What the QT interval is
On an electrocardiogram, the QT interval runs from the start of the QRS complex to the end of the T wave. It covers the time the ventricles take to contract and then recharge electrically. The QT naturally shortens when the heart beats faster, so a raw QT cannot be compared between two heart rates. The corrected QT (QTc) adjusts it to what it would be at 60 beats per minute.
The formulas
All four formulas use the RR interval, the time between two beats, in seconds: RR = 60 ÷ heart rate.
| Formula | QTc |
|---|---|
| Bazett | QT ÷ √RR |
| Fridericia | QT ÷ ∛RR |
| Framingham | QT + 154 × (1 − RR) |
| Hodges | QT + 1.75 × (heart rate − 60) |
QT and QTc are in milliseconds.
Worked example
A QT of 400 ms is measured at a heart rate of 75 bpm.
- RR = 60 ÷ 75 = 0.8 s, or 800 ms.
- Bazett: 400 ÷ √0.8 = 400 ÷ 0.894 = 447 ms.
- Fridericia: 400 ÷ ∛0.8 = 400 ÷ 0.928 = 431 ms.
- Framingham: 400 + 154 × 0.2 = 431 ms.
- Hodges: 400 + 1.75 × 15 = 426 ms.
The spread of about 20 ms between formulas is typical at 75 bpm and grows as the heart rate moves further from 60.
Interpreting the QTc
| QTc | Men | Women |
|---|---|---|
| Normal | Up to 450 ms | Up to 460 ms |
| Borderline to prolonged | 451 to 499 ms | 461 to 499 ms |
| Markedly prolonged | 500 ms or more | 500 ms or more |
A QTc of 500 ms or more, or an increase of 60 ms or more after starting a drug, is the usual threshold for concern about torsades de pointes, a dangerous ventricular rhythm. A short QTc, below about 350 ms, is rare and deserves a check, since very short values are linked to an inherited short QT syndrome.
Which formula to use
Bazett is the oldest and still the most common, and many ECG machines print it. It overcorrects at fast heart rates, labeling some normal tracings as long, and undercorrects below 60 bpm. Fridericia and Framingham are more stable across heart rates, which is why drug-safety studies often prefer Fridericia. Whatever the formula, measure the QT in the same lead each time, usually lead II or V5, and avoid measuring during irregular rhythms such as atrial fibrillation without averaging several beats.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.