This QTc calculator gives you corrected QT interval values using four validated formulas — Bazett, Fridericia, Framingham, and Hodges. Enter the measured QT interval and heart rate from your ECG, and you'll get instant results to help assess arrhythmia risk, evaluate medication safety, or complete pre-procedure cardiac screening.

Bazett remains the most widely used formula in clinical practice, but it tends to overcorrect at low heart rates and undercorrect at high rates. That's why this calculator shows all four methods side-by-side — so you can choose the most appropriate value for your patient's specific situation. Whether you're monitoring a patient on antiarrhythmics, evaluating syncope, or screening before starting a QT-prolonging medication, having multiple QTc values helps you make better clinical decisions.

The QT interval represents the total time for ventricular depolarization and repolarization — essentially, how long it takes your patient's ventricles to electrically reset after each heartbeat. On an ECG, you measure it from the beginning of the QRS complex to the end of the T wave.

A normal QT interval shortens when heart rate increases and lengthens when heart rate decreases. This is completely physiological — faster heart rates mean less time between beats, so the ventricles have less time to repolarize. The problem is that this natural variation makes it difficult to compare QT measurements across different heart rates, or to know whether a particular QT value is actually prolonged.

That's where QTc — the corrected QT interval — comes in.

Imagine two patients: one with a heart rate of 60 bpm and a QT of 400 ms, and another with a heart rate of 100 bpm and a QT of 340 ms. Which one has the longer QT interval?

Without correction, you can't meaningfully compare them. The first patient's heart has more time between beats, so a longer absolute QT is expected. The second patient's QT looks shorter, but relative to their faster rate, it might actually be more prolonged.

QTc formulas mathematically adjust the measured QT to what it would theoretically be at a standardized heart rate (usually 60 bpm). This allows you to:

• Compare values across different heart rates
• Use consistent thresholds for "normal" vs "prolonged"
• Track changes over time even when heart rate fluctuates
• Apply standard drug-safety cutoffs reliably

QTc = QT / √RR

Bazett has been the clinical workhorse since Henry Cuthbert Bazett published it in 1920. You'll find it referenced in FDA drug labeling, research papers, and most automated ECG interpretations. For heart rates between 60-100 bpm, it works reasonably well.

The catch: Bazett overcorrects at slow heart rates (making QTc appear shorter than it really is) and undercorrects at fast heart rates (making QTc appear longer). At a heart rate of 50 bpm, Bazett might underestimate your patient's true QTc by 10-20 ms. At 120 bpm, it might overestimate by a similar margin.

Best used when: Heart rate is 60-100 bpm, or when you need to match historical data and drug labeling that reference Bazett values.

QTc = QT / ∛RR

Fridericia uses the cube root instead of the square root, which provides more stable correction across a wider range of heart rates. Many electrophysiologists prefer it, especially for drug studies.

The FDA has actually started requiring Fridericia in thorough QT studies for new drugs. That tells you something about where the field is heading.

Best used when: Heart rate is below 60 or above 100 bpm, when evaluating QT-prolonging drugs in clinical trials, or when you need more accurate correction at rate extremes.

QTc = QT + 0.154 × (1 - RR)

Derived from the Framingham Heart Study population data, this formula uses linear correction rather than exponential. It tends to give results similar to Fridericia and performs well across varied heart rates.

Best used when: You want an alternative validation of your Fridericia result, or when working with population-based risk assessment.

QTc = QT + 1.75 × (HR - 60)

Hodges directly incorporates heart rate rather than RR interval, making it conceptually straightforward. Some clinicians find it useful as a quick mental check.

Best used when: You want a simple linear estimate or need to quickly verify calculations.

For most clinical situations, start with Bazett — it's what your colleagues expect, what drug labels reference, and what most automated systems report. But always glance at Fridericia too, especially if your patient's heart rate falls outside 60-100 bpm. When the two values diverge significantly, Fridericia usually reflects reality more accurately.

Why the sex difference? Women naturally have slightly longer QT intervals than men, likely due to hormonal influences on cardiac ion channels. This becomes clinically relevant — the same QTc of 460 ms is borderline for a woman but prolonged for a man.

The 500 ms threshold: Above 500 ms, the risk of Torsades de Pointes increases substantially. Most clinicians consider this a hard stop for QT-prolonging medications unless the benefit clearly outweighs the risk.

