Ape index, ape factor, or gorilla index is slang or jargon used to describe a measure of the ratio of an individual's arm span relative to their height. A typical ratio is 1, as identified by the Roman writer, architect and engineer Vitruvius prior to 15 BC. Vitruvius noted that a "well made man" has an arm span equal to his height, as exemplified in Leonardo da Vinci's c. 1492 drawing, the Vitruvian Man. In rock climbing it is believed that an ape index greater than one, where the arm span is greater than the height, provides for a competitive advantage, and some climbers have expressed the belief that exercise can result in an improved ratio, although this view is somewhat controversial.

The ape index is usually defined as the ratio of arm span to height. However, an alternative approach is arm span minus height with the result being positive, 0 or negative. Unlike the dimensionless ratio, this calculation produces a numeric value in the units of measurement used to represent the height and arm span.

Multiple studies have been conducted into the effect of physiological factors, such as anthropometry and flexibility, in determining rock climbing ability, particularly in the area of competition climbing, which is an Olympic sport. A number of these have included the ape index as one of the variables. However, the results have been mixed.

One study found that "untrainable" physical factors, including the ape index, were not necessarily predictors of climbing ability, despite a general tendency identified in previous studies for elite athletes in the sport to share these characteristics. This was supported by a later study that also found that the ape index was not statistically relevant. However, the authors of this second study noted that the findings may have been due to the low variability in the index between the climbers, who all had significantly higher ape indices than those found in the control group. Thus they left open the possibility that the ape index may be more significant when there is a greater degree of equivalence between the other traits under consideration.

Countering these studies are other works that have identified the ape index as a significant (or potentially significant) factor. A 2001 study comparing teenage male and female competition climbers noted that performance differences between the genders could be explained by several factors, one of which was the lower ape index found in the female competition climbers. Similarly, in a later work it was found that the ape index was statistically significant, and thus determined that it was one of several variables that provided the highest diagnostic value in the prediction of climbing performance.

The term was first coined by Frisbee throwers at UC Berkeley in 1975.

It has been noted that swimmers tend to have longer arms in relation to their body size. A notable example is Michael Phelps whose arm span is 10 cm (3.9 in) greater than his height, giving him an index of 1.052.

There is evidence to support that having a higher index will be beneficial to a football goalkeeper ^{[citation needed]}. It can also compensate for being shorter than the recommended norm for a professional goalkeeper^{[citation needed]}. Iker Casillas and Jorge Campos are examples of shorter goalkeepers who possess a higher than average index.