ECG Heart Rate Calculator
Calculate heart rate from ECG strips using R–R interval, large boxes, small boxes, and 6-second rhythm methods with visual step-by-step explanations.
Background
An electrocardiogram (ECG or EKG) records the heart's electrical activity. Heart rate can be estimated from the distance between R waves on a regular rhythm or by counting QRS complexes over a timed strip for irregular rhythms.
How to use this calculator
- Choose an ECG heart-rate method: large boxes, small boxes, R–R interval, timed strip count, or 300 sequence.
- Use box or R–R interval methods for regular rhythms where consecutive R waves are evenly spaced.
- Use the timed-strip method for irregular rhythms by counting QRS complexes across 6, 10, 30, or 60 seconds.
- Check the ECG visual and steps to connect the calculation to ECG paper speed, R waves, and QRS counting.
How this calculator works
At the standard ECG paper speed of 25 mm/s, one small box equals 0.04 seconds and one large box equals 0.20 seconds. Heart rate is calculated by converting the R–R distance into seconds per beat, then dividing 60 seconds by that beat interval.
Formulas & Equations Used
R–R interval: heart rate = 60 / R–R interval in seconds
Large boxes: heart rate = 12 × paper speed / large boxes
Small boxes: heart rate = 60 × paper speed / small boxes
6-second strip: heart rate = QRS count × 10
Timed strip: heart rate = QRS count × 60 / strip seconds
Bazett QTc: QTc = QT / √RR
Fridericia QTc: QTc = QT / RR1/3
Example Problems & Step-by-Step Solutions
Example 1: Large-box method
Problem: There are 4 large boxes between R waves on a standard 25 mm/s ECG.
- At 25 mm/s, use 300 ÷ large boxes.
- Compute 300 ÷ 4.
- Answer: heart rate = 75 bpm.
Example 2: R–R interval method
Problem: The R–R interval is 0.80 seconds.
- Use heart rate = 60 ÷ R–R seconds.
- Compute 60 ÷ 0.80.
- Answer: heart rate = 75 bpm.
Example 3: Irregular rhythm strip
Problem: You count 8 QRS complexes in a 6-second rhythm strip.
- Use heart rate = QRS count × 10.
- Compute 8 × 10.
- Answer: heart rate ≈ 80 bpm.
Example 4: 50 mm/s paper speed
Problem: At 50 mm/s, there are 10 large boxes between R waves.
- At 50 mm/s, one large box is 0.10 seconds.
- 10 large boxes = 1.0 second.
- Answer: heart rate = 60 bpm.
Common mistakes to avoid
- Do not use the 300 method for a clearly irregular rhythm; use a timed strip or R–R averaging instead.
- Do not forget to change the paper speed if the ECG is recorded at 50 mm/s.
- Do not mix milliseconds and seconds when calculating QTc.
- Do not treat heart rate alone as a diagnosis. Rhythm regularity, PR interval, QRS width, QT interval, symptoms, and clinical context also matter.
Frequently Asked Questions
Which ECG heart-rate method should I use?
For regular rhythms, large boxes, small boxes, or R–R interval all work well. For irregular rhythms, a timed strip count is usually more appropriate because one R–R interval may not represent the whole rhythm.
What is the 300 rule on ECG?
At 25 mm/s, divide 300 by the number of large boxes between two R waves. The common sequence is 300, 150, 100, 75, 60, 50.
What is normal adult heart rate on ECG?
A common adult resting reference range is 60–100 beats per minute. Context matters, and ECG interpretation should consider rhythm, symptoms, and clinical setting.
Is this a diagnosis tool?
No. This is an educational calculator for learning ECG heart-rate methods. It does not diagnose arrhythmias or replace clinical judgment.
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