Short cylindrical shells are often used in offshore structures. Such cylinders are loaded by axial compression as well as hydrostatical pressure. The load-carrying capacity is for practical purposes determined for each of the two load cases separately. The determination of the load-carrying capacity for a combined loading is then based on a combination of those two load-carrying capacities. This combination differs from code to code and has a significant influence on the load-carrying capacity. This paper presents a rational way of estimating the capacity by using simple, well-known theories. The elastic, critical stress (fe ) of a perfect cylinder is estimated according to the classic shell theory for the two load cases, and the respective knock-down factors (α) are calculated according to a code or according to Koiter’s classic stability theory. This leads to an estimate of the ratio between actual stress and the elastic, critical stress (fe ·α) of the imperfect cylinder in the two load cases. The membrane stresses and the bending stresses due to the oval imperfection of the cylinder are calculated according to the plate theory, in which the stiffness is reduced corresponding to those ratios. The capacity is defined as the load level at which a point yields according to von Mises’ yield condition. The method is easily applicable for practical purposes and has the advantage that it estimates the capacity at the actual geometry, yield stress, imperfection level and load combination, and thus enables a better estimation. The paper shows that the interaction curves depend severely on the geometry, the level of imperfection, and the size of the yield stress.