ECG heart rate calculator (300, 1500 and 6-second methods)
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Result
For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Work out the heart rate from an ECG strip with the three methods taught in nursing and ECG courses: the 300 method (large squares between two R waves), the 1500 method (small squares) and the 6-second method (QRS complexes in six seconds times ten). The calculator handles paper speeds of 25 mm/s and 50 mm/s and classifies the rate as bradycardia, normal or tachycardia for an adult at rest.
Step-by-step solution
Time of one large square
Heart rate
Key formulas
- Paper (25 mm/s, 10 mm/mV)
- 1 small box = 0.04 s, 1 large box = 0.20 s, 10 mm = 1 mV
- Heart rate, regular rhythm
- 300 ÷ large boxes = 1500 ÷ small boxes = 60 ÷ RR (s)
- Heart rate, irregular rhythm
- QRS complexes in 6 s × 10
How to calculate heart rate from an ECG
- At 25 mm/s one small square is 0.04 s and one large square (5 small squares) is 0.2 s.
- 300 method: count the large squares between two consecutive R waves and divide 300 by that number. Four large squares give 300/4 = 75 beats/min.
- 1500 method: count the small squares between two R waves and divide 1500 by that number. It is more precise: 20 small squares give 75 beats/min.
- 6-second method: count the QRS complexes in a 6-second strip (30 large squares) and multiply by 10. Use it for irregular rhythms.
- At 50 mm/s every square takes half the time, so the numbers become 600 and 3000.
Questions and answers
Which method should I use?
For a regular rhythm the 1500 method is the most accurate and the 300 method the quickest. For an irregular rhythm such as atrial fibrillation, use the 6-second method, which averages over several beats.
Why 300 and 1500?
There are 300 large squares and 1500 small squares in one minute of ECG paper at 25 mm/s (60 s ÷ 0.2 s and 60 s ÷ 0.04 s).
What counts as bradycardia and tachycardia?
For adults at rest, below 60 beats/min is bradycardia and above 100 beats/min is tachycardia. Children have higher normal rates.
How do I find a 6-second strip?
Many ECG papers have marks every 3 seconds along the edge. Otherwise count 30 large squares at 25 mm/s.
Practise it yourself
Practise ECG rhythm strip interpretation
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.