ECG Rate Calculator
Select your calculation method based on the ECG strip data available:
(Small Squares)
(Large Squares)
(R-R Interval)
Understanding ECG Rate Calculation Formulas
Electrocardiography (ECG or EKG) interpretation is a fundamental skill in cardiology and emergency medicine. One of the primary steps in analyzing an ECG strip is determining the heart rate. While modern machines provide automated readings, manual calculation remains a critical skill for verifying accuracy and interpreting rhythms that machines may miscalculate.
The standard ECG paper speed is 25 mm/second. Based on this standardization, the grid on the paper allows us to calculate time and, consequently, heart rate using specific mathematical formulas.
The Grid System
- Small Square: 1mm x 1mm = 0.04 seconds.
- Large Square: 5mm x 5mm (5 small squares) = 0.20 seconds.
- One Minute: 300 Large Squares or 1500 Small Squares.
Method 1: The 1500 Method (Most Precise)
The 1500 method is considered the most accurate manual way to calculate heart rate for regular rhythms. It relies on counting the number of small millimeter squares between two consecutive R waves (the peak of the QRS complex).
Example: If there are 20 small squares between R-R intervals, the heart rate is 1500 / 20 = 75 BPM.
Method 2: The 300 Method (Quick Estimate)
The 300 method is a faster approximation, useful for quick bedside assessment. It involves counting the number of large (5mm) squares between R waves. This method is slightly less precise if the R wave lands in the middle of a large square.
Example: If there are 4 large squares between R waves, the heart rate is 300 / 4 = 75 BPM.
Common sequence to memorize for large squares: 300, 150, 100, 75, 60, 50.
Method 3: The Time Interval Method
If you have measured the exact duration of the cardiac cycle (the R-R interval) in seconds, you can calculate the rate directly. This is mathematically identical to the frequency formula (f = 1/T) adapted for beats per minute.
Example: If the R-R interval is 0.8 seconds, the heart rate is 60 / 0.8 = 75 BPM.
Heart Rate Classifications
Once calculated, the heart rate is generally categorized into three clinical states:
- Bradycardia: Less than 60 BPM.
- Normal Sinus Rhythm: 60 to 100 BPM.
- Tachycardia: Greater than 100 BPM.
Note: This calculator assumes a regular rhythm. For irregular rhythms (like Atrial Fibrillation), the "6-second strip method" is preferred, where you count the number of QRS complexes in a 6-second strip and multiply by 10.