Precise particle size customization is a key requirement for the applications of stimuli-sensitive poly( N -isopropylacrylamide) microgels. No empirically supported mechanism has been presented to explain the microgel particle size dependence on polymerization conditions. In this work, nonstirred precipitation polymerization is employed to address this question. It is argued that the reaction proceeds by radical solution polymerization mechanism rather than as emulsion polymerization. The number of particles in the batch, and the microgel particle volume, is shown to correlate with the charge density of the particles determined by the kinetic chain length. Cross-linking effi ciency is recognized to be an important factor in the particle nucleation. Properties of radical solution polymerization, such as differences in the rate of decomposition of thermally decomposing initiator and redox initiator, are used to tune the particle number density in the batch. Contemporary synthesis approaches, including temperature-programmed synthesis, are discussed in the context of these results. approaches clearly demonstrate that microgel particle synthesis can be advanced, but also the need for thorough understanding of the particle formation mechanism in order to devise new synthesis methods. In this work we investigate the fundamental aspects of microgel particle formation in precipitation polymerization using rapid, convenient, and highly reproducible nonstirred synthesis method and show that a correlation exists between the fi nal volume of the PNIPAM particles in the collapsed state and the average degree of polymerization predicted by the solution polymerization kinetics. As already recognized by Gao and Frisken, [ 14 ] connecting the radical polymerization mechanism with the fi nal particle volume would be an attractive explanation for the experimentally observed microgel size. However, the experimental evidence in support of this mechanism has been nonconclusive and even confl icting [ 14 ] until now.