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Strain is the fractional change in length, so it is dimensionless: 1.2 mm on a 2 m bar is 0.0006, or 600 microstrain. Structural steel yields around 1200 microstrain, so strain gauges are calibrated in microstrain because working values are always small fractions.
Engineering strain
epsilon = change in length / original length
Microstrain
microstrain = strain x 1,000,000
Because real strains are tiny. Writing 0.0006 invites decimal errors, while 600 microstrain reads cleanly on an instrument.
True strain uses the instantaneous length, ln(L/L0). It matters only for large deformations such as metal forming; below a few percent the two agree.