Have you ever wondered what artists are actually doing when they peer at something over an outstretched pencil-top? Now you know: they are measuring up the model (or object). Ok, so a pencil-top is a pretty rough measure, but it is an immense help in getting down the proportions of your subject.
Using this method, it is important to stand in the same place, and to keep your head as still as possible when measuring, and to extend the arm fully with elbow straight, each time a measurement is made. You should not be too close to the model.
Remember that the basic unit in figure drawing is the model's head, from top to chin. Holding your pencil in a fist with the thumb upwards, and arm stretched out fully, close your non-master eye and align the top of your pencil with the top of the model's head, and slide your thumb down the pencil until it aligns with the model's chin. There you have your basic unit of measurement on the pencil. Repeat this step whenever necessary.
Now, to find how many heads tall your model is, drop your hand slightly so that the top of the pencil is at the chin. Observe carefully the point on the figure that aligns with your thumb - this should be roughly below the breastbone. (2 heads - you count the head itself). Drop the top of the pencil to that point, and so on, down to the feet.
To place these measurements on the paper, simply make seven equally spaced horizontal lines down the paper. The actual distance doesn't matter, so long as they are even. You are scaling the observed information to fit the page. Your top division will be the head. As you begin to draw the rest of the figure, check the placement of key points against your head measurements. The armpit begins just above the second head line, the hips at the third, for example. Naturally this will vary depending on the bodyshape and pose of the model. The head unit can also be used to check the size and relative placement of other parts of the body, as demonstrated by the red lines in the diagram above. Use the 'scale' you have established with the height to judge the correct distance on the paper. In this example, the wrist is one head-unit away from the body.
Next Page: How to Measuring Angles in the Figure