ECG criteria
- Find two adjacent R waves in the same lead (preferably lead II or where R waves are clearly visible)
- Make sure the rhythm is regular (RR intervals are visually equal or equal when measured with calipers)
- Count the number of large squares (5 mm, 0.2 s) between the R peaks at a paper speed of 25 mm/s
- For a regular rhythm: Heart rate = 300 / number of large squares between R-R
- More precisely — measure the RR interval in mm, convert to seconds (1 mm = 0.04 s at 25 mm/s) and calculate heart rate = 60 / RR(s)
- For an irregular rhythm (atrial fibrillation, frequent extrasystoles) — count the number of QRS complexes in a 6-second segment (30 large squares) and multiply by 10
- Indicate the average heart rate if the rhythm is irregular, and separately the minimum/maximum heart rate if there is significant variability
How to measure
At the standard recording speed of 25 mm/s, each small square (1 mm) corresponds to 0.04 s, and a large square (5 mm) corresponds to 0.2 s.
The 300 rule is a quick method for regular rhythms: count how many large squares (5 mm each) fit between two adjacent R waves, and divide 300 by this number.
| Number of large squares between R-R | Heart rate, bpm |
|---|---|
| 1 | 300 |
| 2 | 150 |
| 3 | 100 |
| 4 | 75 |
| 5 | 60 |
| 6 | 50 |
If the RR interval does not fit an exact number of large squares, it is more accurate to measure RR with a ruler in millimeters and convert to seconds (1 mm = 0.04 s), then calculate heart rate = 60 / RR(s). Alternatively, you can use the formula "1500 / number of small squares between R-R" — this yields the same result but is more precise for intermediate values.
Before calculating, check the specified recording speed (25 or 50 mm/s) — this is usually indicated in the settings or on the tracing itself. The calibration pulse confirms the amplitude scale (1 mV = 10 mm) and does not reflect the sweep speed, so these two parameters must not be confused. At a speed of 50 mm/s, 1 mm = 0.02 s, and the formulas are halved: Heart rate = 600 / number of large squares between R-R, or Heart rate = 3000 / number of small squares between R-R.
In case of an irregular rhythm (atrial fibrillation, frequent extrasystoles, wandering atrial pacemaker), the 300 rule is not applicable because the RR intervals are of different lengths. Instead, count the QRS complexes over a 6-second segment (30 large squares at 25 mm/s) and multiply by 10 to obtain the average heart rate per minute. The longer the measured segment, the more accurate the average heart rate for an irregular rhythm.
Mechanism
Heart rate on the ECG reflects the ventricular depolarization rate (via the RR interval) rather than directly the rate of impulses from the sinus node: surface ECG evaluates the frequency of atrial and ventricular electrical activity, not the function of the sinus node per se. During sinus rhythm with 1:1 atrioventricular conduction, the ventricular complex rate usually equals the sinus P-wave rate. In other atrial rhythms (ectopic atrial rhythm, atrial flutter), the rate of P or F waves may similarly match the ventricular heart rate with 1:1 conduction, but does not reflect sinus activity. If conduction is impaired (AV block, atrial flutter with 2:1, 3:1 conduction, etc.), the ventricular rate is lower than the atrial wave rate, and therefore the "ECG heart rate" (based on RR) differs from the true atrial activity rate — this must be stated separately.
Normal Variants
In resting adults, the normal heart rate during sinus rhythm is typically 60–100 beats per minute; this is the classic diagnostic range used to define bradycardia and tachycardia. Some epidemiological and preventive sources use a narrower "target" range (e.g., with an upper limit around 80–90 bpm) to assess cardiovascular risk, but this does not change the definition of sinus tachycardia on the ECG. In children, the normal range is higher and age-dependent (physiologically much higher in newborns than in adults). In trained athletes and during sleep, a rate below 60 bpm may be a normal variant in the absence of symptoms.
