The diameter dependence of the collapse of single- and double-walled carbon nanotubes to two- and four-walled graphene nanoribbons with closed edges (CE(x)GNRs) has been experimentally determined and compared to theory. TEM and AFM were used to characterize nanotubes grown from preformed 4.0 nm diameter aluminum-iron oxide particles. Experimental data indicate that the energy equivalence point (the diameter at which the energy of a round and fully collapsed nanotube is the same) is 2.6 and 4.0 nm for single- and double-walled carbon nanotubes, respectively. Molecular dynamics simulations predict similar energy equivalence diameters with the use of ε = 54 meV/pair to calculate the carbon-carbon van der Waals interaction.