QTc Calculator

QTc Calculator: Compute Your Corrected QT Interval With Four Proven Formulas

TL;DR: Enter your QT interval (ms) and heart rate (bpm) or RR interval (s) to get corrected QT values from Bazett, Fridericia, Framingham, and Hodges formulas instantly. A Bazett QTc above 460 ms is borderline prolonged; above 500 ms signals significant arrhythmia risk. This calculator runs all four corrections side by side so you can compare results and spot overcorrection from any single formula.

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Table of Contents

• Why a Raw QT Interval Tells You Almost Nothing
• Six Situations Where a QTc Calculator Saves Time
• Four QT Correction Formulas and How They Differ
• How to Get Your Corrected QT in Seven Steps
• See How the Numbers Play Out: Two Worked Examples
• Where People Go Wrong With QTc Calculations
• FAQ
• Assumptions and Notes
• What to Do Next
• Further Reading

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Why a Raw QT Interval Tells You Almost Nothing

Your ECG printout shows a QT interval of 420 ms. Is that normal? You cannot answer without knowing the heart rate at the time of recording. The QT interval shortens as heart rate increases and lengthens as heart rate drops, so a raw QT measurement is almost meaningless on its own.

The corrected QT interval (QTc) adjusts the measured QT for heart rate, producing a value that can be compared against fixed thresholds regardless of whether the heart was beating at 55 bpm or 100 bpm during the recording. Bazett's formula, published in 1920, remains the most widely cited correction and the one most drug safety trials reference. But Bazett overcorrects at high heart rates and undercorrects at low heart rates. Fridericia, Framingham, and Hodges each address this limitation differently, which is why modern clinical practice often calls for running multiple corrections and comparing the outputs.

Biologically, the QT interval reflects the total time for ventricular depolarization and repolarization. Genetic variants in potassium and sodium channel genes (KCNQ1, KCNH2, SCN5A) directly alter repolarization time, producing congenital long QT syndrome in roughly 1 in 2,000 people. Drug-induced QT prolongation is even more common, affecting patients taking certain antibiotics, antipsychotics, and antiarrhythmics.

Plug your numbers into the calculator above and get all four QTc values in under ten seconds.

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Six Situations Where a QTc Calculator Saves Time

• You are reviewing an ECG and need to verify the machine's automated QTc reading. Automated ECG algorithms use a single correction formula, typically Bazett. At heart rates above 100 bpm, Bazett overcorrects by 10-30 ms compared to Fridericia. Running all four formulas takes the guesswork out of deciding whether a borderline reading at 445 ms is genuinely prolonged or an artifact of the correction method.

• A patient started a new medication known to prolong the QT interval. Drug labels for agents like ondansetron, azithromycin, and haloperidol carry QT prolongation warnings. The FDA recommends monitoring QTc before and after initiation, with a change of more than 30 ms from baseline considered clinically relevant. Comparing baseline to follow-up QTc across all four formulas catches shifts that a single formula might mask.

• You are screening a family for inherited long QT syndrome. Congenital long QT syndrome affects roughly 1 in 2,000 individuals and carries a risk of torsades de pointes and sudden cardiac death. A Bazett QTc above 470 ms in females or 450 ms in males on a resting 12-lead ECG raises suspicion, though genetic testing confirms the diagnosis. Running Fridericia alongside Bazett reduces false positives caused by heart rate-dependent overcorrection.

• You recorded an ECG during exercise or post-exercise recovery. Heart rate during a stress test can reach 150-180 bpm. At 150 bpm the RR interval is 0.4 seconds, and Bazett's square root correction inflates QTc by approximately 15% more than Fridericia's cube root correction at that rate. Hodges, which uses a linear heart rate term, performs best above 100 bpm according to comparative studies.

• You need a quick bedside check on a patient in the emergency department. Measuring the QT interval on a rhythm strip and entering it with the monitor's displayed heart rate takes under 60 seconds. A QTc above 500 ms on any formula warrants immediate clinical attention, as the risk of ventricular arrhythmia rises sharply above this threshold, with some studies reporting a 2-3 fold increase in torsades risk for every 10 ms above 500 ms.

• You are a medical or nursing student learning to interpret ECGs. Running the same QT and heart rate through four different formulas teaches how correction methods diverge. At a heart rate of 60 bpm (RR = 1.0 s), all four formulas return identical or near-identical values. At 120 bpm (RR = 0.5 s), the results can spread by 20-40 ms. Seeing this spread builds intuition faster than memorizing the formulas alone.

