ECG Heart Rate Calculator
Calculate beats per minute (BPM) based on ECG paper intervals.
Formula for Calculating Heart Rate in ECG
Electrocardiograms (ECGs) are printed on standard grid paper that allows medical professionals to calculate heart rate and measure cardiac intervals with precision. Understanding the formulas behind these calculations is essential for interpreting heart rhythms, especially when an automated reading is not available or needs verification.
- 1 Small Square (1mm) = 0.04 seconds
- 1 Large Square (5mm) = 0.20 seconds
- 5 Large Squares = 1.0 second
Common Calculation Methods
1. The 1500 Method (Most Precise)
This method is used for regular heart rhythms and offers the highest level of precision. It relies on counting the small millimeter squares.
Formula: Heart Rate = 1500 / (Number of small squares between R-R intervals)
Why 1500? There are 1,500 small squares in one minute (25mm/sec × 60 sec = 1500mm).
Example: If there are 20 small squares between two R waves:
1500 ÷ 20 = 75 BPM.
2. The 300 Method (Quick Estimation)
This is a faster method for regular rhythms, useful for quick bedside estimation without counting tiny boxes. It relies on the large (5mm) squares.
Formula: Heart Rate = 300 / (Number of large squares between R-R intervals)
Why 300? There are 300 large squares in one minute (300 × 0.20 sec = 60 sec).
Example: If there are 4 large squares between R waves:
300 ÷ 4 = 75 BPM.
3. The 6-Second Method (Irregular Rhythms)
When the heart rhythm is irregular (e.g., Atrial Fibrillation), the R-R intervals vary, making the previous methods inaccurate. The 6-second method provides an average rate.
Formula: Heart Rate = (Number of R waves in a 6-second strip) × 10
A 6-second strip consists of 30 large squares. You count the number of complete QRS complexes within this timeframe and multiply by 10 to estimate the rate for 60 seconds.
Interpreting the Results
| Heart Rate (BPM) | Classification | Clinical Note |
|---|---|---|
| < 60 | Bradycardia | Slow heart rate. Normal in athletes or during sleep. |
| 60 – 100 | Normal Sinus Rhythm | Standard resting heart rate for adults. |
| > 100 | Tachycardia | Fast heart rate. May indicate stress, fever, or arrhythmia. |
Factors Affecting Accuracy
Always verify the paper speed. While 25mm/s is standard, some settings use 50mm/s to broaden waveforms. If the paper speed is 50mm/s, the constants change (e.g., use 3000 instead of 1500 for the small square method).