Calipers provide a more accurate body fat measurement.
A handheld body fat analyzer may seem like a convenient way to measure your body fat, but if you use it at the wrong times, you'll end up with unreliable results. These devices use a technology called bioelectrical impedance, in which the machine sends a light electrical current through your body to calculate total body water, which the device then uses to figure your ratio of fat-free mass to body fat. Because the machine depends on you being adequately hydrated, measuring yourself after a workout isn't recommended.
Hydration Is Key
Even if you were faithful about drinking during your workout, chances are you'll end up slightly deficient in water intake after exercise. Bioelectrical impedance is not a valid way to measure body fat in athletes because even the smallest fluid changes could be perceived by the machine as changes in body fat. Although the technology continues to improve, it still uses the same basic technology of measuring total body water -- and if you're dehydrated from exercise, it will be inaccurate.
Results After a Workout
If you measure your body fat with the handheld analyzer after a workout, your dehydrated status will make the machine read your body fat as higher than it actually is. Your skin temperature, which tends to be warmer after a workout, may also skew the results. The National Institutes of Health notes post-exercise warmed muscle tissue, increased blood flow and sweating further affect the accuracy of bioelectrical impedance measurements. You should wait several hours after exercise and correct any dehydration before using a handheld analyzer.
Obtaining Reliable Results
Every time you measure your body fat with a handheld device, recreate the exact conditions to get the most accurate results. If you measure first thing in the morning, you're also likely to be dehydrated, so aim for a time late morning, before exercise, CNN recommends. Even if the results aren't perfectly accurate, you can assess trends in body fat loss or gain over time -- as long as you measure about the same time every day. Note that certain medications, alcohol and pre-menstrual syndrome can affect hydration levels and distort results.
Body fat calipers, water-weighing, air displacement plethysmography and dual energy X-ray absorptiometry are more accurate methods of determining body fat. Of all these methods, body fat calipers are the most convenient, as you can have this measurement done at most fitness facilities by a personal trainer -- the other methods tend to require a clinical setting. Bioelectrical impedance devices that measure leg, arm and belly fat are also more accurate than the handheld devices. These more sophisticated machines are more accurate but are currently only available to government agencies, the armed forces and Division I collegiate athletic programs, CNN reports.