Pathological changes
Sinus bradycardia is generally defined as a heart rate less than 60 bpm, and sinus tachycardia as a heart rate greater than 100 bpm; the clinical significance of these findings is assessed based on symptoms and context, not just the number. A heart rate of 90–100 bpm is not tachycardia in itself. The specific cause of heart rate abnormalities (sinus, junctional, block-related, tachyarrhythmia) is determined by the P-wave morphology, QRS width, and rhythm regularity.
Clinical significance and management
Heart rate is one of the first parameters a physician evaluates when interpreting an ECG because the subsequent algorithm depends on it: assessment of the PQ interval, frequency-corrected QT duration (QTc), and the urgency of intervention for bradycardia or tachycardia with hemodynamic instability. The heart rate itself is indicated in the report as an averaged figure, while detailed rhythm characteristics (regularity, source, conduction) are described separately. In clinically significant heart rate abnormalities, the physician relies not only on the numerical value, but also on the patient's symptoms and hemodynamics.
Common interpretation errors
- Applying the 300 rule during an irregular rhythm yields an incorrect result; counting over 6 or 10 seconds is required.
- Calculating based on the RR interval in a single lead without checking rhythm regularity across the entire tracing.
- Ignoring the recording speed (25 or 50 mm/s) is a twofold calculation error.
- Calculating the ventricular heart rate instead of the true atrial rate in AV block or atrial flutter with non-1:1 conduction.
- Using a short segment (2–3 s) to estimate average heart rate in marked rhythm irregularity yields an unstable result.
300-150-100-75-60-50 — Heart rate by number of large squares between R and R
1. There are 4 large squares between two adjacent R waves on an ECG at a speed of 25 mm/s. What is the heart rate?
According to the 300 rule: 300 / 4 = 75 bpm.
2. Which heart rate calculation method is appropriate for atrial fibrillation?
In an irregular rhythm, RR intervals vary in length, so counting the number of QRS complexes over a 6-second segment and multiplying by 10 is used to obtain the average heart rate.
3. A patient has atrial flutter with an F-wave rate of 300 bpm and 2:1 AV conduction. What is the ventricular heart rate on the ECG?
With 2:1 conduction, only every second impulse reaches the ventricles, so the QRS rate is half the atrial wave rate: 300 / 2 = 150 bpm.
Frequently asked questions
How to quickly calculate heart rate from an ECG without a calculator?
Count the number of large squares (5 mm) between adjacent R waves and divide 300 by this number. For intermediate values, it is more accurate to measure RR in millimeters and use the formula 1500 / number of small squares.
How to calculate heart rate in atrial fibrillation?
The 300 rule is not applicable because the RR intervals are of different lengths. You need to count the number of QRS complexes in a 6-second segment (30 large squares at 25 mm/s) and multiply by 10 to get the average heart rate per minute.
Why might the heart rate on an ECG and the pulse rate at the wrist differ?
During certain extrasystoles and tachyarrhythmias, not every electrical contraction of the ventricles produces a full peripheral pulse wave, so the palpated pulse may be slower than the ECG heart rate.
What resting heart rate is considered normal in an adult?
Classically, sinus rhythm is 60–100 bpm; this is the range used to define bradycardia and tachycardia. Some preventive sources use a narrower target range, but it does not replace the diagnostic criterion for tachycardia. For athletes and during sleep, a heart rate below 60 without symptoms is often a normal variant.
Does heart rate calculation depend on the ECG paper speed?
Yes. At 25 mm/s, 1 mm corresponds to 0.04 s, and at 50 mm/s, 0.02 s. At 50 mm/s, the 300 rule is replaced by Heart rate = 600 / number of large squares between R-R or Heart rate = 3000 / number of small squares. Recording speed is verified by its label on the tracing or in the settings, independently of the amplitude calibration pulse.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- Hampton. The ECG Made Easy
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization of the Electrocardiogram, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.