QTc Calculator (Bazett)
Bazett QTc with sex based flag.
What it does
QT tracks ventricular recharge. Because it shortens as rate rises, raw QT misleads across rates. Bazett divides by the square root of the RR interval so tracings compare fairly. Male and female limits then flag the result.
The tool above runs Bazett. With QT 400 ms at 75 bpm, RR is 800 ms and QTc is 447 ms, Normal for male. A female tracing at QT 460 ms and 60 bpm gives 460 ms QTc, Normal for female against the 470 line. Borderline spans the next 20 ms before prolonged.
Rate extremes and wide beats distort the correction. Fast rates overcorrect upward, slow rates undercorrect, and bundle branch block widens QT for non repolarization reasons. Drugs, potassium, magnesium, and calcium all move the number, so the med list travels with the tracing.
This page estimates information only. It does not diagnose long QT, clear drugs, or set doses. Prolonged flags need prompt clinical overread.
How to use
- Enter the measured QT in ms.
- Enter the heart rate in bpm.
- Pick male or female for the flag.
- Read QTc first, then the flag.
- Read RR to check the rate context.
- Confirm prolonged flags with a clinician overread.
Formula
- QT
- Measured QT interval in ms.
- HR
- Beats per minute at the same tracing.
- RR
- 60 / HR in seconds, shown as ms RR.
Bazett correction in milliseconds. At very fast or slow rates other formulas fit better. Bundle branch block and paced rhythms need specialist reading, not this line.
Worked example
Example: QT 400 ms at 75 bpm
Defaults: QT 400 ms at 75 bpm for male. Female limit is 470. U waves and bundles distort Bazett.
- Inputs: QT 400 ms at 75 bpm, male.
- RR: 60 / 75 is 0.8 s, or 800 ms RR.
- Root: square root of 0.8 is 0.8944.
- QTc: 400 / 0.8944 is 447 ms QTc.
- Male limit 450 gives Normal for male. Tool shows the same.
Reference table
| Tracing | QTc | Flag |
|---|---|---|
| 400 ms at 75 bpm male | 447 ms QTc | Normal for male |
| 460 ms at 60 bpm female | 460 ms QTc | Normal for female |
| 400 ms at 60 bpm male | 400 ms QTc | Normal for male |
| 500 ms at 60 bpm male | 500 ms QTc | Prolonged |
| 470 ms at 70 bpm female | 508 ms QTc | Prolonged |
Limits
- Bazett only. Other formulas fit extremes better.
- Estimate, not an overread. Morphology matters.
- Wide QRS and pacing distort results.
- One complex assumed. Irregular rhythms need averaging.
Common mistakes
- Mixing ms with seconds in the root.
- Grading against the wrong sex limit.
- Ignoring meds that lengthen QT.
- Reading one noisy complex as final.
- Skipping electrolytes with a long flag.
Try it yourself
Quick checks. Answers sit tucked under each question.
QT 460 ms at 60 bpm, female?
RR is 1000 ms. QTc is 460 ms QTc, Normal for female under 470.
QT 420 ms at 90 bpm, male?
RR is 667 ms. QTc is 514 ms QTc, Prolonged for male. Seek care with med list.
Questions
How is QTc figured?
QTc = QT / root RR with seconds. Here 447 ms QTc.
What counts as normal?
Male below 450 normal, female below 470 normal. Here Normal for male.
What is borderline?
Between limit and limit plus 20. Male 450 to 469, female 470 to 489.
What is prolonged?
At or above limit plus 20. Needs prompt review with meds and electrolytes.
Can Bazett mislead?
Yes. QRS widening, U waves, and fast rates all distort Bazett. Use clinical overread.
Do medicines matter?
No. Many drugs lengthen QT. Bring the full med list with the numbers.