Abstract.This paper reports on theoretical and experimental investigations into the buckling characteristics of a series of six ring-stiffened circular cylinders that experienced general instability when subjected to external hydrostatic pressure. Each study used between 3-5 designs with the same internal and external diameters, but with different numbers and sizes of ring-stiffeners. Four used designs that were machined to a high degree of precision from steel, while the other two were machined from aluminium alloy. The theoretical investigations focused on obtaining critical buckling pressure values, namely P cr , for each design from the well-known Kendrick's Part I and Part III theories, together with an ANSYS finite element prediction. The thinness ratio λ 1 , which was originally derived by the senior author, was calculated together with a dimensionless quantity called the plastic knockdown factor (PKD), for each model. The plastic knockdown factor was calculated by dividing the theoretical critical buckling pressures P cr, by the experimental buckling pressures P exp . The thinness ratio was used because vessels such as these, which have small but significant random out-of-circularity, defy "exact" theoretical analysis and it is because of this that the design charts were produced. Three design charts were constructed by plotting the reciprocal of the thinness ratio (1/ λ 1 ) against the plastic knockdown factor (P cr / P exp ), using results from Kendrick Part I, Kendrick Part III, and ANSYS. Comparison of the results obtained using Kendrick's theories and experimentally obtained results was good.