ECG Boxes to Seconds Calculator

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Count the boxes between the two points you measured with calipers (or by eye) on the ECG tracing.
A large box contains 5 small boxes. Small boxes are 1 mm wide; large boxes are 5 mm wide on standard ECG paper.
Most ECGs use 25 mm/s. If the strip is labelled 50 mm/s or recorded in a country where that is standard, select 50 mm/s - every duration will be halved.
Duration (seconds)Normal PR range (120-200 ms)
0.2s

Time represented by the measured ECG boxes

Duration (milliseconds)200ms
Duration per box0.04s
Equivalent small boxes5
200 ms
Very short<80Normal range80-200Prolonged200-300Markedly prolonged300+

5 small boxes at 25 mm/s = 0.200 s (200 ms).

  • At 25 mm/s, each small box = 0.040 s (40.0 ms).
  • This duration falls within the normal PR interval range (120-200 ms), consistent with normal atrioventricular conduction.
  • A QRS duration greater than 120 ms would suggest bundle branch block or aberrant conduction if measuring that complex.

Next stepCompare this value against the reference table below to identify which ECG interval or segment it corresponds to. Abnormal findings should always be interpreted alongside the full clinical picture by a qualified clinician.

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