Phlebotomy (CPT) and EKG (CET)
Heart rate from an EKG strip
Count boxes or complexes, type the paper speed printed on the strip, and read the rate in beats per minute with the length of each box.
Heart rate from an EKG strip
Heart rate
75 bpm
- Small box
- 0.04 s
- Large box
- 0.20 s
- Adult resting range 60 to 100
- 60 to 100
Type the speed printed on the strip; a box lasts its width in mm divided by that speed. Exam arithmetic, not a reading of a real tracing.
Working out the heart rate from a printed EKG strip is one of the few calculations on the NHA CET exam, and it also appears on the medical assistant exams. The method rests on the grid: a small box is 1 mm wide and a large box 5 mm, so their duration is their width divided by the paper speed. At the usual speed of 25 mm per second a small box lasts 0.04 seconds and a large box 0.2 seconds. Three methods follow. For a regular rhythm, divide 1,500 by the number of small boxes between two R waves, or 300 by the number of large boxes. For an irregular rhythm, count the QRS complexes on a six-second strip and multiply by ten. A rate below 60 is called slow and above 100 fast for an adult at rest, the range MedlinePlus gives. The calculator above applies those rules at any paper speed, for exam practice only.
CET · practice
0 / 0
correct out of answered
Pick an answer to start
| Study target: 4 of 5 (80%), not the official passing score |
Practice questions written by Radif Partners from the public outline; not questions from the real exam. Progress stays in this browser.
Questions, formats and fees by Radif Partners · Editorial policy · Method and sources
The three methods side by side
Small boxes: the 1,500 rule
Count the small boxes between two consecutive R waves and divide 1,500 by that number. Twenty small boxes give 75 beats a minute; fifteen give 100. It is the most precise method for a regular rhythm, and the one exam questions use when they give a count.
Large boxes: the 300 rule
Divide 300 by the number of large boxes between two R waves. Four large boxes give 75; three give 100. Many technicians learn the sequence 300, 150, 100, 75, 60, 50 to read a rate at a glance: the R wave after one heavy line, then the next heavy lines.
Six-second strip: count and multiply
Count the QRS complexes in six seconds and multiply by ten. Seven complexes give 70; nine give 90. It is the method for irregular rhythms such as atrial fibrillation, and a quick check of the other two.
When the paper speed is not 25 mm per second
Some machines and some stress tests run the paper at 50 mm per second, which stretches every complex to twice its width. The boxes are the same size, but each one lasts half as long, so the same count means twice the rate. The table shows it.
| Between two R waves | Rate at 25 mm/s | Rate at 50 mm/s |
|---|---|---|
| 15 small boxes | 100 bpm | 200 bpm |
| 20 small boxes | 75 bpm | 150 bpm |
| 25 small boxes | 60 bpm | 120 bpm |
| 30 small boxes | 50 bpm | 100 bpm |
An exam question that mentions the speed is testing exactly this. Read the speed first, then count.
Intervals use the same arithmetic
The duration of a wave or interval is its number of small boxes times the duration of one box. At 25 mm per second, a PR interval of four small boxes lasts 0.16 seconds and a QRS of two small boxes 0.08 seconds. CET questions ask for that conversion and for the order of the waves (P, QRS, T); the normal limits of each interval are taught in your program and tested in context, so learn them from your course material.
Common errors on rate questions
- Counting the boxes from the start of the strip instead of between two R waves.
- Using the 1,500 rule on an irregular rhythm and reporting a single beat-to-beat rate.
- Forgetting a paper speed of 50 mm per second written in the question.
- Counting the complexes in three seconds and multiplying by ten.
The five questions above come from the analysis domain of the CET test plan. For acquisition and safety, continue with the EKG practice test, then the full CET mock. This page trains exam arithmetic; it is not a tool for reading a patient's EKG, which belongs to qualified clinicians (see MedlinePlus on the electrocardiogram).