ECG Heart Rate Calculator

The RR interval is the distance (in millimeters or time in milliseconds) between two consecutive R-wave peaks in the QRS complexes on an ECG strip. It represents one full cardiac cycle and is the key measurement used to calculate heart rate.

The standard formula is: divide the RR interval in mm by the paper speed (25 mm/s) to get the RR duration in seconds, then divide 60 by that duration to get beats per minute. Alternatively, if you have the cycle length in milliseconds, divide 60,000 by the cycle length to get bpm.

The '300 rule' uses large ECG boxes: divide 300 by the number of large boxes (5 mm each) between two R-waves to estimate heart rate. The '1500 rule' uses small boxes: divide 1500 by the number of small boxes (1 mm each) between R-waves. Both are quick bedside methods at standard 25 mm/s paper speed.

The standard paper speed is 25 mm per second (mm/s). At this speed, each small square (1 mm) represents 0.04 seconds and each large square (5 mm) represents 0.2 seconds. Some countries or specific clinical situations use 50 mm/s — always verify the paper speed before calculating heart rate.

A normal adult heart rate (sinus rhythm) falls between 60 and 100 beats per minute. A rate below 60 bpm is called bradycardia, and a rate above 100 bpm is called tachycardia. These thresholds guide clinical decision-making during ECG interpretation.

This calculator assumes a regular rhythm. If the RR intervals vary between complexes, the patient may have an arrhythmia (such as atrial fibrillation), and a single RR interval measurement may not represent the true average heart rate. In such cases, count the number of QRS complexes over a 6-second strip and multiply by 10 for a better estimate.

In the 6-second method, you count the number of QRS complexes within a 6-second segment of the ECG strip (marked by tick marks or counting 30 large boxes at 25 mm/s) and multiply by 10 to estimate beats per minute. This method is especially useful when the rhythm is irregular.

Cycle length is the time in milliseconds between two successive R-wave peaks. It is the inverse of heart rate: Heart Rate (bpm) = 60,000 / Cycle Length (ms). For example, a cycle length of 1000 ms equals exactly 60 bpm, while 600 ms equals 100 bpm.