First in situ quantitative synchrotron X-ray diffraction (XRD) study of plastic strain-induced phase transformation (PT) has been performed on α−ω PT in ultra-pure Zr as an example under different compression-shear pathways in rotational diamond anvil cell (RDAC). Radial distributions of pressure in each phase and in the mixture, and concentration of ω-Zr, all averaged over the sample thickness, as well as thickness profile were measured. The yield strength of both phases is estimated to be practically the same, in strong contrast to known estimates. Minimum pressure for the strain-induced α−ω PT, 1.2 GPa, is smaller by a factor of 4.5 than under hydrostatic loading. Theoretically predicted plastic strain controlled kinetic equation was quantified and verified; it is found to be independent of the loading path. Thus, strain-induced PTs under compression in DAC and torsion in RDAC do not fundamentally differ. Obtained results open new opportunity for quantitative study of strain-induced PTs and reactions with applications to material synthesis and processing, mechanochemistry, and geophysics.