Heart Rate from R-R Interval Calculator
How to Calculate Heart Rate from the R-R Interval
The R-R interval represents the time elapsed between two successive R-waves of the QRS signal on an electrocardiogram (ECG). It is the reciprocal of the heart rate. While heart rate is typically expressed in Beats Per Minute (BPM), the R-R interval is measured in time (milliseconds or seconds) or distance (ECG grid squares).
The Formulas
Depending on how you measure the R-R interval, different formulas are used to convert the value into BPM.
1. Using Time Units
If you have measured the time duration between peaks:
- Milliseconds (ms): Heart Rate = 60,000 / R-R Interval
- Seconds (s): Heart Rate = 60 / R-R Interval
2. Using ECG Grid Squares
On standard ECG paper (paper speed of 25 mm/s):
- Small Squares (1mm): Heart Rate = 1,500 / Number of Small Squares
- Large Squares (5mm): Heart Rate = 300 / Number of Large Squares
Calculation Example
Let's assume a patient undergoes an ECG and you measure the distance between two consecutive R-waves.
Scenario A: Milliseconds
The interval is measured as 800 ms.
Math: 60,000 ÷ 800 = 75 BPM.
Scenario B: Large Squares
The interval spans exactly 4 large squares on the grid.
Math: 300 ÷ 4 = 75 BPM.
Why Calculate from R-R Interval?
While digital ECG machines calculate heart rate automatically, manual calculation from the R-R interval is crucial for verification and detailed analysis. This method calculates the instantaneous heart rate based on a single cardiac cycle, which is essential for detecting arrhythmias.
For example, in cases of Heart Rate Variability (HRV) analysis or sinus arrhythmia, the heart rate changes from beat to beat. Averaging beats over a minute would hide these fluctuations, whereas R-R interval calculation reveals the exact timing of specific beats.
Understanding the Results
- Bradycardia: Heart rate < 60 BPM (R-R interval > 1000 ms).
- Normal Sinus Rhythm: Heart rate 60–100 BPM (R-R interval 600–1000 ms).
- Tachycardia: Heart rate > 100 BPM (R-R interval < 600 ms).