Weight is directly related to BMI, but not as important as % Body Fat or Waist/Hip Ratio which indicate whether the person is over or under weight.

S3X Fitness tracks three important body measurement:

• Butt - The measurement around the largest part of your butt. This is used to calculate the Waist/Hip Ratio.
• Belly - The measurement around the largest part of your belly. Will probably be at/near your belly button. People who are trying to lose abdominal fat will want to watch this measurement.
• Waist - The measurement around the smallest part of your waist. Typically this occurs about half way between the bottom of your rib cage and the top of your hip bone. This is not the measurement around where your jeans sit on your hips! Waist is used to calculate your Waist/Hip Ratio.

Don't worry too much if your lastest measurement is too high or too low. The measurements will naturally fluctuate. Your goal is just to get the trend line going in the right direction. To do that you need more dots on the "right" side of the trend line than on the "wrong" side.

S3X Fitness tracks 4 or 5 other measurements for members who are trying to build muscle:

• Forearms - The measurement around the largest part of your forearm (while flexing). The forearm is often neglected, but big forearms are hot!
• Biceps - The measurement around the largest part of your bicep (while flexing). The triceps play a big role in this measurement as well, but they're not what are being flexed.
• Chest (Men Only) - The measurement around the largest part of your chest while flexing your pecs. This measurement is used to calculate your Waist/Chest Ratio.
• Quads - The measurement around the largest part of your upper leg (while flexing) – usually right at your groin.
• Calves - The measurement around the largest part of your calf muscle (while flexing).

One interesting thing is that in many men Biceps = Calves = Neck. So to be "well-proportioned" you should shoot for the bicep and calf lines shown above to be very close to each other.

And don't worry too much if one of your lastest measurements is too high or too low. The measurements will naturally fluctuate. Your goal is just to get the trend line going in the right direction. To do that you need more dots on the "right" side of the trend line than on the "wrong" side.

Your lean mass is everything that's not fat. So muscle, bones, the food you just ate, the water in your body, etc. It's very important to monitor your lean mass when you're losing weight since you can lose muscle if your body decides to burn muscle instead of fat. (For example, when you do a lot of constant-state moderate cardio.) You want to see the trend line above staying flat or going up. But even if your trend line is going down – remember that lean mass is based on your percent body fat, which may not be measured perfectly. What matters most is what you see in the mirror.

Segmented body fat scales give a much clearer picture of where the fat is in your body. Unfortunately, with the exception of liposuction, there's no way to target fat loss in a particular part of your body. Working out an area, doesn't increase fat loss in that area (at least not any more than fat loss elsewhere).

If you think data is missing in the graph, it's probably because it overlaps with the data for the same body part on the other side of the body. For example, left arm may cover up the data for right arm.

Segmented body fat scales typically give details on the muscle mass you have in different parts of your body. This is useful if you're trying to build particular muscles. Realize that some of the values above have been divided to make the graph tighter and better able to show trends.

If you think data is missing in the graph, it's probably because it overlaps with the data for the same body part on the other side of the body. For example, left arm may cover up the data for right arm.

Body fat is the ultimate measure of fitness. Your body is made up of lean mass and fat. Together they are your weight. Since it's possible for your pounds of fat to stay constant while your percent of body fat changes (e.g. if you gain only muscle), there are times when you should consider body fat separately from percent body fat. For overall fitness and health you want to look at percent body fat, but if you're trying to lose weight you'll want to look at pounds of fat. However, always keep an eye on your lean mass to make sure when you lose weight, you're not losing muscle.

If you ever wondered how you get 6-pack abs - that's all about your body fat level. Get down to 15% (for men, not sure about women) and the top two abs will become visible. Get down to 11-13% and you'll have a full 6-pack. Of course, you need to work your abs as well, but you can work them all day long and they won't show if you've got a big layer of fat over them.

The problem is getting an accurate body fat reading. The body fat percent given on scales is only a rough estimate. Caliper tests are sometimes more accurate than scales, but are often quite inaccurate as well. The important thing is to always measure body fat with the same device. It may not be the most accurate number, but it will be good at showing you general trends.

The Waist/Hip ratio is one of the better measures of overall health – everything from your cardiac health, to a woman's fertility, even the intelligence of a woman's kids is tied to Waist/Hip Ratio (see Wikipedia).

It's interesting that "healthy" for men is so close to borderline obesity.

The Waist/Chest ratio is primary determining factor in whether one man will perceive another as attractive. The lower this number is, the better – especially for gay men. Overall, the most ideal Waist/Chest Ratio is approximately 0.77. (see Today.com)

Women's perception of a men is largely related to the Waist/Shoulder Ratio, which is closely related to the Waist/Chest Ratio. (see Wikipedia). Because shoulders are very difficult to measure (especially on your own), we use the Waist/Chest Ratio.

Body Mass Index is directly related to a person's weight and is a rough indicator of whether the person needs to lose or gain weight:

However, % Body Fat and Waist/Hip Ratio are better measures of fitness and health. For example, bodybuilders are often obese according to BMI, but (other than their testosterone use) they're quite fit and healthy.