LängezuDurchmesserVerhältnis

Länge-zu-Durchmesser-Verhältnis, often abbreviated L/D, is a dimensionless quantity used in mechanical and civil engineering to describe the elongation of cylindrical or tubular objects relative to their cross-section. It is defined as L/D, where L is the object’s length and D its diameter. The concept is commonly referred to in English as the length-to-diameter ratio or aspect ratio, and it characterizes slenderness.

A higher L/D indicates a more slender shape. In structural design, slender members with large L/D are more prone to buckling under compressive loads, while short, stubby members with small L/D are more stable. In fluid dynamics, L/D influences flow patterns, drag, and boundary layer development; elongated cylinders experience different drag coefficients and wake structures than short ones.

Applications include drill rods, pipelines, structural poles, masts, rockets, and fibers. Designers use L/D to anticipate performance, ease of handling, and stability, balancing manufacturing constraints, material strength, and service conditions. For example, a rod of length 2 m and diameter 5 cm has L/D = 40.

The term originates from engineering practice in German-speaking contexts and has equivalents in many languages as length-to-diameter ratio or aspect ratio. See also aspect ratio, slenderness ratio, and drag coefficient.