Step 1: Measure the QT Interval

On your ECG, identify a lead with a clear T wave ending — lead II or V5 often works well. Measure from the very beginning of the QRS complex to the point where the T wave returns to baseline. If you're uncertain where the T wave ends, use the tangent method: draw a line along the steepest part of the T wave's downslope and mark where it crosses the baseline.

Step 2: Enter Your Values

Input the measured QT interval in milliseconds. Most ECG paper runs at 25 mm/sec, so each small box represents 40 ms. Count the small boxes from QRS onset to T wave end and multiply by 40.

Enter the heart rate in beats per minute. You can use the rate displayed on the monitor, or calculate it from the RR interval (HR = 60,000 / RR in ms).

Step 3: Review the Results

The calculator displays:

• RR Interval: Automatically calculated from your heart rate input
• QTc (Bazett): The primary result most clinicians reference
• Alternative formulas: Fridericia, Framingham, and Hodges values for comparison

If Bazett and Fridericia differ by more than 20 ms, consider which formula better fits your patient's heart rate range.

A 45-year-old man has a routine pre-operative ECG showing QT = 380 ms at a heart rate of 72 bpm.

Interpretation: Both values fall well within normal limits for a man (< 430 ms). No concerns — proceed with surgery.

A 68-year-old woman on metoprolol has QT = 440 ms at a heart rate of 48 bpm.

Interpretation: Bazett gives a reassuringly normal value, but Fridericia shows borderline prolongation. At this slow heart rate, Fridericia is more reliable. Her true QTc is likely around 425 ms — still acceptable for a woman, but worth noting before adding any QT-prolonging medications.

A 32-year-old man presents with palpitations. ECG shows QT = 320 ms at a heart rate of 115 bpm.

Interpretation: Bazett flags this as borderline prolonged, which might trigger unnecessary concern. Fridericia correctly identifies this as normal. The short absolute QT is appropriate for the fast heart rate — once corrected properly, there's no prolongation.

A 55-year-old woman on sotalol for atrial fibrillation has QT = 480 ms at heart rate of 65 bpm.

Interpretation: Both formulas agree — this QTc exceeds 470 ms (prolonged for women) and approaches the critical 500 ms threshold. Check potassium and magnesium levels, review other medications, and consider dose reduction or alternative therapy. This requires action.

A 60-year-old man starts ondansetron for chemotherapy-induced nausea. Baseline QT = 400 ms at heart rate of 75 bpm.

Interpretation: Baseline QTc is at the upper edge of normal for a man. Ondansetron can prolong QT, particularly at higher doses. This patient warrants closer monitoring — consider limiting to single doses rather than scheduled dosing, and recheck ECG if symptoms develop.

Before starting drugs known to prolong QT — antiarrhythmics (sotalol, dofetilide, amiodarone), certain antibiotics (fluoroquinolones, azithromycin), antipsychotics (haloperidol, ziprasidone), and antiemetics (ondansetron, droperidol) — baseline QTc helps stratify risk. A QTc already above 450 ms in men or 470 ms in women warrants extra caution.

For patients on chronic QT-prolonging therapy, periodic QTc checks catch gradual prolongation before it becomes dangerous. Many protocols call for repeat ECG 3-7 days after starting therapy and again at steady state.

Unexplained syncope, especially with exertion or startle, can indicate inherited long QT syndrome. A resting QTc above normal ranges should prompt further evaluation, family history review, and possible genetic testing.

Hypokalemia, hypomagnesemia, and hypocalcemia all prolong the QT interval. When you find an unexpectedly long QTc, check electrolytes — correction often normalizes the interval without other intervention.

This isn't exhaustive. CredibleMeds (crediblemeds.org) maintains a regularly updated database of QT-prolonging drugs categorized by risk level — worth bookmarking for clinical reference.

If Bazett and Fridericia give QTc values more than 20-30 ms apart, trust Fridericia for heart rates below 60 or above 100 bpm. In the 60-100 bpm range, both are generally reliable.

U waves, artifact, and atrial fibrillation can make QT measurement challenging. In A-fib, average the QT across multiple beats (typically 10) with varying RR intervals and use the mean RR for correction. Be aware that QTc variability is higher in irregular rhythms.

Some patients have paradoxical QT responses to heart rate changes — the QT doesn't shorten appropriately with faster rates. These patients may need stress testing or Holter monitoring for complete evaluation, as resting QTc alone can miss dynamic abnormalities.

Genetic long QT syndrome can produce normal resting QTc values in some affected individuals. If clinical suspicion is high (family history of sudden death, exertional syncope), a normal calculator result doesn't rule out the diagnosis.