About

The ape index quantifies the relationship between arm span and standing height. Formally, it is the ratio r = SH, where S is fingertip-to-fingertip span and H is barefoot height. For most adults, r clusters near 1.0, but deviations of ±5cm are common. In rock climbing, a positive ape index (r > 1.0) correlates with improved reach on dynamic moves and reduces reliance on footwork compensation. In competitive swimming, longer spans increase stroke catch area. Misunderstanding your proportions can lead to poor route beta selection or suboptimal stroke mechanics. This calculator computes both the ratio and the absolute difference, classifying results against population and sport norms.

Measurements should be taken with arms fully extended horizontally at shoulder height. Use a wall-mounted tape for height (no shoes) and a partner or wall marks for span. Accuracy degrades if elbows are bent or shoulders protracted. Note: this tool assumes bilateral symmetry. Significant left-right arm length discrepancy (common after fractures) will skew the span reading. Re-measure twice and average for reliable input.

Formulas

The ape index is expressed in two standard forms. The ratio method produces a dimensionless scalar. The difference method yields a signed length value in the input unit.

r = SH

d = S − H

Where r is the ape index ratio (dimensionless), d is the ape index difference (cm or in), S is the arm span measured fingertip to fingertip with arms fully abducted at 90° shoulder flexion, and H is standing height measured barefoot against a vertical surface.

For imperial input, the total height and span are first converted to inches: x_in = ft × 12 + in. The conversion factor between systems is 1in = 2.54cm.

Reference Data

Classification	Ratio Range	Difference (cm)	Typical Occurrence	Sport Relevance
Very Negative	< 0.96	< −7	Rare (~3%)	Disadvantage in reach sports
Negative	0.96 - 0.98	−7 to −3.5	Uncommon (~8%)	May compensate with footwork
Slightly Negative	0.98 - 0.995	−3.5 to −1	Common (~18%)	Neutral impact
Neutral	0.995 - 1.005	−1 to +1	Most common (~30%)	Average baseline
Slightly Positive	1.005 - 1.02	+1 to +3.5	Common (~20%)	Mild climbing advantage
Positive	1.02 - 1.05	+3.5 to +9	Uncommon (~12%)	Notable advantage in climbing & swimming
Very Positive	> 1.05	> +9	Rare (~5%)	Elite climber / swimmer trait
Notable Athletes
Michael Phelps	1.052	+10cm	-	Swimming (23 Olympic golds)
Adam Ondra	1.035	+6.5cm	-	Rock climbing (5.15d)
Janja Garnbret	1.028	+4.5cm	-	Competition climbing
Chris Sharma	1.04	+7.5cm	-	Rock climbing (5.15b)
Ian Thorpe	1.046	+9cm	-	Swimming (5 Olympic golds)
Alex Megos	1.015	+2.5cm	-	Rock climbing (5.15c)
Average Adult Male	1.001	+0.2cm	-	Population baseline
Average Adult Female	0.997	−0.5cm	-	Population baseline

Frequently Asked Questions

A measurement error of just 1cm in either height or span shifts the ratio by approximately 0.006 for a 175cm individual. This means a 2cm combined error can move you across classification boundaries. Always measure twice, average the readings, and ensure arms are fully extended at shoulder height without protraction or retraction of the scapulae.

The skeletal ratio is fixed after the growth plates close, typically by age 18 - 25. However, measured span can appear to decrease with age due to thoracic kyphosis (forward rounding of the upper spine), which shortens effective reach by 1 - 3cm. Conversely, height loss from disc compression can artificially increase the ratio. Training does not alter bone length but can improve functional reach through shoulder mobility.

Spinal disc decompression during sleep means you are tallest in the morning. Height can decrease by 1 - 2cm throughout the day due to axial loading. For consistent results, measure at the same time. Morning measurements will yield a slightly lower ratio than evening measurements. Arm span is unaffected by diurnal variation.

Not unconditionally. A positive ape index helps on long reaches between holds but can be a disadvantage in compression problems and roof climbing where shorter, more powerful lever arms are biomechanically efficient. Climbers with r > 1.04 sometimes report difficulty on squeeze chimneys. The advantage is most pronounced on vertical to slightly overhanging faces with spread-out hold placements.

The difference method (d = S − H) is more intuitive for practical use: a climber with +5cm immediately knows their reach advantage. The ratio method is preferred in anthropometric research because it normalizes for body size, allowing comparison across populations. A +5cm difference is more significant on a 155cm person (ratio 1.032) than a 195cm person (ratio 1.026). This calculator reports both.

Leonardo da Vinci's Vitruvian Man (c. 1490) posits that arm span equals height, i.e., r = 1.0. Modern anthropometric studies confirm that the population mean is very close to 1.0, with a standard deviation of approximately 0.02. The Vitruvian model is a reasonable approximation but does not account for sexual dimorphism: males average slightly above 1.0 and females slightly below.