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Four QT Correction Formulas and How They Differ

Each formula corrects the QT interval for heart rate using a different mathematical relationship between QT and the RR interval.

RR(s) = 60 / Heart Rate   (or input directly)

Bazett:      QTc = QT / sqrt(RR)
Fridericia:  QTc = QT / cbrt(RR)
Framingham:  QTc = QT + 154 × (1 - RR)
Hodges:      QTc = QT + 1.75 × (Heart Rate - 60)

QTc Interval Interpretation (Bazett)

Formula Comparison at Different Heart Rates (QT = 400 ms)

The spread between Bazett and the other three formulas widens as heart rate rises. At 120 bpm, Bazett reads 566 ms while Framingham reads 477 ms. That 89 ms gap is the difference between "significantly prolonged" and "borderline prolonged," which is why relying on Bazett alone at high heart rates leads to unnecessary clinical concern.

Genetic variation also plays a role in baseline QT duration. Women have QTc values approximately 10-20 ms longer than men on average, partly due to hormonal effects on cardiac ion channel expression. The interpretation thresholds used clinically reflect this: some guidelines set the upper normal limit at 450 ms for men and 460 ms for women.

The main limitation of all four formulas: none accounts for autonomic tone, electrolyte levels, or body temperature, all of which independently affect repolarization time. A QTc reading is a rate-corrected snapshot, not a complete assessment of repolarization risk.

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How to Get Your Corrected QT in Seven Steps

1. Obtain a clean ECG recording. A standard 12-lead ECG or a single-lead rhythm strip works. The QT interval is typically most visible in leads II and V5. Paper speed must be 25 mm/s (the standard), because a different speed changes the apparent interval width.

2. Measure the QT interval from the onset of the QRS complex to the end of the T wave. Count the number of small squares (each = 40 ms at 25 mm/s) or large squares (each = 200 ms). If the T wave merges into a U wave, measure to the nadir between T and U, not to the end of U.

3. Record the heart rate or RR interval from the same ECG strip. The heart rate displayed on the monitor works if the rhythm is regular. For irregular rhythms like atrial fibrillation, average the RR interval over 3-5 beats and convert to heart rate using HR = 60 / average RR.

4. Select your input method in the calculator. Choose "Heart Rate" if you have bpm from the monitor, or "RR Interval" if you measured the RR distance directly from the strip and converted to seconds.

5. Enter the QT interval in milliseconds and either heart rate or RR interval. The default values (QT = 400 ms, HR = 75 bpm) provide a normal baseline for comparison. Replace them with your measured values.

6. Read all four QTc outputs and the interpretation. The calculator returns Bazett, Fridericia, Framingham, and Hodges values simultaneously. If three formulas read normal and only Bazett reads borderline, the prolongation is likely a Bazett overcorrection artifact rather than true prolongation.

7. Document the results with the clinical context. Record QTc values alongside the patient's current medications, electrolyte levels (potassium, magnesium, calcium), and whether the ECG was taken at rest or post-exertion. A QTc value without context is just a number.

Non-obvious insight: If a patient has atrial fibrillation with a highly irregular ventricular rate, none of these formulas performs reliably from a single beat. Average the QT and RR over at least 10 consecutive beats. Some clinicians prefer using the longest QT paired with the shortest preceding RR interval as a worst-case estimate.

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See How the Numbers Play Out: Two Worked Examples

Example 1: 67-Year-Old Man on Sotalol, Resting ECG

Gerald takes sotalol for atrial fibrillation. His cardiologist orders a follow-up ECG. The strip shows: QT interval = 440 ms, heart rate = 62 bpm.

RR = 60 / 62 = 0.968 s

Bazett:     QTc = 440 / sqrt(0.968) = 440 / 0.9839 = 447 ms
Fridericia: QTc = 440 / cbrt(0.968) = 440 / 0.9892 = 445 ms
Framingham: QTc = 440 + 154 × (1 - 0.968) = 440 + 4.9 = 445 ms
Hodges:     QTc = 440 + 1.75 × (62 - 60) = 440 + 3.5 = 444 ms

All four formulas agree: Gerald's QTc is borderline prolonged. Because his heart rate is close to 60 bpm, the corrections barely adjust the raw QT. His cardiologist should compare this to his pre-sotalol baseline. If baseline QTc was 410 ms, the 35 ms increase exceeds the 30 ms threshold for clinical concern, and a dose reduction or alternative antiarrhythmic should be discussed.

