In aeronautics, the load factor is the ratio of the lift of an aircraft to its weight and represents a global measure of the stress ("load") to which the structure of the aircraft is subjected:

where

Since the load factor is the ratio of two forces, it is dimensionless. However, its units are traditionally referred to as g, because of the relation between load factor and apparent acceleration of gravity felt on board the aircraft. A load factor of one, or 1 g, represents conditions in straight and level flight, where the lift is equal to the weight. Load factors greater or less than one (or even negative) are the result of maneuvers or wind gusts.

The fact that the load factor is commonly expressed in g units does not mean that it is dimensionally the same as the acceleration of gravity, also indicated with g. The load factor is strictly non-dimensional.

The use of g units refers to the fact that an observer on board an aircraft will experience an apparent acceleration of gravity (i.e. relative to their frame of reference) equal to load factor times the acceleration of gravity. For example, an observer on board an aircraft performing a turn with a load factor of 2 (i.e. a 2 g turn) will see objects falling to the floor at twice the normal acceleration of gravity.

In general, whenever the term load factor is used, it is formally correct to express it using numbers only, as in "a maximum load factor of 4". If the term load factor is omitted then g is used instead, as in "pulling a 3 g turn".

A load factor greater than 1 will cause the stall speed to increase by a factor equal to the square root of the load factor. For example, if the load factor is 2, the stall speed will increase by a ratio of ${\displaystyle {\sqrt {2}}}$, or about 140%.^{[citation needed]}

The load factor, and in particular its sign, depends not only on the forces acting on the aircraft, but also on the orientation of its vertical axis.