The long-standing issue-the ferromagnetic coupling of adjacent Nd2Fe14B matrix grains across grain boundaries severely impedes further coercivity enhancement of sintered Nd-Fe-B permanent magnets. However, the quantitative role of the ferromagnetic coupling across grain boundaries on the coercivity degradation of sintered Nd-Fe-B magnets is still not well understood. Here, we quantified the nucleation effect of correlated (ferromagnetically coupled) matrix grains in sintered Nd-Fe-B magnets by integrating electron backscatter diffraction, energy dispersive spectroscopy, atom probe tomography, and high-resolution magnetic force microscopy in conjunction with micromagnetic simulations. We revealed that (1) the de-nucleation effect of correlated matrix grains (αψ) across intergranular grain boundaries is linearly enhanced with the increasing thickness (d) of grain boundaries (αψ = kd + l) and k is reduced with increasing exchange stiffness of these grain boundaries when the d is comparable to the domain wall width of the Nd2Fe14B matrix grains in sintered Nd-Fe-B magnets; (2) The thickness variation of the ferromagnetic grain boundary at the nanoscale is harmful to achieving high intrinsic coercivity of sintered Nd-Fe-B magnets; (3) The nucleation field is reduced if there is a little misalignment between correlated (ferromagnetically coupled) matrix grains. These findings shed light on grain boundary engineering at the nanoscale for enhancing the coercivity of Nd-Fe-B permanent magnets.