subjects, also minimizes overcorrection at extreme heart rates. When Bazett gives a prolonged result in a tachycardic patient but Fridericia and Framingham are normal, the Bazett result may represent formula overcorrection rather than true pathological QT prolongation. Document which formula was used and why when recording QTc in clinical notes.
Why is the QTc threshold different for men and women?
Women naturally have a longer QTc than men due to hormonal influences — particularly the role of testosterone in shortening cardiac repolarization in men. This sex difference is well established in large epidemiological studies and is reflected in gender-specific clinical thresholds from the ACC/HRS and ACEP. Normal QTc for men is generally accepted as below 440–450 ms, while for women it is below 460 ms. This means a value of 455 ms might be borderline in a man but normal in a woman. Women are also more susceptible to drug-induced TdP than men, accounting for approximately 65–70% of drug-induced TdP cases despite representing only 50% of the population. This biological predisposition makes gender-specific QTc thresholds clinically essential.
Can medications affect QTc even if the value is still within the normal range?
Yes. The absolute QTc value is only part of the picture. Drug-induced QT prolongation is typically assessed by the change from baseline QTc (delta QTc), not just the final value. The FDA Intensive QT monitoring guidelines define a QTc increase of more than 10 ms above baseline as concerning, and an increase above 20 ms as requiring further investigation. A patient whose baseline QTc is 420 ms and rises to 450 ms after starting a QT-prolonging antibiotic has experienced a 30 ms delta — clinically significant even though 450 ms may fall within the borderline range. Always obtain a baseline QTc before initiating high-risk medications and monitor for QTc change during treatment, not just absolute values.
What are the most common causes of acquired QT prolongation?
Acquired (non-congenital) QT prolongation has multiple common causes that often act together to increase TdP risk. Medications are the most frequent cause — particularly antibiotics (macrolides and fluoroquinolones), antipsychotics (haloperidol, quetiapine), antidepressants (citalopram, tricyclics), antiemetics (ondansetron, droperidol), and antiarrhythmics (amiodarone, sotalol, quinidine). Electrolyte disturbances are the second most common cause: hypokalemia impairs cardiac repolarization directly, as does hypomagnesemia and hypocalcemia. Bradycardia itself prolongs the QT interval independently of the corrected value. Systemic conditions including hypothyroidism, hypothermia, and structural heart disease also contribute. Female sex, older age, and underlying subclinical genetic LQTS variants increase individual susceptibility to all these triggers.