QTc Calculator (Bazett + Fridericia + Framingham)
QTc calculator with Bazett, Fridericia and Framingham corrections. Enter QT and heart rate for the corrected QT and a normal/borderline/prolonged band. Educational only.
QTc Calculator (Bazett, Fridericia, Framingham)
Please tick the acknowledgement above before calculating.
How to use the QTc calculator
Enter the QT and heart rate
Type the measured QT interval in milliseconds and the heart rate in beats per minute. The calculator converts heart rate to an RR interval (RR = 60 ÷ HR, in seconds) before correcting.
Choose a primary correction
All three corrections are shown, but the one you pick drives the headline and the band. Bazett is the classic default; Fridericia or Framingham are more accurate away from 60–100 bpm.
Select sex
Thresholds for a prolonged QTc differ by sex, so choose male or female to get the correct band.
Acknowledge, then read the band
The result is graded normal, borderline, or prolonged. A prolonged QTc raises the risk of a dangerous arrhythmia, especially with QT-prolonging drugs — interpretation and any action belong with a clinician reading the actual ECG.
QTc — correcting the QT interval for heart rate
Why the QT has to be corrected
The QT interval on an electrocardiogram measures the time the heart's ventricles take to depolarise and then recover — electrically, to fire and reset. It matters because an abnormally long QT can let the heart slip into torsades de pointes, a chaotic rhythm that can cause fainting or sudden cardiac death. The problem is that the raw QT naturally shortens as the heart speeds up and lengthens as it slows, so a single QT value means little without knowing the heart rate. The corrected QT, or QTc, normalises the interval to a standard rate of 60 beats per minute, making values comparable across different heart rates. The correction uses the RR interval (the time between beats, equal to 60 divided by the heart rate in seconds). Three formulae dominate: Bazett divides the QT by the square root of RR; Fridericia divides by the cube root of RR; and Framingham adds a linear term, 154 times (1 minus RR), to the QT.
Bazett's formula, published in 1920, is the historical default and what most ECG machines print, but it has a well-known flaw: it over-corrects at fast heart rates (flagging false prolongation) and under-corrects at slow ones (missing real prolongation). Fridericia and Framingham behave better outside the 60–100 beats-per-minute range, which is why guidelines increasingly favour them, especially when screening for drug-induced QT prolongation in tachycardic patients. Showing all three side by side, as this tool does, makes the disagreement visible and helps avoid being misled by a single number.
"A QT means nothing without a heart rate. QTc normalises the interval to 60 beats per minute — and which formula you trust depends on how fast the heart was going."
What a prolonged QTc means
Broadly, a QTc under about 430 ms in men and 450 ms in women is normal; values up to roughly 450 and 470 respectively are borderline; and beyond those a QTc is considered prolonged, with the risk of arrhythmia climbing as it lengthens — a QTc above 500 ms is a recognised danger threshold. Many things prolong the QT: inherited long-QT syndromes, low potassium or magnesium, and a long list of common drugs (certain antibiotics, antipsychotics, antiemetics, and antiarrhythmics), often in combination. That is why the QTc is checked before and during treatment with QT-prolonging medicines. But the number is only meaningful alongside the actual ECG: the QT must be measured correctly — in the right lead, avoiding the U wave, ideally averaged over several beats — and the rhythm and clinical context taken into account. Accurate QT measurement is itself a skill, and automated machine readings can be wrong. Treat this calculator as an educational aid for understanding and converting between the corrections, not as a substitute for a clinician's interpretation of a real tracing.
10 Facts About the QTc
QTc corrects the QT interval for heart rate.
RR interval = 60 ÷ heart rate (in seconds).
Bazett: QT ÷ √RR — the classic default since 1920.
Fridericia: QT ÷ ∛RR — better at fast rates.
Framingham: QT + 154 × (1 − RR).
Bazett over-corrects when the heart is fast.
Prolonged ≈ > 450 ms (men), > 470 ms (women).
A QTc > 500 ms markedly raises arrhythmia risk.
Many drugs prolong the QT — checked before prescribing.
Low potassium or magnesium also prolongs the QT.
Frequently asked questions
QTc is the QT interval corrected for heart rate. The raw QT shortens as the heart speeds up and lengthens as it slows, so it can't be interpreted alone. The QTc normalises the interval to a heart rate of 60 beats per minute, allowing comparison across rates and assessment of whether the interval is dangerously long.
Bazett is the traditional default and is what most ECG machines display, but it over-corrects at fast heart rates and under-corrects at slow ones. For heart rates outside about 60–100 bpm, Fridericia or Framingham are more accurate and are increasingly preferred, especially when screening for drug-induced prolongation. The calculator shows all three so you can compare; pick the primary one in the dropdown.
As a guide, a QTc above roughly 450 ms in men and 470 ms in women is considered prolonged, with values up to those thresholds borderline and below them normal. Risk of dangerous arrhythmia rises as the QTc lengthens, and a QTc above 500 ms is a widely recognised high-risk threshold. Exact cut-offs vary slightly between guidelines, so the bands here are indicative.
A prolonged QT means the ventricles take longer to reset electrically, creating a window in which an early beat can trigger torsades de pointes — a fast, chaotic ventricular rhythm. Torsades can cause fainting and, occasionally, degenerate into ventricular fibrillation and sudden cardiac death. The danger rises with the degree of prolongation, which is why a long QTc prompts review of medications and electrolytes.
Inherited long-QT syndromes; electrolyte disturbances, especially low potassium, magnesium, or calcium; and a long list of drugs, including some antibiotics (macrolides, fluoroquinolones), antipsychotics, antiemetics, methadone, and antiarrhythmics — often additively when combined. Bradycardia and certain heart conditions also contribute. Because so many common medicines are involved, the QTc is frequently checked before and during their use.
The QT is measured from the start of the QRS complex to the end of the T wave, in the lead where it is longest (often lead II or V5–V6), taking care not to include a following U wave. Averaging over several beats improves accuracy, particularly with an irregular rhythm. Automated machine measurements can be unreliable, so an experienced reader often re-measures by hand. Accurate measurement is essential — feed a wrong QT in and you get a wrong QTc out.
At a heart rate near 60 bpm, where RR is close to 1 second, all three formulae give almost the same answer, because the correction term is minimal. The differences grow as the heart rate moves away from 60 — at fast rates Bazett reads noticeably higher than Fridericia or Framingham. So the choice of formula matters most in tachycardia or bradycardia, exactly where Bazett is least reliable.
Automated QTc values are a useful starting point but not infallible. Machines can misplace the end of the T wave, include the U wave, or mis-handle wide QRS complexes and irregular rhythms, all of which distort the QT and therefore the QTc. When the QTc matters clinically — for example before a QT-prolonging drug — a clinician should verify the measurement on the tracing rather than trust the printout alone.
No. This tool is educational, to help you convert between QT corrections and understand the thresholds. Interpreting a QTc requires the actual ECG, accurate measurement, and clinical context, and any change to medication is a decision for a doctor. If you've been told you have a long QT or take QT-prolonging drugs, follow your clinician's advice rather than a calculator.
No. The QT, heart rate, and sex you enter are processed in your browser only. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs entirely on your device.
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