Plasmon rulers relate the shift of resonance wavelength, λl, of gold agglomerates to the average distance, s, between their constituent nanoparticles. These rulers are essential for monitoring the dynamics of biomolecules (e.g., proteins and DNA) by determining their small (<10 nm) coating thickness. However, existing rulers for dimers and chains estimate coating thicknesses smaller than 10 nm with rather large errors (more than 200%). Here, the light extinction of dimers, 7- and 15-mers of gold nanoparticles with diameter dp = 20-80 nm and s = 1-50 nm is simulated. Such agglomerates shift λl up to 680 nm due to plasmonic coupling, in excellent agreement with experimental data by microscopy, dynamic light scattering, analytical centrifugation, and UV-visible spectroscopy. Subsequently, a new plasmon ruler is derived for gold nanoagglomerates that enables the accurate determination of sub-10 nm coating thicknesses, in excellent agreement also with tedious microscopy measurements.