ECG Heart Rate Calculator
Large Squares Rule (300 Rule)
Small Squares Rule (1500 Rule)
6-Second Method (Irregular Rhythms)
Estimated Heart Rate:
0 BPM
How to Calculate Heart Rate from an ECG Strip
Electrocardiogram (ECG) interpretation is a critical skill for medical professionals and students. While modern ECG machines provide an automated heart rate (HR) calculation, manual verification is essential, especially when the rhythm is irregular or the paper speed is non-standard.
Heart rate calculation on an ECG is based on the standard paper speed of 25 mm/second. This means that 1 second of time is represented by 5 large squares or 25 small squares on the grid.
1. The 300 Rule (Large Squares Method)
This is the fastest way to estimate heart rate for regular rhythms. You count the number of large squares (5mm boxes) between two consecutive R-waves (the R-R interval).
- Formula: 300 / Number of Large Squares
- When to use: Best for quick estimation of regular rhythms.
- Example: If there are 4 large squares between R-waves, the heart rate is 300 / 4 = 75 BPM.
2. The 1500 Rule (Small Squares Method)
For a more precise heart rate calculation in regular rhythms, use the small squares. Since there are 1,500 small squares in one minute of ECG paper at standard speed, this provides a more granular result.
- Formula: 1500 / Number of Small Squares
- When to use: Best for highly accurate measurement of regular rhythms.
- Example: If there are 20 small squares between R-waves, the heart rate is 1500 / 20 = 75 BPM.
3. The 6-Second Method
If the patient's heart rhythm is irregular (such as in Atrial Fibrillation), the square-counting methods will fail because the interval between beats changes. In these cases, we use a 6-second strip.
- Formula: Number of R-waves in 6 seconds × 10
- How to identify 6 seconds: On standard ECG paper, 6 seconds is equal to 30 large squares. Count the number of complexes in that span and multiply by 10.
- When to use: Mandatory for irregular rhythms.
Note: Standard ECG paper speed is 25mm/sec. If the paper speed is set to 50mm/sec, you must double your result or adjust the constant in the formulas (e.g., 600 instead of 300).
Quick Reference Table
| Large Squares (R-R) | Small Squares (R-R) | Heart Rate (BPM) |
|---|---|---|
| 1 | 5 | 300 |
| 2 | 10 | 150 |
| 3 | 15 | 100 |
| 4 | 20 | 75 |
| 5 | 25 | 60 |
| 6 | 30 | 50 |
Summary of Cardiac Rhythms
- Bradycardia: Heart rate less than 60 BPM.
- Normal: Heart rate between 60 and 100 BPM.
- Tachycardia: Heart rate greater than 100 BPM.