ECG Heart Rate Calculator
Calculate heart rate from an ECG strip using the RR interval. Measure in millimeters or count boxes to quickly find beats per minute.
What Is an ECG Heart Rate Calculator?
An ECG (electrocardiogram) heart rate calculator helps healthcare professionals quickly determine a patient's heart rate from an ECG strip. By measuring the distance between two consecutive R wave peaks (the RR interval), you can calculate how many times the heart beats per minute.
The R wave is part of the QRS complex, which represents the depolarization of the ventricles. It is defined as the first upward deflection after the P wave (which represents atrial depolarization).
How to Measure the RR Interval
You can measure the RR interval using two methods:
- With a ruler: Measure the distance between two R wave peaks in millimeters. Try to keep your ruler parallel to the horizontal lines on the ECG paper.
- With a caliper: Place each measuring tip on peaks of subsequent R waves. Then, without changing the angle, place one tip on an intersection of ECG paper lines and count the number of boxes between the tips. A small box = 1 mm, a big box = 5 mm.
The ECG Heart Rate Formula
ECG paper typically runs at 25 mm/s (occasionally 50 mm/s). The distance on the horizontal axis between two points corresponds to a certain time duration:
Once you know the duration of one RR interval, you can calculate the heart rate:
The 300 and 1500 Rule
A quick shortcut for calculating heart rate from ECG paper at standard 25 mm/s speed:
- 300 Rule: Count the number of big boxes (5 mm each) between two R peaks. Divide 300 by this number to get the heart rate.
- 1500 Rule: Count the number of small boxes (1 mm each) between two R peaks. Divide 1500 by this number to get the heart rate.
The 6-Second ECG Method
For patients with irregular heart rhythms, the single-interval method is not reliable. Instead, use the 6-second ECG method:
- Count the number of R waves in a 6-second strip (30 big boxes at 25 mm/s).
- Multiply the count by 10.
- This gives you an estimated heart rate in bpm.
Heart Rate ≈ R waves in 30 boxes × 10
Normal Heart Rate Ranges
A physiological resting heart rate for adults should be between 60 and 100 bpm:
| Classification | Heart Rate (bpm) |
|---|---|
| Bradycardia | < 60 |
| Normal | 60 – 100 |
| Tachycardia | > 100 |
In children, normal heart rate ranges change as the child gets older. During physical activities, the heart beats faster to ensure muscles receive enough oxygen.
RR Interval Calculator (Reverse)
You can also use this calculator in reverse: if you know the heart rate (e.g., from a pulse check), you can determine the expected RR interval on the ECG paper. Simply enter the heart rate and see the corresponding RR interval distance in mm or boxes.
Worked Example
- Measure the RR interval: 20 mm
- Paper speed: 25 mm/s
- Calculate RR duration: 20 ÷ 25 = 0.8 seconds
- Calculate heart rate: 60 ÷ 0.8 = 75 bpm
- Classification: Normal (between 60–100 bpm)
Frequently Asked Questions
What is the 300 rule in ECG?
The 300 rule is a quick method to calculate heart rate from an ECG. Count the number of big boxes (5 mm each) between two consecutive R waves, then divide 300 by that number. For example, if there are 4 big boxes, the heart rate is 300 ÷ 4 = 75 bpm. This works at standard 25 mm/s paper speed.
How do I calculate heart rate from small boxes?
Count the number of small boxes (1 mm each) between two R wave peaks. Divide 1500 by that number. For example, 20 small boxes → 1500 ÷ 20 = 75 bpm. This is also known as the 1500 rule.
Can I use this for irregular heartbeats?
No. For irregular rhythms (arrhythmias), use the 6-second ECG method instead: count R waves in 30 big boxes (6 seconds) and multiply by 10 for an estimated heart rate.
What is an RR interval?
The RR interval is the time between two consecutive R wave peaks on an ECG. It represents one complete cardiac cycle and is used to calculate heart rate. A shorter RR interval means a faster heart rate.