What this calculator does
The QT interval on an ECG shortens as the heart speeds up, so a raw measurement cannot be compared against a fixed threshold. Correction formulas adjust it to what it would be at 60 beats per minute.
Which formula is used matters. At 100 beats per minute a 400 ms QT corrects to 516 ms by Bazett but only 474 ms by Fridericia, a 42 ms difference that can move a reading from clearly abnormal to borderline.
The formula
The RR interval in seconds is 60 divided by the heart rate. Bazett divides the QT by the square root of RR; Fridericia divides by the cube root, which corrects less aggressively at fast rates.
| Term | Meaning |
|---|---|
| QT interval | The measured time from the start of the Q wave to the end of the T wave. |
| RR interval | The time between heartbeats, being 60 divided by heart rate. |
| Bazett and Fridericia | Two correction formulas. Bazett is the most widely used and over-corrects at high heart rates. |
The inputs explained
| Field | What to enter |
|---|---|
| Measured QT interval (ms) | The measured QT interval in milliseconds, read from an ECG. |
| Heart rate (bpm) | Heart rate in beats per minute at the time of the recording. |
When to use it
Reading an ECG report
Knowing which formula produced a quoted QTc explains disagreements between readings.
Comparing recordings at different heart rates
Correction is what makes two ECGs taken at different rates comparable.
Understanding a medication warning
Many drugs carry QT prolongation cautions, and monitoring is done on the corrected value.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How do the two formulas diverge?
The same QT interval at four heart rates.
| Heart rate | QTc (Bazett) | QTc (Fridericia) |
|---|---|---|
| 50 bpm | 365 ms | 376 ms |
| 70 bpm | 432 ms | 421 ms |
| 100 bpm | 516 ms | 474 ms |
| 120 bpm | 566 ms | 504 ms |
Questions
Which formula should be used?
Bazett is the most widely reported and is what most ECG machines print, but it performs poorly outside a normal heart rate range. Fridericia is generally preferred in drug safety studies. Clinical practice varies, and the formula used should be stated alongside any value.
What counts as a prolonged QTc?
Roughly above 450 ms in men and 460 ms in women is the commonly cited threshold, with values above 500 ms attracting more concern. Thresholds vary between guidelines and must be interpreted in clinical context.
Why does the QT interval need correcting at all?
Because it naturally shortens as heart rate rises. Without correction, a fast heart rate would make every QT look reassuringly short and a slow one would make every QT look long.
Can I interpret my own ECG with this?
No. Measuring the QT interval accurately is itself difficult and depends on which lead is used and where the T wave is judged to end. This calculator performs the arithmetic only; the reading and its interpretation belong with a clinician.
For average circulatory pressure, see the mean arterial pressure calculator. For cardiovascular risk from a lipid panel, see the cholesterol ratio calculator.