The lift-to-drag ratio of an aircraft is the ratio of the lift force to the drag force. This number is dimensionless, and is commonly abbreviated to `L/D`, pronounced "l over d".

The L/D of an aircraft is one of its more important design variables. The value of L/D, in the case of commercial aircraft, is typically maximized, which results in the greatest aerodynamic efficiency. This means better fuel economy and thus lower costs.

Early in the design, L/D is estimated by using the coefficient of lift (C_{L}) and coefficient of drag (C_{D}) calculated from similar types of aircraft and the designer's personal experience. The ratio of these dimensionless coefficients is the same as the ratio of the actual forces, because each coefficient is scaled to the same reference dimension, namely, (1/2)*ρ*V^{2}*S, where S is the planform area of the wing.

It can be shown that the (L/D)_{max} occurs when the parasite drag of the aircraft is equal to its induced drag. This makes sense because parasite drag increases (at an increasing rate) with velocity, and induced drag decreases (at a decreasing rate) with velocity, therefore, the speed at which they intersect on a graph is going to be the smallest sum of the two values.