D:100 = d:X, whence X = 100×d/D
Instead of working out the calculations, we may find the required index in the tables already compiled.
Volume.—The volume of the cranium cannot be taken directly, except in the case of a skull. After the various osseous foramina have been closed, the cranial cavity is filled through the occipital foramen with any one of a number of substances (millet, shot, water, etc.), which is afterward measured. The method of taking this measurement is practised on a facsimile of a cranium already calculated, and usually made of metal.
But in the case of a living person the direct calculation of the volume is impossible. Nevertheless various empirical methods have been sought for obtaining this measurement, even though imperfect and approximate. Recently renewed use has been made, especially in France, of an approximate calculation made by means of Broca's cubic index. The volume of the cranium is equal to half the product of the three diameters, divided by an index which varies according to age.
This index is as follows:
| Adults from 25 years upward. | men | 1.20 |
| women | 1.15 | |
| Young persons from 25 to 20 years. | men | 1.15 |
| women | 1.10 | |
| Young persons from 20 to 16 years. | men | 1.10 |
| women | 1.08 | |
| Children of both sexes. | 15-10 years | 1.07 |
| 10-5 years | 1.06 | |
| 5 years and below | 1.05 |
An index of cranial development is afforded by the maximum circumference. The average volume of the normal adult cranium is about 1,500 cubic centimetres: mesocephalic cranium.
When the cranium is much inferior in volume, it is called microcephalic (from 1,200 down to 700 cubic centimetres). When on the contrary it is much superior (from 1,900 up to 2,200 cubic centimetres), it is called macrocephalic or megalocephalic.
For the face, the following craniometric points should be noted: