The human head

The human head
The measurements of the human head based upon an adult looking directly forward.
The eye line falls roughly half way between the top of the head and the chin.
An important thing to remember is to leave a space of eyes width between the two eyes.
The base of the nose is half way between the eyes and the chin.
the mouth is approximately one-third of the distance from the nose to the chin.
The ears fall approximately between the eye line and the nose line. Since People’s ears size varies widely this is not always true but it gives you a starting point.
Some other measurements that may be helpful but again vary according to your subject are:
The sides of the nose usually lineup directly under the tear ducts.
The corners of the mouth generally lineup directly underneath the pupils.

Although these measurements are not always completely accurate, being that people are different, it gives you a general idea of where the features may lay. Therefore besides using these measurements it is also good to draw by using negative space, seeking out shapes, and learning to control your thought process as you draw. I find the biggest obstacle people have when trying to draw a human being is not being able to really draw exactly what they are seeing. In other words people tend to draw what they know. They tend to think of things as features and gender. focusing on drawing an accurate eye, drawing an accurate mouth, making someone look pretty because they are a woman, or masculine because they are a man. Thinking like this does not help us because all eyes come in different shapes and sizes, a mouth has thousands of expressions. It is almost impossible to learn to draw every expression there is. Therefore it is much easier to learn to draw what you see instead of drawing what you know. It also does not help to think of gender. Some men have more feminine features and some women have more masculine features. You must try to let go of symbols of what you think something should look like. To open your mind to all possibilities. sometimes you might be drawing something and you’ll actually see a certain shape and think "that can’t be that shape, or lashes can be that shape, so I’ll make them the shape they should be. It is that kind of thinking exactly which keeps us from drawing the way we would like to draw. You have to keep your mind open to all possibilities. Start looking for shapes in things that you draw. Instead of looking at an eye look at the shape of the space around the iris. (the negative space) Start to name the shapes. You might say the shape around the right side of the iris looks like a half moon and the lid forms the shape of a banana. This takes your mind away from thinking of the subject as an eye and starts you seeing what is really there.