There is a very wide variation in the hand spans of pianists. Hand spans among adult men and women can vary by as much as 5 inches (12.7 cm).
The available data indicate that, on average, the hand spans of adult male (thumb to fifth finger) are about one inch (or 2.5 centimetres) greater than those of adult females. This means men can reach, on average, more than one extra white key on the current keyboard. For more results of the Australian study*, see: http://smallpianokeyboards.org/hand-span-data-recent-australian-study/
An important benchmark separating ‘small’ from ‘large’ hands is a span of 8.5 inches. Up to this point, the pianist cannot normally play a tenth, and more importantly, fast passages of octaves and large chords can be uncomfortable and involve pain or tension. From the available data, we can estimate that about 76% of adult men have hand spans that can reach 8.5 inches or more. This leaves about 24% of men who cannot play a tenth. For women, the situation is much worse, as an estimated 87% of adult females do not have hands large enough to play a tenth.
The photos below compare ‘average’ female and male hands:- the male hand is very relaxed playing octaves and can accomplish legato ocatves. The male 9th is equivalent to the female octave, with thumb near the front of the keys. This means reduced power due to the flat hand and fingers outstretched, and tension in the hand during extended passages. The female 9th is equivalent to the male 10th – just playable on the front edge.
For further information about the proportions of pianists with small hands, see: http://smallpianokeyboards.org/how-many-adults-have-small-hands/
*This study was presented to, and published by the Australasian Piano Pedagogy Conference 2015: https://www.appca.com.au/proceedings/
Ethnic and age differences
The conventional piano keyboard further disadvantages some races with generally smaller hands. Based on the recent Australian study, it appears that over 90% of female Asian pianists have spans below 8.5 inches.
Human hand span variation is even greater when we include children. Unlike string players, they do not have the advantage of learning on a keyboard that suits their hands. Older people may find that their span is reduced with age.
A smaller study of the hand spans of children indicates that there is a significant overlap between the spans of adult women and those of young children (under 12 years). In other words, a significant proportion of women (almost one third) have ‘child-sized’ hands.