What Bioimpedance Scales Can and Cannot Measure

A body-composition scale sends a current too small to feel through your feet and then reports body fat, muscle, water and bone. This guide explains what the technology, bioelectrical impedance analysis or BIA, really measures, how it compares with a DEXA scan in the studies we read, and what that means for how you use the numbers. It is general information for adults, not medical advice. For the buying checks, see our guide to choosing a smart scale.

What the scale measures

Garmin's manual for its Index S2 describes the principle: the scale sends a small current from one foot through the body to the other, measures the signal that comes out, and uses it to estimate body composition. A 2019 review by Marra and colleagues in Contrast Media and Molecular Imaging says the basic idea is that how a low-voltage current passes through the body depends on its composition. The measurement is used to estimate total body water, which the review puts at approximately 73% of fat-free mass in healthy individuals.

The scale does not measure fat. It measures electrical resistance and then applies an equation. The authors of a 2021 study of smart scales explain that whole-body fat-free mass is calculated from a regression equation using resistance, height, weight and age, and that researchers do not know the equations smart scales use. Foot-to-foot scales also measure the lower body and extrapolate to the whole body, which the same authors say makes them more sensitive to differences in body shape. The Marra review adds that such equations are generally population-specific and device-specific.

What changes the reading

• Water. Garmin's manual says activities that temporarily affect your weight, hydration and distribution of water in your body can affect accuracy, and recommends waiting two hours after eating, drinking, exercising, bathing or visiting a sauna. The Marra review lists non-standard body position, previous exercise and food or fluid intake among the factors.
• Your feet. The 2021 authors cite earlier technical work showing that foot size and width, sweat, foot position and bent legs can all change a foot-to-foot reading. Garmin's troubleshooting page asks for a dry scale, bare feet, standing still with feet flat on the glass and no bouncing.
• Timing and equation. Garmin asks you to use the scale at about the same time of day each time. Two different scales can also give one person different body-fat figures, because each applies its own equation.

The DEXA scan, the usual reference

The patient-education site RadiologyInfo.org says a DEXA scan is commonly used to diagnose osteoporosis and assess fracture risk, uses a very small dose of ionizing radiation, and should be mentioned to the doctor and technologist if you are pregnant. The Marra review calls DXA the reference technique in clinical practice for body composition and says no more than two body scans a year are currently advised, so it is not something you repeat weekly.

What the studies found

Three smart scales against DEXA. Frija-Masson and colleagues, in JMIR mHealth and uHealth in 2021, weighed patients on one of three smart scales just after a DEXA scan at a French hospital. The median errors for body weight were 0.3 kg, 0 kg and 0.25 kg. For fat mass the scales read 2.2 kg, 4.4 kg and 3.7 kg too low at the median (about 5, 10 and 8 lb), with wide spread between people. The authors concluded that smart scales are not accurate for body composition and should not replace DEXA in patient care. This was one hospital sample and three models, so it does not describe every scale.

BIA against a four-compartment laboratory model. A systematic review published in 2026 by Oliver and colleagues covered twelve studies of healthy, weight-stable adults, comparing impedance devices with a four-compartment laboratory model. Mean bias in body-fat percentage ranged from 3.5 points too low to 4.4 points too high, with limits of agreement typically spanning 15 to 20 percentage points. For fat-free mass the limits of agreement often exceeded 6 kg either way. The authors said the wide limits show non-equivalence at the individual level despite small average differences.

Fifteen devices over time. The 2023 study by Siedler and colleagues in the British Journal of Nutrition included fourteen consumer-grade devices. Repeat readings agreed closely, but agreement with the laboratory model varied, with each device's average error in body fat ranging from 3.5 percentage points too low to 11.7 too high. Errors in the change over 12 to 16 weeks were smaller, and the authors suggested that select eight-electrode and foot-to-foot models may be useful for tracking body composition over time.

What an FDA mention does and does not tell you

The federal rule that names these devices is 21 CFR 870.2770, FDA's classification of the impedance plethysmograph. It places the device in Class II (special controls) and says a body composition analyzer "which is not intended to diagnose or treat any medical condition" is exempt from premarket notification, subject to the limits in 21 CFR 870.9. As we read it, the exemption concerns a filing before sale, not a review of any scale's body-fat readings. We did not find an FDA page giving consumers a verdict on home body-composition scales. Treat "FDA" wording in a listing as a statement to check against the maker's own documents.

What that means for how you use the numbers

• Weight is the dependable number. It was close to the reference in the French study.
• Read body fat as that scale's trend. Take it under the same conditions each time, and look at change over weeks rather than a single reading.
• Keep to one scale. The 2021 authors say a first DEXA result and a later home reading may differ, and that follow-up should use the same connected device.
• Do not treat a label as a diagnosis. Garmin's manual attributes its body-fat ranges to the American Council on Exercise, labeling 25 to 31% as "Acceptable" for women and 18 to 25% for men and using "Obese" from 32% and 26%, and says the data is not intended to diagnose any medical condition. Those are a fitness organization's bands, not the CDC's definition, which uses BMI.
• Do not swap it for BMI. NICE, the guidance body for England, tells health professionals not to use bioimpedance as a substitute for BMI as a measure of general adiposity in adults. Our guide to BMI, waist and body-fat numbers explains the measures the CDC and NHLBI use.

When to leave the function off

The American Heart Association says electronic body fat scales likely interfere with an implanted defibrillator or pacemaker and advises avoiding them. Withings says not to use the Body Smart with a pacemaker or other internal medical device and offers a Weight-Only Mode, Garmin says the Index S2 should not be used by individuals with pacemakers or other internal medical devices and advises consulting a physician if you are pregnant, and Tanita says pregnant individuals should use weight-only mode. A 2023 Heart Rhythm study of smart scales, watches and rings found interference with cardiac implants in simulations and on the bench, lower for scales than for watches, and its authors do not recommend these devices for that group. The National Institute of Mental Health says some people become fixated or obsessed with weight loss, body weight or shape and that this may be a sign of an eating disorder; if body-composition numbers feed that worry, switch the function off and talk to a doctor. If you are in crisis, call or text 988.

Choosing with this in mind

If you want estimates anyway, look for a manual that defines each measurement, a weight-only mode and an app that shows trends. Body composition scales on Amazon (opens in a new tab) and smart scales with Bluetooth on Amazon (opens in a new tab) are searches for them. If weight is enough, digital bathroom scales on Amazon (opens in a new tab) is the simpler search. Our buying guide lists the other checks.

The manufacturer's manual is the authority on your scale, and your doctor, pharmacist or the clinic that looks after you is the right person to ask about your own health.