The corrected QT interval (QTc) adjusts the QT interval for heart rate. Bazett’s formula is QTc = QT ÷ √RR, with RR in seconds. A QT of 400 ms at 75 bpm gives a QTc of 447 ms by Bazett and 431 ms by Fridericia. QTc above 450 ms in men or 460 ms in women is usually considered prolonged, and above 500 ms carries a higher risk of torsades de pointes.
What is the QTc?
The QT interval on an ECG spans the start of ventricular depolarisation (the Q wave) to the end of repolarisation (the T wave). It shortens when the heart beats faster, so doctors correct it for heart rate and call the result QTc. A prolonged QTc is a risk marker for the dangerous arrhythmia torsades de pointes, and it matters when prescribing drugs that affect repolarisation, such as some antibiotics, antipsychotics, antiemetics and antiarrhythmics.
The four formulas
| Formula | Calculation | Notes |
|---|---|---|
| Bazett | QT ÷ √RR | Most used, over-corrects above about 90 to 100 bpm |
| Fridericia | QT ÷ ∛RR | Better at fast and slow rates |
| Framingham | QT + 154 × (1 − RR) | Linear, derived from a large cohort |
| Hodges | QT + 1.75 × (HR − 60) | Linear correction by heart rate |
RR is the interval between heartbeats in seconds, 60 ÷ heart rate. At 75 bpm RR = 0.8 s, so Bazett divides 400 by 0.894 to give 447 ms. At 120 bpm Bazett would greatly overestimate the QTc, which is why many cardiologists prefer Fridericia for tachycardia.
Interpreting the number
- Normal: generally below 450 ms in men and below 460 ms in women (some guidelines use 440 and 460).
- Prolonged: above those limits. Review drugs, electrolytes (potassium, magnesium, calcium) and congenital long QT syndrome.
- High risk: above 500 ms, or an increase of more than 60 ms from baseline after starting a drug.
- Short QT: below about 340 to 360 ms is rare and may be linked to arrhythmia risk.
Measuring the QT properly
Use lead II or V5, measure several beats and take the longest typical value. With atrial fibrillation, average over at least 5 to 10 beats. When a bundle-branch block widens the QRS, the QT is overestimated and special corrections apply. Automated ECG machines are generally accurate, but a manual check is wise when the result changes treatment.
This calculator is a clinical support tool and does not replace clinical judgment. For another clinical scoring tool, see the TI-RADS calculator.
How to use this calculator
- Measure the QT interval on the ECG in milliseconds.
- Enter the heart rate in beats per minute.
- Choose the sex for the upper limit of normal.
- Compare the four corrected values, preferring Fridericia at fast or slow heart rates.
Key figures at a glance
| Prolonged QTc | > 450 ms (men), > 460 ms (women) |
|---|---|
| High risk | > 500 ms |
| RR interval | 60 ÷ heart rate (s) |
| Better at fast rates | Fridericia |
Frequently asked questions
Divide the QT interval by the square root of the RR interval in seconds (Bazett), or by its cube root (Fridericia).
Roughly 350 to 450 ms in men and 360 to 460 ms in women.
Fridericia is generally more accurate across heart rates. Bazett is common because it is simple, but it overcorrects at fast rates.
Several drugs prolong repolarisation. Clinicians check the baseline QTc before and after starting them.
Consumer devices are not accurate enough for QT decisions. A 12-lead ECG read by a clinician is the standard.