ECG Heart Rate Calculator
Calculate BPM using Large Squares, Small Squares, or the 6-Second Method
How to Calculate Heart Rate on ECG Paper
Electrocardiogram (ECG) interpretation is a critical skill for medical professionals. At a standard paper speed of 25 mm/sec, the grid lines on the ECG paper represent specific units of time. Understanding these units allows you to calculate the heart rate (BPM) accurately.
1. The Large Square Method (The 300 Rule)
This is the fastest method for regular rhythms. You count the number of large squares (5mm) between two consecutive R waves (the R-R interval). Since there are 300 large squares in a minute at standard speed, the formula is:
BPM = 300 / Number of Large Squares
2. The Small Square Method (The 1500 Rule)
This is the most accurate method for regular rhythms. Since each small square (1mm) represents 0.04 seconds, there are 1,500 small squares in one minute. Use this formula for precise measurement:
BPM = 1500 / Number of Small Squares
3. The 6-Second Strip Method
For irregular rhythms (like Atrial Fibrillation), the square methods are inaccurate. Instead, count the number of QRS complexes within a 6-second interval (usually 30 large squares) and multiply by 10.
BPM = Number of QRS Complexes × 10
Quick Reference Chart
| Large Squares (R-R) | Heart Rate (BPM) | Classification |
|---|---|---|
| 1 | 300 | Extreme Tachycardia |
| 2 | 1500 | Tachycardia |
| 3 | 100 | Normal (Upper Limit) |
| 4 | 75 | Normal |
| 5 | 60 | Normal (Lower Limit) |
| 6 | 50 | Bradycardia |
Standard ECG Paper Constants
- 1 Small Square: 1mm (0.04 seconds / 40ms)
- 1 Large Square: 5mm (0.20 seconds / 200ms)
- Standard Speed: 25mm per second
- Normal HR: 60 – 100 Beats Per Minute