ECG Heart Rate (Rhythm Strip) Calculator
The 1500 method is the most precise for regular rhythms. Count the small squares between two consecutive R-waves.
How to Calculate Heart Rate on an ECG Rhythm Strip
Accurately determining the heart rate from an electrocardiogram (ECG) rhythm strip is a fundamental skill in clinical medicine. Standard ECG paper moves at a speed of 25 mm/second. This means that time can be measured by counting the boxes on the grid. One small square represents 0.04 seconds, and one large square (containing 5 small squares) represents 0.20 seconds.
- 1 Small Square = 1mm = 0.04 seconds
- 1 Large Square = 5mm = 0.20 seconds
- 5 Large Squares = 1 second
- 300 Large Squares = 1 minute
- 1500 Small Squares = 1 minute
The Three Primary Methods
1. The 1500 Method (Most Accurate)
This is the preferred method for regular rhythms. You count the number of small squares between two consecutive R waves (the R-R interval) and divide 1500 by that number.
Formula: 1500 ÷ (Number of small squares between R waves) = Heart Rate (BPM)
2. The 300 Method (Large Square Method)
For a quick estimation of regular rhythms, count the number of large squares between two R waves and divide 300 by that number.
Formula: 300 ÷ (Number of large squares between R waves) = Heart Rate (BPM)
3. The 6-Second Method (For Irregular Rhythms)
When the rhythm is irregular (like Atrial Fibrillation), the R-R interval varies. To find the average heart rate, count the number of QRS complexes in a 6-second strip and multiply by 10.
Formula: (Number of QRS Complexes in 6 seconds) × 10 = Heart Rate (BPM)
Heart Rate Interpretations
| Heart Rate (BPM) | Classification |
|---|---|
| Below 60 | Bradycardia |
| 60 – 100 | Normal Sinus Rhythm |
| Above 100 | Tachycardia |
Practical Example
If you are looking at a strip and find exactly 4 large squares between two R waves:
- Using 300 Method: 300 / 4 = 75 BPM.
- Using 1500 Method: (4 squares × 5 small boxes) = 20 small boxes. 1500 / 20 = 75 BPM.
Both methods yield the same result for regular rhythms, but the 1500 method allows you to account for fractional large squares (e.g., 4.2 large squares).