How body composition scales work
A body composition scale does more than report weight. Many models also show estimates for body fat, muscle mass and body water. Those extra figures usually come from a technique called bioelectrical impedance analysis, or BIA.
What the scale actually measures
During a reading, the scale sends a very small electrical signal through your body and measures how much it is slowed down, a property called impedance. Water-rich tissue such as muscle lets the signal pass more easily than fat does. The scale then combines the impedance with details you enter, such as age, sex and height, and uses formulas to estimate body composition.
Feet-only versus hands-and-feet designs
Basic models measure through the feet only. Some models add handles with electrodes so the signal also travels through the arms, which allows separate estimates for each limb and the trunk. More electrodes can give a more detailed picture, but the figures are still estimates.
Why readings vary
Hydration, a recent meal, exercise, skin dryness and time of day can all change impedance. Different devices also use different formulas, so numbers from two scales should not be compared directly. A published review of BIA discusses how accuracy and clinical usefulness depend on the device, the measurement conditions and standardisation.
Getting more consistent readings
Measure at the same time of day, under similar conditions, with clean bare feet and dry hands. Focus on the trend across several weeks rather than a single reading.
Who should be careful
The signal is weak, but impedance scales are generally not recommended for people who are pregnant or who have an implanted electronic medical device such as a pacemaker, unless a doctor has said it is fine. Follow the manufacturer’s safety instructions.
This article is for general information only and is not medical advice. Consult a qualified healthcare professional about your health.