You can't tell what your body fat weight is just by looking in the mirror. You need to work out the percentage of your body fat by knowing your weight and your height. It is good to know your body fat percentage if you want to lose weight. Body fat weight is also called BMI or Body Mass Index. When you know your BMI then you will know if you are in a normal weight range, overweight or obese. Body Mass Index can sometimes be inaccurate if you have a lot of muscle, so people such as body builders will have a lot of muscle weight and therefore can't work out an accurate body fat percentage.
Even people who are overweight may still have a lot of muscle sitting under the fat. Those who lose weight by dieting and exercising will begin to build muscle as their body becomes more toned from the exercise. This is why some weeks you may not be happy with the amount of weight you lose, however the weight may not be an accurate way to measure your weight loss as when you build muscle, that muscle will weigh more than the fat you have lost. Muscle weighs more than fat so the more muscle you build you are adding some weight to your body. You can still monitor your weight but you should also take measurements to see how much you are actually losing. Some weeks you may notice only a small change in weight but a large difference in your waist measurement. When you build muscle your BMI may increase a little because BMI is calculated with your weight, so the result might not be 100% accurate.
The scale doesn't always show an accurate progress when you lose body fat, especially when you are exercising and eating a healthy diet. The weight that a person loses in fat is being replaced by muscle weight. This isn't a bad thing as your body will look great when it is slim and toned. It is perfectly normal when you start an exercise program to gain muscle but unfortunately many people don't realize that they are gaining muscle weight and when they see little loss on the scales they become discouraged. This is why you shouldn't only rely on the scales to know if you are losing weight.
If you have started a weight lifting program then you will really build up a lot of muscle mass and that will make your weight loss on the scales appear low. Don't be concerned if this is the case as muscle weight is healthy weight. Muscle uses energy and helps you to burn calories faster so the more muscle you have the faster you will burn off fat. Fat is an unhealthy weight and does nothing for your body except take up space. The more muscle you have the faster your metabolism will be and the more calories you will burn. When you want to know if you are losing weight then try taking measurements of your waist, chest, arms and thighs and compare those measurements regularly. Often you will notice that you are losing weight by the feel of your clothes, when your clothes become very loose you know you have lost weight.
There are a number of ways in which body fat weight is measure and one is with callipers. The skin is pinched and measured. The callipers will grip the skin and fat to get a measure of it. How much flesh you have within the callipers will determine your body fat weight percentage.