Large Squares Method (Standard)
Small Squares Method (Precise)
6-Second Strip Method (Irregular Rhythm)
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How to Calculate Heart Rate from an ECG Strip
Electrocardiogram (ECG) interpretation is a vital skill in clinical medicine. Determining the heart rate is one of the first steps in analyzing a cardiac rhythm. Depending on whether the rhythm is regular or irregular, different mathematical approaches are required.
1. The 300 Method (Large Squares)
This is the most common method for regular rhythms. ECG paper usually moves at a standard speed of 25mm/s. A large square represents 0.2 seconds.
Formula: 300 / (Number of large squares between two R waves)
Best for: Quick estimation of regular rhythms.
2. The 1500 Method (Small Squares)
For a more precise measurement, count the small squares (1mm each) between two R waves (the R-R interval). Since there are 1,500 small squares in one minute of ECG paper tracing:
Formula: 1500 / (Number of small squares between two R waves)
Best for: High-precision calculations in regular rhythms.
3. The 6-Second Method
If the heart rhythm is irregular (like in Atrial Fibrillation), the square-counting methods will give inaccurate results. Instead, count the number of QRS complexes within a 6-second strip (usually marked by 3-second intervals at the top or bottom of the paper).
Formula: Number of QRS complexes in 6 seconds × 10
Best for: Irregular rhythms (Arrhythmias).
Practical Examples
Scenario
Measurement
Calculation
Result
Regular Rhythm
3 Large Squares
300 / 3
100 BPM
Bradycardia Case
6 Large Squares
300 / 6
50 BPM
Precise Measurement
18 Small Squares
1500 / 18
83 BPM
Irregular Pulse
7 QRS in 6s
7 × 10
70 BPM
Understanding Normal vs. Abnormal Ranges
Once you have calculated the heart rate, it is important to categorize the findings based on standard clinical thresholds for adults:
Normal: 60 to 100 Beats Per Minute (BPM).
Bradycardia: Less than 60 BPM. This can be normal in highly trained athletes or during sleep, but may indicate heart block in others.
Tachycardia: Greater than 100 BPM. This may occur due to exercise, stress, fever, or underlying conditions like hyperthyroidism or cardiac distress.
Note: While this calculator provides numerical accuracy based on standard ECG rules, it does not replace medical diagnosis. Always consult a healthcare professional for interpretation of clinical results.