Built and fact-checked by the DocNectar team — see our editorial standards
Key Features
Instant Calculation
Get accurate results in real time with our optimized algorithm.
Mobile Friendly
Fully responsive design. Works on all devices & screen sizes.
Privacy Focused
Your data stays on your device. We don't store any inputs.
100% Free
No hidden costs. This tool is completely free forever.
The corrected QT interval (QTc) adjusts the measured QT interval for heart rate. Bazett's formula (the default here) is QTc = QT / sqrt(RR in seconds); Fridericia's formula, offered as an alternative that corrects less aggressively at extreme heart rates, is QTc = QT / cbrt(RR in seconds). Commonly cited normal QTc reference ranges are roughly under 440ms for men and under 460ms for women. This tool reports the corrected value only, with no interpretation of rhythm risk. For general reference and educational purposes only — not a substitute for a qualified professional's ECG interpretation.
How it works
Choose a correction formula (Bazett's or Fridericia's), then enter the QT interval and R-R interval (both in milliseconds). The tool converts R-R to seconds and applies the chosen formula.
- Enter correction formula.
- Enter qT interval (ms).
- Enter r-R interval (ms).
- Click Calculate to see your results.
Examples
QT 400ms, RR 800ms, Bazett's formula
QTc = 400 / sqrt(0.8) = 400 / 0.894 = 447.2 ms.
Who should use it
- Calculating QTc from a manually measured QT and R-R interval during ECG interpretation practice.
- Cross-checking a monitor-calculated QTc against a manual calculation.
Industry applications
- Cardiology and emergency medicine education
- Pharmacology education (QT-prolonging medication monitoring)
Advantages
- Supports both of the most widely published correction formulas.
- Simple arithmetic from two directly measured intervals.
Limitations
- Both formulas are approximations; no correction formula is perfect at all heart rates.
- Does not interpret rhythm risk — reports the corrected value only.
Common mistakes to avoid
- Entering the R-R interval in seconds instead of milliseconds (this tool expects milliseconds).
- Using Bazett's formula at very high or low heart rates, where it is known to over- or under-correct.
Best practices
- Measure the QT interval from the same lead consistently, and consider Fridericia's formula at heart rate extremes.
Tips
- Use our ECG Boxes to Seconds Calculator if you need to convert box counts to milliseconds first.