A quantitative metallography method is described to obtain size and number per unit volume of martensite units from linear intercept measurements. The entailed relationship between the number per unit volume of martensite plates and the volume fraction transformed is consistent with the autocatalytic nature of martensite. Application to the athermal and the isothermal martensite reactions allowed development of a unified microstructure-kinetic model. Validation of the model equations was achieved with data pertaining to FeNiC and FeNiMn alloys found in the literature. The apparent activation energy for propagation of isothermal martensite yielded by the transformation curve is compatible with the value obtained from the initial transformation rate. The defect redistribution process austenite/martensite established during the thickening of the plates has a crucial role in autocatalysis.