Last updated October 12, 2026 · Report an error
This calculator applies the standard ECG "shortcut" heart rate formulas taught throughout ECG interpretation training: HR = 1500 / (R-R interval in small boxes), or HR = 300 / (R-R interval in large boxes), both derived from the standard 25mm/s paper speed. The R-R interval can also be entered directly in milliseconds. This calculator performs pure arithmetic on the R-R interval — it does not interpret rhythm. For general reference and educational purposes only — not a substitute for a qualified professional's ECG interpretation.
How it works
Select how the R-R interval is given (small boxes, large boxes, or milliseconds), enter the value, and the tool applies the corresponding shortcut formula.
- Enter r-R interval given as.
- Enter r-R interval value.
- Click Calculate to see your results.
Examples
R-R interval of 4 large boxes
HR = 300 / 4 = 75 bpm.
Who should use it
- Quickly estimating heart rate from a measured R-R interval during ECG interpretation practice.
- Cross-checking a monitor-displayed heart rate against a manual strip measurement.
Industry applications
- Cardiology and emergency medicine education
- Nursing and paramedic ECG training
Advantages
- Fast, standard shortcut formulas taught throughout ECG interpretation training.
- Accepts small boxes, large boxes, or milliseconds directly.
Limitations
- Assumes a regular rhythm with a consistent R-R interval.
- Does not interpret rhythm or diagnose arrhythmias.
Common mistakes to avoid
- Applying the regular-rhythm shortcut formulas to a strip with an irregular rhythm.
- Mixing up the small-box (1500) and large-box (300) constants.
Best practices
- Confirm the rhythm is regular before applying the box-count shortcut formulas.
Tips
- Use our ECG Boxes to Seconds Calculator alongside this tool if you need the R-R interval in milliseconds first.