This info is taken from one of my favorite books on drawing. It is “ Drawing the Head and Hands” by Andrew Loomis
William Andrew Loomis (1892–1959), was an American illustrator, author, and art instructor. His commercial work was featured prominently in advertising and magazines; however, Loomis is best known as author of a series of instructional art books printed throughout the 20Th century. Long after his death, Loomis' realistic style has continued to influence popular artists.
If you are serious about drawing the human form this man is the best teacher there is on the subject. He has covered everything one need to know about how to do it!
Getting the proportions right is the most important aspect of painting the human.
I have found that even the tiniest mistake can make a portrait not look like the person. It is important to know the structure below the skin and the measurements of the face of a man, woman, child and an older person as each one will be different.
Here I will explore the woman's face as Andrew Loomis has explained in his book.
If you look at the cranium it actually looks like a ball.
First Mr. Loomis would establish an axis with a nail through the top of the ball.
Then he would divided the ball into quarters and again at the equator.
Then he would slice off a thin slice on each side you have produced a basic shape that resembles the cranium.
The equator line is where you draw the brow. One of the lines through the axis becomes the middle line of the face.
Here we see how Mr. Lommis divided the female head.
The hair line is 1/3 from the brows.
The eyes are about half way.
The end of the nose from the brows is 2/3
The area in between the end of the nose the top of the mouth is 1/3
the mouth is 2/5
the Area from the edge of the bottom of the lower lip to the chin is 3/3
The eyebrow will usually lineup with the top of the ear.
The females head will vary only slightly from those of the male head. The bone and muscle structure is lighter and less prominent. The eyebrows are usually a little higher then the mens are and the mouth is smaller. The lips are more full and rounded and the eyes are slightly larger.
Here are some examples for Mr. Loomis work