Which unit does the "strain" have?

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
10-6m/m=1ppm=1μm/m=1μϵ=10-3‰=10-4%