Lesson 3 of 21 · Fundamentals
EKG Paper & Measurements
Standard EKG paper runs at 25 mm/s with a gain of 10 mm/mV. Every measurement you make, from rate to intervals to ST elevation, comes from counting boxes.
01Boxes
- Small box: 1 mm = 0.04 s horizontally, 0.1 mV vertically
- Large box: 5 mm = 0.20 s horizontally, 0.5 mV vertically
- 5 large boxes = 1 second; 30 large boxes = 6 seconds
02Calibration
The calibration pulse at the start of every tracing should be 10 mm tall (two large boxes). If it is 5 mm (half gain), every voltage you measure is halved, so LVH and ST elevation can be missed.
Clinical pearl. Always check calibration and paper speed before calling low voltage or high voltage.
Key takeaways
- 1 small box = 0.04 s; 1 large box = 0.20 s
- 10 mm = 1 mV
- Check calibration first