Ape Index Calculator

Background.

Stretch your arms out sideways and measure from one fingertip to the other, and the number usually lands close to your standing height. Leonardo's Vitruvian Man is drawn on exactly that rule, the figure filling the same square from head to heel as from hand to hand. Real bodies drift from it in both directions, and ape index is the name climbers use for the drift.

It comes in two forms, and both are shown here. The ratio divides arm span by height, so 1.00 means the two match and anything higher means a longer reach than your height alone would suggest. The difference subtracts height from span, and it is the version climbers quote: plus five means a span 5 cm longer than the climber is tall. The ratio scales with body size and the difference does not, so an extra 5 cm is a bigger share of a short frame than of a tall one.

Outside sport the comparison has practical uses. Arm span changes little in adult life, while standing height shrinks as the spine compresses with age, so span is sometimes used in place of height for people who cannot stand straight or at all, including in lung-function testing. An unusually long span for a person's height is also one of the physical features a doctor examines when Marfan syndrome is suspected.

The page works out the two numbers and stops there. It labels no result normal or abnormal and predicts nothing about athletic ability. The span-to-height relationship differs with age, sex and ancestry, and a span measured with bent elbows can shift the answer by more than the effect you are trying to see.

What is ape index calculator?

The ape index compares arm span, the distance between the tips of the two middle fingers with both arms stretched out horizontally, with standing height. As a ratio it is span divided by height, a pure number with no unit. As a difference it is span minus height, in centimetres here. A ratio of 1 or a difference of 0 means the two measurements are equal. Neither form is a medical test in its own right; both simply describe body proportion.

How to use this calculator.

1. 01Measure arm span standing with your back flat against a wall and both arms raised sideways to shoulder height, from the tip of one middle finger to the tip of the other. A helper holding the tape gives a far more reliable figure than measuring alone.
2. 02Measure height barefoot, heels against the same wall and eyes looking straight ahead, marking the top of the head with a flat object such as a hardback book.
3. 03Enter both values in centimetres. If you only have inches, multiply each by 2.54 first; entering both in inches still gives the right ratio, but the difference will then be in inches despite the cm label.
4. 04Read the ratio and the difference. Take the span two or three times and use the average, because a slight bend at the elbow or a dropped shoulder shortens it.

The formula.

Ape index ratio = arm span ÷ height, and ape index difference = arm span − height. Both use the two measurements exactly as entered, with no adjustment for age, sex or ancestry and no reference table behind them. In the ratio the centimetres cancel, leaving a dimensionless number: 1.03 means the span is 3% longer than the height. The difference keeps the unit of the inputs. For a person around 175 cm tall, a 1 cm error in either measurement moves the ratio by roughly 0.006 and the difference by a full centimetre, so careful measuring matters more than any decimal places. Both inputs must be positive numbers; there is no upper or lower cut-off and no category attached to the result.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Society for the Advancement of Kinanthropometry, International Standards for Anthropometric Assessment, 2019 (PRINT).
2. [2]Quanjer PH et al. All-age relationship between arm span and height. European Respiratory Journal. 2014.
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.