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How to calculate body fat

How to use a body fat analyzer scales?
Body fat percentage is simply the percentage of fat that your body contains. 
For most body fat/water scales, you will have to enter your profile to accurately assess your body fat content. This includes entering your height, age and gender. 
You will need to stand on the scale barefoot to get a reading. If your feet are too cold, the scale will register an error reading and you will have to remedy the situation to get a reading. When each heel is correctly positioned as noted in the scale’s user manual, the scale will pass a very weak and mild current through your body to get a reading.
The scale will then calculate your body fat percentage and weight. Many scales also include a chart on the digital readout that maps whether you fall into underweight, normal, overweight or obese categories. If your scale does not have this functionality, its user manual should have this information for you to reference and determine which category you fall into. Some scales also offer the Body Mass Index (BMI) measurement as part of the reading.
Comparing with other methods of measuring body fat, there is no physical imposition to the user; no need for a trained technician to operate the equipment; and the entire procedure takes less than one minute. 
What will interfere the weighing results when measuring body fat scales and water scales?
Unclean foot pads may interfere with conductivity.
Nylons interfere with conductivity. If it is absolutely necessary to measure in nylons, use a drop of isopropyl (rubbing) alcohol on the foot pads to act as a conductor.
Early morning is not recommended because the body is often dehydrated after a night's sleep. Once you have established your baseline, monitor body fat about twice a month. Checking body fat more frequently is not beneficial as changes occur slowly over time.
What the body fat scales and water scales helps you?
Many modern scales have a body fat and water analyzer functionality that measures your body fat percentage in addition to measuring your weight. This added functionality can be helpful in determining how much weight you need to lose to be within a healthy body fat percentage range and track how well your weight loss is progressing.
This is where a body fat scale may play a useful role. It is possible that your body composition may be changing while your body weight remains the same. This occurs when you are losing body fat and gaining muscle (a good thing!). If you replace a pound of fat with a pound of muscle your weight will stay the same, but you will look leaner because the muscle is more dense (takes up less space) and your body fat percentage will drop.So, you actually may be making great progress and a body fat scale will reflect this more accurately than a typical scale.
How do body fat scales and hydration scales work?  
Body fat scales use a technique called Bioelectrical Impedance Analysis (BIA). A very small, harmless electrical current passes through your body tissues, slower through fat and quicker through muscle. The signal is conducted through the water contained in the body. Lean muscle has much more water than fat tissue and allows the signal to pass easily. Fat causes impedance or resistance to the signal. Resistance is what is being measured. So, actually it is not analyzing your body fat percentage so much as your body density. The scale then takes this reading and uses a formula to give you a body fat percentage reading. 
Things that can affect hydration include:
strenuous exercise
recent food intake
diuretics such as caffeine, alcohol, certain medications
Are there people for whom the monitor is not appropriate? 
A: People with pacemakers are advised not to use the monitors. Although there are no known health risks, this is a precaution that is advocated by all manufacturers of BIA. Accuracy is an issue for certain categories of people, but the units may still be used to monitor trends and accurately show degree of change.
They include:
Pregnant women
People who exceed the weight capacity of the scale (models vary) and/or exceed 75% body fat
Professional athletes and bodybuilders.
Are body fat scales accurate?  
Following phase is extract from Tanita website for your reference.:
“How accurate and reliable are Tanita's Body Fat Monitor/Scales? 
A: Independent research at several major universities (including Columbia University in New York City) has confirmed that in clinical settings, the Tanita Body Fat Monitor is accurate within +/- 5 percentage of the institutional standard of body composition analysis--Dual Energy X-ray Absorptiometry (DEXA). It should be made clear that there is only one method of calculating body composition that is close to 100% accurate, and that is an autopsy. Tanita believes its method to be the most convenient and accessible to accurately predict body composition. Tanita's Body Fat Monitor Series results are repeatable to within +/- 1 percent variation when used under consistent conditions.”
"To obtain the greatest accuracy and repeatability from our product, Tanita suggests the following protocol.
Take readings at least 3 hours after rising, eating a large meal, or exercising.
Take readings once a week at the same time of day under the same conditions. (Note: Taking readings more than once per week is not beneficial since body fat does not change from day to day.)
Average your readings for the month.
Compare averages from month to month.
By following this format, hydration fluctuations throughout the month will average out, and you will be able to better assess any real change in your body fat percentage over time."
Generally we can estimate that there’s no very accurate body fat scales convenient in house use. Factors such as body type, body temperature, hydration, and recent exercise can have a noteworthy impact on the number you see on the scale. Even wet or sweaty feet can skew the results. 
Whether or not you get a true analysis, what you can do is use that number to gauge your progress. But you must be consistent when you weigh yourself, so do it at the same time of day, drink/eat approximately the same amount, don't do it directly after exercising, and use it in a room that is generally a stable temperature.
Utilize the body fat scale as a means of tracking trends rather than focusing on what that number is or comparing that number to what you think your body fat should be. 
Look for features such as multiple-user memory where you can store data for several users. 
Body fat scales can be a useful device enables one to establish a baseline and use the product for "trending purposes" to assess loss or gain in body fat over time. They may not always be exact so try not to get too obsessive with the numbers they display. Use a body fat scale to help stay motivated, but the best way to know how you are doing is awareness of how your clothes fit and how your body feels.
Fat/Hydration Content Fitness Assessment Chart
After your weighing, you can use the following assessment chart for reference.
Besides normal digital display of fat % ,hydration % and calorie,the device also uses 5 different colors of LCD backlight to reflect 5 different fat content levels, giving a visual effect to the interpretation of your body fat and hydration.
The above information is only for reference.