Spellings and units for "strain"
The unit of strain is meters per meter: [m/m]
Since the change in length Δl is very small compared to the original length l0, the strain ε is often given in microns per meter: [μm/m].
If the unit m/m is reduced, 10-6 or "parts per million" (ppm) remain. The elongation is also expressed in percent (‰) or in percent (%).
In the US, the unit "microepsilon" or "microstrain" [με] stands for 1 μm/m