Ape index compares your arm span to your height to describe your relative reach. Climbers, and occasionally basketball scouts, use it as a quick, informal way to see whether someone's reach runs long, short, or proportional for their height.
How the ape index is calculated
Ape index has two common expressions. The difference form subtracts height from arm span (arm span − height), giving a number in centimeters or inches. The ratio form divides arm span by height, giving a unitless number centered near 1.00. Both describe the same underlying relationship, just on different scales — the difference is easier to read at a glance, while the ratio makes it easier to compare people of very different heights.
Inputs and what they mean
The calculator needs two measurements in the same unit: arm span (fingertip to fingertip with arms stretched to the sides) and height (standing, barefoot). Small measurement errors of a centimeter or two are common and expected — take the measurement a couple of times and use the average if your numbers vary. The unit toggle lets you enter either centimeters or inches without converting by hand.
Limits and edge cases
Ape index is a popular rule-of-thumb in climbing gyms, not a scientific predictor of performance — technique, strength, flexibility, and route reading matter far more than a few centimeters of reach. A positive ape index can help on spaced-out holds, while a negative one often favors compact, powerful climbers on technical, close-together sequences. Treat the result as a fun data point, not a verdict on climbing potential.