There are a number of ways of estimating one's body fat, but the simplest of these is the body mass index (BMI). The BMI scale is not, however, appropriate for children, the elderly and pregnant women, so these people will need to consult a doctor for advice; but others can use the scale to assess their own situation.
It is important to have an idea of one's body fat before beginning any weight loss program because it is the most accurate assessment of the situation, and the best way of determining whether a weight problem exists.
BMI is calculated with the formula: [weight] / [height]². (Weight and height can be entered in either imperial or metric measurements, but must both be given in the same form.) For most people, this will produce a score of 20-25; this is entirely normal and indicate a healthy weight. Less than 20 suggests the individual is underweight; more than 25 overweight; and over 30 obese.
Most doctors use the BMI scale to estimate the body fat of their patients, but this calculation can be done easily online. Entering the term into a search engine will bring up thousands of hits, and most of the sites will allow you to enter your measurements and be given your score. Many of these sites will also contain useful information and tips, as well as links to other dieting and health sites.