If you are a patient, this page is for technician exam prep and is not a way to interpret your own EKG. Ask your clinician about your results, or read MedlinePlus on electrocardiograms.
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Educational exam prep, not medical advice. This page teaches what certification exams test. Follow your facility’s policies, your supervising clinician and the manufacturer’s instructions in practice. If you are a patient with a health question, talk to your clinician or see MedlinePlus.
EKG paper basics
| Measure | Value at 25 mm/s |
|---|---|
| Small box (1 mm) | 0.04 seconds |
| Large box (5 mm) | 0.20 seconds |
| 5 large boxes | 1 second |
| Standard calibration | 10 mm = 1 mV |
The 5-step method
- 1. Rate
Regular rhythm: 300 / large boxes between R waves, or 1,500 / small boxes. Irregular: count QRS complexes in a 6-second strip and multiply by 10.
- 2. Rhythm
Are R-R intervals regular, regularly irregular or irregularly irregular?
- 3. P waves
Is there one upright P wave before every QRS in lead II?
- 4. PR interval
Normal 0.12-0.20 seconds (3-5 small boxes).
- 5. QRS
Normal is narrow - under 0.12 seconds (fewer than 3 small boxes).
Normal sinus rhythm vs. rate variants
| Rhythm | Rate | Other criteria |
|---|---|---|
| Normal sinus rhythm | 60-100/min | Regular; one P per QRS; PR 0.12-0.20 s; QRS < 0.12 s |
| Sinus bradycardia | < 60/min | Same as sinus otherwise |
| Sinus tachycardia | > 100/min | Same as sinus otherwise |
What the technician does
An EKG technician acquires a clean tracing, recognizes rhythms the exam lists, and reports urgent findings to the licensed provider according to facility policy. Technicians do not diagnose or treat.
Test yourself on the EKG
Take the free practice test and get a score report by topic in a few minutes.
CNA, PTCB and CCMA prep course - $19
1,297 original practice questions for CNA, PTCB and CCMA, with explanations, timed exams and domain score reports. One-time $19 for 90 days, emailed to you after purchase - no subscription.
Your course access is emailed to you after purchase. 14-day money-back guarantee. Refund policy
Frequently asked questions
How do you calculate heart rate on an EKG?
300 divided by large boxes between R waves, 1,500 divided by small boxes, or QRS count in 6 seconds x 10 for irregular rhythms.
What is a normal PR interval?
0.12-0.20 seconds.
What is a normal QRS duration?
Less than 0.12 seconds.
How long is one small box on EKG paper?
0.04 seconds at the standard 25 mm/s paper speed.
Sources
- StatPearls (NCBI Bookshelf) - Electrocardiogram (accessed 2026-09-23)
- NHA - CET Test Plan (2017 job analysis) (accessed 2026-09-23)
- MedlinePlus - Electrocardiogram (accessed 2026-09-23)