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Example 2: 24-Year-Old Female Nurse, Post-Exercise ECG

Amara completed a treadmill stress test. At peak recovery (3 minutes post-exercise), her ECG shows: QT interval = 320 ms, heart rate = 130 bpm.

RR = 60 / 130 = 0.462 s

Bazett:     QTc = 320 / sqrt(0.462) = 320 / 0.6794 = 471 ms
Fridericia: QTc = 320 / cbrt(0.462) = 320 / 0.7732 = 414 ms
Framingham: QTc = 320 + 154 × (1 - 0.462) = 320 + 82.9 = 403 ms
Hodges:     QTc = 320 + 1.75 × (130 - 60) = 320 + 122.5 = 443 ms

Bazett flags Amara's QTc as prolonged at 471 ms, but the other three formulas place her in the normal to borderline range. At 130 bpm, Bazett's square root correction inflates the result. Fridericia and Framingham, which handle tachycardia more accurately, both read normal. Her actionable step: repeat the ECG at rest. If resting QTc is below 440 ms on all four formulas, the elevated Bazett reading during tachycardia was a correction artifact.

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Where People Go Wrong With QTc Calculations

Trusting only Bazett at heart rates above 90 bpm. Bazett overcorrects progressively as heart rate rises. At 100 bpm, the overcorrection is roughly 10-15 ms compared to Fridericia. At 130 bpm, the gap balloons to 50+ ms. Always run a second formula when heart rate exceeds 90 bpm. Fridericia is the most widely recommended alternative.

Measuring QT to the end of a U wave instead of the T wave. U waves are common in hypokalemia and bradycardia. Including the U wave in the measurement inflates QT by 40-80 ms, which can push a normal QTc into the prolonged range. Measure to the end of the T wave only. If T and U merge, use the tangent method to define T wave end.

Using heart rate from a different time point than the QT measurement. In a patient whose heart rate fluctuates between 70 and 95 bpm, pairing a QT measured at 70 bpm with a heart rate reading of 95 bpm produces a QTc error of approximately 30-50 ms. Both values must come from the same cardiac cycle or the same short ECG segment.

Forgetting to convert RR interval units. RR interval must be in seconds for these formulas. An RR measured as 800 ms (0.8 seconds) entered as "800" produces nonsensical results. The Bazett formula would divide QT by the square root of 800, returning a QTc near 14 ms. Double-check the unit before hitting calculate.

Ignoring electrolyte context when interpreting results. Hypokalemia (potassium below 3.5 mEq/L) alone prolongs the QT interval by 10-30 ms independent of any drug effect. A QTc of 480 ms in a patient with potassium of 2.8 mEq/L may normalize to 450 ms after potassium correction. Treat the electrolyte abnormality before attributing QT prolongation to a medication.

Applying these formulas during atrial fibrillation without averaging. In atrial fibrillation, beat-to-beat RR variation can exceed 300 ms. A single-beat QTc calculation is unreliable because the QT interval adapts to the preceding RR interval with a delay of several beats. Average QT and RR over at least 10 beats, or use dedicated QT-RR regression methods designed for irregular rhythms.

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Assumptions and Notes

• Margin of error: QTc accuracy depends on precise QT and heart rate measurement. Manual QT measurement carries an inter-observer variability of approximately 20-30 ms. Combined with formula-specific correction differences of 10-50 ms at non-resting heart rates, total QTc uncertainty can reach 40-70 ms. Always interpret borderline results in clinical context rather than as absolute thresholds.
• Professional disclaimer: This QTc calculator is an informational tool for educational and screening purposes. It does not replace clinical judgment, professional ECG interpretation, or diagnostic evaluation. QTc values should be interpreted alongside patient history, medications, electrolyte levels, and full ECG morphology. Consult a cardiologist or qualified physician for any clinical decisions related to QT prolongation.

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What to Do Next

Amara's example showed a 68 ms gap between Bazett and Framingham at 130 bpm. That gap is the reason no single formula should be the final word on QT prolongation. The math behind each correction takes seconds. The clinical decision that follows requires the full picture: medications, electrolytes, family history, and serial ECGs over time.

Enter your QT interval and heart rate above to compare all four corrections instantly.

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Further Reading

• Heart Rate Calculator: Find Your Resting and Target Heart Rate Zones
• Blood Pressure Calculator: Check Your Cardiovascular Risk Profile
• BMI Calculator: Assess Your Baseline Health Metrics