QTc Calculator (Corrected QT Interval)
By Santaji GadepatilinUpdated October 27, 2026
The QT interval on an ECG changes with heart rate, so it is corrected (QTc) to compare readings fairly. A long QTc can signal a risk of dangerous heart rhythms.
How to use the QTc calculator
Enter the QT interval in milliseconds and the heart rate in beats per minute. The calculator shows the RR interval and the QTc by four common formulas.
This tool is for education and does not replace interpretation of an ECG by a qualified clinician.
QTc formulas
| Formula | QTc |
|---|---|
| Bazett (1920) | QT ÷ √RR |
| Fridericia (1920) | QT ÷ ∛RR |
| Framingham (1992) | QT + 154 × (1 − RR) |
| Hodges (1983) | QT + 1.75 × (HR − 60) |
QT is in ms and RR in seconds (RR = 60 ÷ heart rate).
How the formulas compare (QT 400 ms)
| Heart rate | Bazett | Fridericia | Framingham | Hodges |
|---|---|---|---|---|
| 50 bpm | 365 | 376 | 369 | 382 |
| 60 bpm | 400 | 400 | 400 | 400 |
| 75 bpm | 447 | 431 | 431 | 426 |
| 90 bpm | 490 | 458 | 451 | 452 |
| 110 bpm | 542 | 490 | 470 | 488 |
| 130 bpm | 589 | 518 | 483 | 522 |
At 60 bpm all formulas agree. At faster rates Bazett gives noticeably higher values.
Interpreting QTc (adults)
| QTc | Men | Women |
|---|---|---|
| Normal | under 450 ms | under 460 ms |
| Borderline | 450–470 ms | 460–480 ms |
| Prolonged | over 470 ms | over 480 ms |
| High risk | over 500 ms | over 500 ms |
Thresholds vary between guidelines. A short QTc (under about 350 ms) can also be significant.
Measuring QT
Measure from the start of the QRS complex to the end of the T wave, usually in lead II or V5, and average over 3–5 beats. In atrial fibrillation, average several RR intervals.
Frequently asked questions
What is a normal QTc?
Commonly under about 450 ms in men and 460–470 ms in women. Above 500 ms is associated with a higher risk of dangerous arrhythmias.
Which QTc formula is best?
Bazett is the most widely used but over-corrects at high heart rates and under-corrects at low ones. Fridericia and Framingham are often preferred when the heart rate is outside 60–100 bpm.
Why correct the QT interval?
QT naturally shortens as heart rate rises. Correcting it to a standard heart rate of 60 bpm lets measurements be compared.
What can prolong the QT interval?
Some medicines (certain antibiotics, antiarrhythmics, antipsychotics and antiemetics), low potassium or magnesium, and inherited long-QT syndromes.