How to Calculate Heart Rate on an EKG
Updated October 27, 2026
Calculating the heart rate is the first step in reading any rhythm strip, and it’s tested on the NHA CET exam. There are three common methods. Which one to use depends on whether the rhythm is regular. Here’s how each works, followed by free practice questions. All three assume the standard paper speed of 25 mm per second.
The grid
- One small box = 0.04 seconds
- One large box (5 small boxes) = 0.20 seconds
- 5 large boxes = 1 second, so 30 large boxes = 6 seconds
The three methods
| Method | How | Best for |
|---|---|---|
| 300 method | Count the large boxes between two R waves, then divide 300 by that number | Regular rhythms, quick estimates |
| 1,500 method | Count the small boxes between two R waves, then divide 1,500 by that number | Regular rhythms, most precise |
| 6-second method | Count the QRS complexes in a 6-second strip, then multiply by 10 | Irregular rhythms |
Quick sequence for the 300 method: if the next R wave lands on the 1st, 2nd, 3rd, 4th, 5th or 6th large-box line, the rate is about 300, 150, 100, 75, 60 or 50.
Free practice questions
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On a regular rhythm, there are 3 large boxes between R waves. What is the heart rate using the 300 method?Answer: B300 ÷ 3 = 100 beats per minute.
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On a regular rhythm, there are 20 small boxes between R waves. What is the heart rate using the 1,500 method?Answer: C1,500 ÷ 20 = 75 beats per minute.
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Which method is best for an irregular rhythm?Answer: DBecause the R-to-R intervals vary in an irregular rhythm, counting the complexes over 6 seconds and multiplying by 10 gives a better estimate.
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How many large boxes make up a 6-second strip at standard paper speed?Answer: CEach large box is 0.20 seconds, so 5 large boxes equal 1 second and 30 large boxes equal 6 seconds.
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A 6-second strip shows 12 QRS complexes. What is the approximate heart rate?Answer: D12 × 10 = 120 beats per minute.
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How long is one small box at standard paper speed?Answer: BAt 25 mm per second, each 1-mm small box represents 0.04 seconds.
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