QTc Calculator - Free Online Tool

Corrected QT interval using all 5 formulas with gender-specific interpretation and TdP risk assessment

Welcome to our free QTc Calculator, the most comprehensive corrected QT interval tool available online. Unlike other calculators that show only a single formula result, ours computes all five clinically validated correction formulas simultaneously — Bazett, Fridericia, Framingham, Hodges, and Rautaharju — so you can compare results and choose the most appropriate correction for your patient's heart rate range. The corrected QT interval, or QTc, is a measurement derived from the electrocardiogram (ECG) that represents the duration of ventricular depolarization and repolarization, adjusted for heart rate. The raw QT interval shortens at faster heart rates and lengthens at slower ones, making direct comparison between patients — or the same patient at different heart rates — unreliable without correction. QTc standardizes the QT interval as though the heart rate were exactly 60 beats per minute, enabling clinically meaningful interpretation. QTc prolongation is one of the most important safety signals in clinical medicine. A prolonged QTc predicts risk of a life-threatening arrhythmia called Torsades de Pointes (TdP), a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. According to multiple guidelines, QTc values above 500 ms carry a particularly high risk of TdP and should prompt immediate clinical attention. QTc values between the normal upper limit and 500 ms represent a spectrum of increasing risk that requires careful clinical judgment. This tool is designed for clinicians, pharmacists, medical students, and nurses who regularly assess QTc intervals on ECGs. It is equally useful for patients with known Long QT Syndrome (LQTS) who monitor their own QTc, and for pharmacists screening for drug-induced QT prolongation before prescribing medications from high-risk categories. The calculator accepts either a heart rate (in bpm) or a direct RR interval measurement (in milliseconds), making it compatible with both manual ECG reading and data from cardiac monitors. Gender selection is required because women have a naturally longer QTc than men, and clinical thresholds differ by sex. Normal QTc for men is less than 440 ms (ACEP) or less than 450 ms (ACC/HRS); for women, normal is less than 460 ms. Values above 500 ms are considered severely prolonged for both sexes. Our tool also includes a built-in drug-induced QT prolongation reference panel listing the major drug classes associated with QT prolongation — antibiotics, antipsychotics, antidepressants, antihistamines, antiemetics, antiarrhythmics, antimalarials, methadone, antifungals, and antiretrovirals — along with the CredibleMeds risk tier framework (Known, Possible, and Conditional TdP risk). This information helps clinicians quickly assess whether a patient's QTc prolongation may be medication-related. All five formulas are presented with individual classification labels (Normal, Borderline, Prolonged, Severely Prolonged, or Short QT) based on sex-specific thresholds. The visual bar chart compares all five QTc values side by side, making it immediately apparent which formula produces the most extreme correction. Bazett's formula — the most widely used — is displayed as the primary result, but formula accuracy guidance notes that Bazett overcorrects at heart rates above 100 bpm and undercorrects below 60 bpm. For patients with extreme heart rates, Fridericia or Framingham may provide more accurate correction. All calculations run entirely in your browser. No patient data is transmitted to any server, making this tool safe to use in clinical environments with strict data privacy requirements.

Understanding the QTc Interval

What Is the QTc Interval?

The QT interval, measured on an electrocardiogram from the beginning of the Q wave to the end of the T wave, represents the total time required for ventricular myocardial cells to depolarize and then repolarize. Because this duration is strongly influenced by heart rate — shortening at faster rates and lengthening at slower rates — clinicians use rate-corrected formulas to standardize the measurement. The result is the QTc (corrected QT interval), expressed in milliseconds (ms). A QTc above gender-specific thresholds indicates prolonged ventricular repolarization, which increases susceptibility to early afterdepolarizations and, ultimately, Torsades de Pointes arrhythmia. QTc is one of the most clinically important measurements derived from the 12-lead ECG and is monitored routinely for patients on QT-prolonging medications, those with congenital Long QT Syndrome, and anyone presenting with syncope or palpitations.

How Are the Five Formulas Calculated?

Five correction formulas are widely used in clinical practice, each with different mathematical approaches and optimal use cases. The Bazett formula (1920) divides the QT interval by the square root of the RR interval in seconds: QTcB = QT / sqrt(RR). It remains the most widely used formula in clinical practice and electronic medical records. The Fridericia formula (1920) divides QT by the cube root of RR: QTcFri = QT / cbrt(RR). It corrects more linearly than Bazett and is FDA-recommended for drug safety trials. The Framingham formula (1992) uses a linear correction: QTcFra = QT + 154 x (1 − RR), derived from the Framingham Heart Study and considered superior to Bazett for mortality prediction. The Hodges formula (1983) uses heart rate directly: QTcHod = QT + 1.75 x (HR − 60), providing a linear additive correction. The Rautaharju formula (2014) was derived from 57,595 subjects: QTcR = QT x (120 + HR) / 180, offering improved performance at extreme heart rates.

Why Does QTc Prolongation Matter?

QTc prolongation is dangerous primarily because it predisposes the heart to Torsades de Pointes (TdP), a polymorphic ventricular tachycardia that can produce syncope, cardiac arrest, and sudden death. The 2022 ESC Guidelines define QTc at or above 480 ms as a diagnostic criterion for Long QT Syndrome, and QTc above 500 ms as especially high risk for TdP. Numerous medications can prolong the QTc interval by blocking cardiac potassium channels — especially the IKr (hERG) channel — and the risk is compounded by electrolyte imbalances (particularly hypokalemia and hypomagnesemia), bradycardia, hypothyroidism, and female sex. Pharmacists and prescribers routinely check QTc before initiating antibiotics, antipsychotics, antiarrhythmics, and antiemetics known to carry TdP risk. Monitoring QTc during treatment with these agents is a key patient safety practice that prevents preventable sudden cardiac deaths.

