The scales used to measure your body-fat percentage send a harmless electric signal through your body to measure the ratio of fat to lean mass. Muscles draw in water and are highly conducive, while fat is less conducive and holds less water. Therefore, the speed at which the signal moves through your body helps to calculate your body-fat percentage. Certain factors -- which you can control to an extent -- can cause your body fat reading to vary.
While your weight generally fluctuates from day to day, changes in water balance are the primary reason for fluctuating body-fat percentages, as scales rely heavily on your body's water content and distribution. Sweating, taking baths and the amount of water you drink within a few hours of measuring your body fat are factors that affect your reading for the day. Exercising within a few hours of measuring your body fat can also alter your reading, because muscles draw in more water during and immediately after exercise.
Time of Day
The time of day plays a role in a fluctuating body-fat percentage reading. If you weigh yourself in the morning one day and in the afternoon the next, you are more likely to get a different number than the day before. Physicians fail to agree on the best time of day to obtain a more accurate and consistent body-fat percentage reading but they do concur that you need to take your body-fat reading at the same time each day for a more reliable measurement.
Hormonal fluctuations cause the body to redistribute body fat or draw in more water, resulting in fluctuating body-fat percentages over the course of several days to a week. Women are particularly sensitive to these fluctuations, so much that it is commonly recommended that you avoid measuring your body fat a few days before, during and immediately after your menstrual period.
Because body-fat percentage scales can only provide a close estimate, it's unnecessary to get hung up on the exact number that the scale shows. Depending on the type of scale that you use, the margin of difference could be as wide as plus or minus 5 percent or as small as plus or minus 2 percent. Therefore, if you notice a slight fluctuation, there is typically nothing to worry about. So use body-fat scales merely as a way of tracking your progress.