How this instrument works
Ape index is the difference between arm span (fingertip to fingertip with both arms held straight out to the sides) and standing height, measured in the same units. A positive number means your arm span exceeds your height — reach beyond what your height alone would suggest — while a negative number means your arms are shorter than your height. Most people land close to zero, since arm span and height track each other fairly closely across the general population.
The name is informal, not a scientific or medical term: it references the long relative arm span of great apes, and gained its foothold in rock climbing and boxing communities where extra reach at a given height can matter tactically. According to Wikipedia's entry on the metric, the term itself was coined by a group of frisbee players at UC Berkeley in 1975, well before it was picked up by climbing and combat-sports culture.
How much a positive ape index actually helps varies a lot by sport and even within a sport. In climbing, some studies have found arm span statistically linked to performance while others found no meaningful effect, and several elite climbers — including Brooke Raboutou and Lynn Hill — compete successfully with a negative index, so a long reach clearly isn't a prerequisite. In combat sports it's more straightforwardly useful for keeping distance and landing strikes first, though research specifically on MMA has found ape index alone doesn't predict who wins a fight.
- Enter Arm span — stand with both arms held straight out to the sides at shoulder height and measure fingertip to fingertip.
- Enter Height — your standing height in the same unit as arm span.
- Read Ape index — arm span minus height. A positive number means your reach exceeds your height; a negative number means the reverse.
- For the most consistent reading, measure against a wall with a helper, and keep both values in the same unit (don't mix inches and centimetres).
Worked example — a 72 in arm span, 70 in height
Enter an arm span of 72 in and a height of 70 in. The ape index is 72 minus 70, or +2.0 in — this person's fingertip-to-fingertip reach is two inches longer than they are tall, a modestly positive index in the range often discussed in climbing and boxing circles as a mild reach advantage.
A person with the reverse measurements — a 70 in arm span and a 72 in height — would come out at -2.0 in instead, the same two-inch gap but running the other direction. Ape index only reports that difference; it says nothing on its own about whether the difference actually helps in a given sport.
Questions
What counts as a 'good' or high ape index?
There's no official threshold — it's an informal metric, not a medical or standardized sport statistic. As a rough anchor, an index within a couple of inches (or a few centimetres) of zero is common for most people, so a positive index in the range of two to four inches is often described casually as a noticeably long reach, while anything larger is comparatively rare.
Does a positive ape index actually improve climbing performance?
The evidence is mixed. Some studies have found a statistically meaningful link between longer relative arm span and climbing performance, while others found no significant effect, and several elite competitive climbers — Brooke Raboutou and Lynn Hill among them — have negative ape indices. Reach can help reduce the number of moves between holds, but technique, strength-to-weight ratio and route-reading matter far more overall.
Where does the term 'ape index' come from?
It's an informal term, not a scientific one — according to its Wikipedia entry, it was coined by ultimate frisbee players at UC Berkeley in 1975, referencing the comparatively long arm span of great apes relative to their height. Climbing and combat-sports communities later adopted the phrase to describe the same arm-span-minus-height comparison.
Is there an alternative way to express ape index besides subtraction?
Yes — a ratiometric version divides arm span by height instead of subtracting, giving a ratio around 1.0 for someone whose span equals their height, above 1.0 for a positive index and below 1.0 for a negative one. This calculator uses the subtraction (absolute) version, which is the more commonly quoted form and reports a length rather than a unitless ratio.
Does ape index matter in sports other than climbing and boxing?
It shows up informally in a few others. A longer reach is often cited as helpful for basketball defense and rebounding and for swimmers' stroke length — Michael Phelps is frequently mentioned with an index around 1.05 on the ratiometric scale — while it's sometimes described as a disadvantage in the bench press, where a longer arm span increases the bar's travel distance.
How do I measure my arm span accurately at home?
Stand with your back against a wall, arms held straight out to your sides at shoulder height, palms facing forward, and have someone mark or measure fingertip to fingertip in a straight line. Measuring against a flat wall or door frame with a tape measure gives a more consistent result than eyeballing it freehand, and repeating the measurement twice helps catch any slouching or arm-angle error.