sorry for the cut and paste, but this seemed to be the best answer I found:
You can do it approximately by the following method: if you assume that the average density of tissue and bone in your head is similar to that in the rest of your body (taken as a whole), then all you need to do is determine the volume of your head, the volume of the rest of your body, and then multiply the ratio of the two by your total body weight.
To determine the volume of your head, fill a bathtub up to the rim and carefully immerse yourself in it completely - including your head. Collect the water that spills over the rim and measure its volume. This is the volume of your entire body.
Then repeat the exercise, but this time keeping just your head above water. Measure the overspill again, and subtract this from the volume of overspill on the previous occasion. The amount remaining is equal to the volume of your head.
Now take the ratio of the volume of your head alone, divded by the volume of your entire body. Multiply your total body wieght by this number. The result is the weight of your head. Voila!
Here's some other techniques:
· 1 decade ago