Bioimpedance analysis (BIA)
How smart scales use electrical impedance to measure body fat, muscle mass, and water content.
How a bathroom scale reads your body composition
Step on a smart scale, and within seconds it tells you your body fat percentage, muscle mass and water content. How? Bioimpedance analysis (BIA) sends a tiny, completely safe electrical current through your body via electrodes in the footpads. You won't feel a thing: the current is a fraction of a milliampere at a frequency of 50 kHz, far below anything a nerve or muscle can register.
The science is simple
Different tissues conduct electricity differently:
- Muscle and water, excellent conductors (low resistance)
- Fat, poor conductor (high resistance)
- Bone, minimal conduction
The scale measures the impedance the current meets and feeds it, together with your height, weight, age and sex, into a regression equation that was fitted on a reference population measured by DEXA or underwater weighing. The equation returns total body water first; lean mass follows from the assumption that lean tissue is about 73 % water, and fat is what remains. Every number the scale shows is that chain of assumptions, which is why hydration matters so much.
Advanced models like the Withings Body Scan go further with segmental BIA: a handle adds electrodes for the arms, so the device reads each arm, each leg and the trunk separately instead of assuming the whole body from the legs. Multi-frequency devices add readings at higher frequencies, which pass through cell membranes and let the scale separate water inside cells from water outside them.
How accurate it is
Consumer BIA is about ±3 to 5 percentage points for body fat compared to DEXA scans on a single reading, and the error is not random: foot-to-foot scales systematically under-read fat in lean people and over-read it in people with obesity, because the current only travels through the legs and the equation guesses the rest. Segmental devices narrow the gap but do not close it. Muscle mass estimates are less reliable than fat estimates, and "bone mass" on a consumer scale is a modelled figure, not a measurement.
But consistency matters more than absolute accuracy. Measured the same way every time, the day-to-day noise of a good scale is around 1 percentage point, which is enough to see a real change over a month.
Tips for accurate readings
- Same time, same conditions. Morning, after using the bathroom, before eating or drinking, bare feet on a hard floor
- Hydration is the biggest variable. A dehydrated body reads as fatter, an over-hydrated one as leaner. Skip the reading the morning after alcohol, a sauna, a long flight or a hard evening session
- Not straight after exercise or a shower. Warm skin and shifted fluids move the number by a point or two for an hour
- Women: expect a monthly cycle. Water retention around menstruation shows up as one to two points of "fat" that are not fat
- Read the weekly average, not the day. MyBodyAI smooths the weight chart over seven days; body fat is shown as recorded, so read it against last week yourself
One safety note: manufacturers advise people with a pacemaker or other implanted electronic device not to use BIA scales, and the body-composition function should be switched off during pregnancy, when the equations do not apply.
What the numbers are good for
A single reading answers little. A series answers a lot: whether a diet is costing fat or muscle, whether a strength block is adding lean mass, whether the weight that came back after a holiday is water or fat. The reference method for the absolute number is a DEXA scan, available at sports clinics for the price of a dinner out; measure once by DEXA if you want to calibrate what your scale calls 20 %.
In MyBodyAI
Body fat and muscle mass from a connected smart scale feed the body composition domain of biological age, scored against age- and sex-matched norms from the latest reading, and visceral fat joins them in the long-term trend views. When a scale and a wearable both report a weight, MyBodyAI keeps the measurement, not the mirrored copy, so the trend is built from the device that actually weighed you.
Frequently asked questions
How accurate are smart scales for body fat?
Within about 3 to 5 percentage points of a DEXA scan on a single reading, with a bias that under-reads fat in lean people and over-reads it in people with obesity. Day-to-day repeatability is far better, around one point, so the trend over weeks is trustworthy even when the absolute number is not.
Why does my body fat reading change from day to day?
Because the scale is really measuring water. Dehydration after alcohol, a sauna or a hard session reads as more fat; a big dinner or salty food reads as less. Measure at the same time under the same conditions and judge the weekly average, not the morning's number.
Is bioimpedance safe?
For almost everyone, yes: the current is a fraction of a milliampere and cannot be felt. The exceptions are people with a pacemaker or another implanted electronic device, who are advised by manufacturers not to use BIA, and pregnancy, when the body-composition function should be switched off because the equations do not apply.