Limitations and Clinical Caveats

No single correction formula perfectly adjusts QT across all heart rates and patient populations. The Bazett formula, despite its widespread use, is known to overcorrect at heart rates above 100 bpm (tachycardia) and undercorrect at heart rates below 60 bpm (bradycardia) — errors that can lead to false prolongation or false reassurance. For patients with very fast or very slow heart rates, Fridericia or Framingham formulas generally provide more accurate correction. Additionally, QTc measurement itself is subject to intraobserver and interobserver variability of 10–20 ms depending on the measurement technique (tangent versus threshold method), ECG quality, and presence of bundle branch blocks, Wolff-Parkinson-White, or pacing artifacts. Automated QTc measurements from ECG machines may differ from manual measurements. Clinical decisions should never be made on a single QTc value in isolation — trends over time, clinical context, medication history, and electrolyte status all contribute to accurate QTc interpretation.

QTc Correction Formulas

Bazett Formula

QTcB = QT / √(RR)

The most widely used QTc correction formula (1920). Divides the measured QT interval by the square root of the RR interval in seconds. Accurate at heart rates of 60–100 bpm but overcorrects in tachycardia and undercorrects in bradycardia.

Fridericia Formula

QTcFri = QT / ∛(RR)

Uses the cube root of the RR interval for correction (1920). FDA-recommended for drug safety clinical trials. Provides more linear correction than Bazett and performs better at elevated heart rates.

Hodges Formula

QTcHod = QT + 1.75 × (HR − 60)

A linear additive correction using heart rate directly (1983). Adds 1.75 ms for each bpm above 60. Good performance across typical clinical heart rate ranges without the nonlinear distortion seen in Bazett.

Framingham Formula

QTcFra = QT + 154 × (1 − RR)

Linear correction derived from the Framingham Heart Study (1992). Considered superior to Bazett for mortality prediction. Uses the RR interval in seconds for a straightforward additive adjustment.

QTc Reference Tables

Normal QTc Ranges by Biological Sex

Gender-specific QTc classification thresholds based on ACC/HRS and ACEP guidelines. Women have naturally longer QTc intervals due to hormonal influences on cardiac repolarization.

Classification	Men (ms)	Women (ms)
Normal	< 440	< 460
Borderline	440–460	460–480
Prolonged	460–500	480–500
Severely Prolonged (High TdP Risk)	> 500	> 500
Short QT (possible SQTS)	≤ 340	≤ 340

Drug Classes Associated with QT Prolongation

Major medication categories known to prolong the QTc interval via cardiac potassium channel (hERG/IKr) blockade, organized by CredibleMeds risk classification.

Drug Class	Examples	CredibleMeds Risk
Antiarrhythmics	Amiodarone, sotalol, quinidine, dofetilide	Known Risk
Antibiotics	Azithromycin, fluoroquinolones, clarithromycin	Known / Possible
Antipsychotics	Haloperidol, quetiapine, ziprasidone, thioridazine	Known Risk
Antidepressants	Citalopram, escitalopram, tricyclics (amitriptyline)	Known / Possible
Antiemetics	Ondansetron, droperidol, metoclopramide	Known / Conditional
Antimalarials	Chloroquine, hydroxychloroquine	Known Risk
Methadone	Methadone (opioid)	Known Risk
Antifungals	Fluconazole, voriconazole	Known / Possible

Worked Examples

Bazett QTc from QT 380 ms at HR 80 bpm

A patient's ECG shows a QT interval of 380 ms with a heart rate of 80 bpm. Calculate the corrected QT using Bazett's formula.

Convert HR to RR interval: RR = 60,000 / 80 = 750 ms = 0.75 seconds

Apply Bazett formula: QTcB = QT / √(RR) = 380 / √(0.75)

Calculate √(0.75) = 0.866

QTcB = 380 / 0.866 = 438.8 ms

QTcB = 439 ms. For a male patient, this is within the normal range (< 440 ms). For a female patient, this is also normal (< 460 ms).

Fridericia vs Bazett at Tachycardic HR 120 bpm

A tachycardic patient has QT 340 ms at HR 120 bpm. Compare Bazett and Fridericia corrections to demonstrate overcorrection at high heart rates.

Convert HR to RR: RR = 60,000 / 120 = 500 ms = 0.50 seconds

Bazett: QTcB = 340 / √(0.50) = 340 / 0.707 = 481 ms

Fridericia: QTcFri = 340 / ∛(0.50) = 340 / 0.794 = 428 ms

Difference: Bazett gives 481 ms (prolonged) vs Fridericia 428 ms (normal) — a 53 ms gap

At HR 120 bpm, Bazett overcorrects by approximately 53 ms compared to Fridericia. The Bazett result (481 ms) falsely suggests prolongation, while Fridericia (428 ms) is within normal limits. This is why Fridericia is preferred for tachycardic patients.

Hodges QTc from QT 400 ms at HR 72 bpm

A patient has QT 400 ms at HR 72 bpm. Calculate QTc using the Hodges linear formula.

Apply Hodges formula: QTcHod = QT + 1.75 × (HR − 60)

QTcHod = 400 + 1.75 × (72 − 60)

QTcHod = 400 + 1.75 × 12 = 400 + 21 = 421 ms

QTcHod = 421 ms. This is within the normal range for both sexes. At near-normal heart rates (60–100 bpm), all five formulas tend to produce similar results.

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.