ECG Heart Rate Calculator
Calculate heart rate directly from an ECG recording using the method that fits your strip. Choose between the RR-interval distance in millimetres, the 300-rule for large ECG boxes, the 1500-rule for small boxes, the 6-second R-wave count, or a known cycle length in milliseconds. Select your paper speed (25 mm/s or 50 mm/s) and the result updates instantly. The worked steps panel shows exactly how the number was reached.
How to calculate heart rate from an ECG
An ECG records the electrical activity of the heart on graph paper moving at a standard speed of 25 mm per second (or 50 mm/s for some recordings). Because the paper speed is fixed, the distance between any two corresponding points on successive heartbeats can be converted directly to a time interval, and that interval can be used to calculate beats per minute. The most precise measurement uses the distance in millimetres from the peak of one R wave to the peak of the next (the RR interval). Dividing 60 by the duration of that interval in seconds gives the heart rate. For regular rhythms where only a quick estimate is needed, the 300-rule and 1500-rule use the number of large or small ECG grid boxes instead, saving time without sacrificing much accuracy.
The 300-rule, 1500-rule, and 6-second method explained
The 300-rule is a mental shortcut for regular rhythms: count the large boxes (each 5 mm wide, equal to 0.2 seconds at 25 mm/s) between two adjacent R peaks and divide into 300. The sequence is easy to memorise - one box = 300 bpm, two boxes = 150, three = 100, four = 75, five = 60, six = 50. The 1500-rule does the same with small boxes (each 1 mm, equal to 0.04 seconds at 25 mm/s): divide 1500 by the small-box count. It is more accurate for fast regular rhythms because a small measurement error spans fewer boxes. The 6-second method is best for irregular rhythms such as atrial fibrillation, where a single RR interval does not represent the average rate: count every R wave in a 6-second strip (which spans 30 large boxes at 25 mm/s) and multiply by 10 to extrapolate to beats per minute.
Paper speed: why it matters
Standard ECG paper runs at 25 mm per second in most of the world. Some recordings - particularly paediatric studies and recordings of rapid arrhythmias - use 50 mm/s to spread the complexes out and make timing intervals easier to measure. At 50 mm/s, each large box represents 0.1 seconds rather than 0.2 seconds, and the 300-rule must be replaced by direct time calculation. This calculator automatically adjusts its RR-interval method for whichever paper speed you select. The box rules (300-rule and 1500-rule) are defined for 25 mm/s paper and are not shown when 50 mm/s is selected, because their standard divisors do not apply.
Normal ranges and when to act
The normal adult resting heart rate is 60-100 beats per minute. A rate below 60 bpm is called bradycardia and a rate above 100 bpm is tachycardia. Both terms describe rate only - they say nothing about rhythm, the origin of the impulse, or whether the rate is dangerous. A trained endurance athlete may have a resting rate of 40-50 bpm with normal sinus rhythm, while a patient with complete heart block may have a rate of 40 bpm with dissociated P waves and wide QRS complexes. Context is everything: always integrate the rate with the rhythm, P-wave morphology, PR interval, QRS width, QT interval, and the patient's clinical picture before drawing conclusions.
Heart rate ranges and ECG classification
| Rate (bpm) | Classification | Common causes |
|---|---|---|
| Below 40 | Severe bradycardia | Complete heart block, inferior MI, sick sinus syndrome |
| 40-59 | Bradycardia | Athlete physiology, high vagal tone, beta-blockers, hypothyroidism |
| 60-100 | Normal sinus rate | Healthy adult resting heart rate |
| 101-150 | Tachycardia | Sinus tachycardia, SVT, atrial flutter (2:1), fever, dehydration |
| 151-250 | Rapid tachycardia | SVT, atrial flutter, ventricular tachycardia |
| Above 250 | Life-threatening | Ventricular fibrillation, torsades de pointes |
Reference ranges for adults at rest. Athletes may have a normal resting rate below 50 bpm. Children have higher normal ranges - a newborn may have a rate of 110-150 bpm.
Frequently asked questions
What is the RR interval on an ECG?
The RR interval is the time between two consecutive R-wave peaks on an ECG. The R wave is the tall positive deflection in the middle of the QRS complex, the main electrical signal produced when the ventricles contract. Because R waves are easy to identify on most recordings, the RR interval is the standard measure of beat-to-beat timing. At a paper speed of 25 mm/s, each millimetre of distance on the paper equals 0.04 seconds, so a 20 mm RR interval represents 0.8 seconds and a heart rate of 75 bpm.
Which method is most accurate for calculating heart rate from an ECG?
The most accurate method for regular rhythms is to measure the RR interval in millimetres and divide by the paper speed, then divide 60 by the result. Using electronic calipers and a cycle-length readout in milliseconds is equally precise. The 1500-rule (small boxes) is more accurate than the 300-rule (large boxes) for fast rhythms because counting small boxes in a short interval introduces less relative error. The 6-second method is the best choice for irregular rhythms, where a single RR interval would misrepresent the average rate.
What is bradycardia and what causes it?
Bradycardia is a heart rate below 60 beats per minute. It can be normal - well-trained athletes often have resting rates of 40-50 bpm because their hearts are efficient and stroke volume is high. Pathological bradycardia can result from increased vagal tone, inferior myocardial infarction (which can affect the SA node), sick sinus syndrome, high-degree atrioventricular block, hypothyroidism, hypothermia, or medications such as beta-blockers and digoxin. Symptomatic bradycardia (causing dizziness, syncope, or haemodynamic compromise) requires urgent assessment.
What is tachycardia and when is it dangerous?
Tachycardia is a heart rate above 100 beats per minute. It is not a diagnosis but a finding that requires further characterisation. Sinus tachycardia (regular rate of 100-180 bpm, normal P waves before each QRS) is usually a response to physiological demand such as fever, pain, anaemia, or dehydration, and treatment targets the underlying cause. Supraventricular tachycardias such as AVNRT or atrial flutter can reach 150-250 bpm and may need vagal manoeuvres or medication. Ventricular tachycardia (wide complex, rate above 120 bpm) is a medical emergency requiring immediate assessment.
Can I use this calculator for atrial fibrillation?
For atrial fibrillation (AF) and other irregular rhythms, use the 6-second method. In AF the RR intervals vary beat to beat, so measuring a single interval and applying the 300-rule or RR-interval formula gives only an instantaneous snapshot, not the average ventricular rate that guides clinical decisions. The 6-second method averages multiple beats across the strip, which better represents the mean ventricular response. On a standard 10-second rhythm strip, you can also count all QRS complexes and multiply by 6 to achieve the same result.
Does paper speed affect the heart rate calculation?
Yes. At 25 mm/s (standard), a 25 mm RR interval equals 1.0 second and 60 bpm. At 50 mm/s (fast), the same 25 mm strip spans only 0.5 seconds, meaning the rate is 120 bpm. If you apply the 25 mm/s formula to a 50 mm/s recording your result will be exactly half the true rate. Always check the paper speed printed at the side of the ECG before measuring.