How do you know if you should lose weight is a question people often ask. For the most part the need for weight loss should be determined by how you feel in your own body. There are some calculations that doctors use to determine if you need to lose weight. One of those calculations is BMI or Body Mass Index. BMI is based upon a fairly difficult equation. BMI is figured by multiplying your weight by 703 then dividing by your height squared. A body mass index over 25 is considered overweight. Another way to figure out whether you need to lose weight is to look at your height and waist size. If your waist is less than 50% of your height you are okay. If it is more than 50% of your height you need to lose weight.