Consumer bioelectrical-impedance scales report body fat to one decimal place. Compared to DEXA reference scans, the error margin is typically 3-5 percentage points in either direction — sometimes more. The precision in the display is theater. The technology is genuinely useful, but only for trends and only if you weigh in consistently. Here is how to use one without misreading what it tells you.
How BIA actually works
The scale sends a tiny imperceptible electrical current up through your body and measures the resistance. Water conducts well. Fat does not. From the resistance pattern, the device estimates the percentages of fat, muscle, and water that would produce that signal — given a model. The model is the catch.
Why accuracy is limited
Two problems drive the error:
- Hydration changes the reading meaningfully. Same body, dehydrated vs well-hydrated, several percentage points difference.
- The model has to generalize. Individual body composition varies enough that any one-size-fits-all model will be wrong for plenty of individuals.
What that means in practice
If your scale reads 22% body fat one morning and 26% three days later, that is not a four-point change in body fat. It is hydration. Do not draw conclusions from any single reading.
Where smart scales are genuinely useful
Trends, not absolute numbers. If you weigh in consistently — same time, same hydration state, same clothing — the relative direction of body composition changes is reasonably informative over weeks and months.
Weight trends specifically are more reliable than body-fat trends, because weight is the directly measured signal. Fat percentage is inferred from a model on top of that signal.
What the apps add
Visualization and history. A scale without an app is just a number that you will forget. The platform value of Withings, Garmin Connect, Apple Health, and Google Fit is in seeing patterns over months and integrating with other tracking.
Specific scales worth considering
Withings Body Comp / Body Smart
The most polished hardware and app in the category. Multi-user, Wi-Fi syncing, integrates with practically everything. Body Comp adds heart rate and vascular age estimates — both feel more marketing than medical.
Renpho
The budget choice. Same BIA accuracy as the premium scales (the hardware is roughly equivalent across price points) at a third of the price. App is unobjectionable. We would buy this one.
Garmin Index S2
Integrates with Garmin Connect. Useful if you already use a Garmin watch. Otherwise nothing special.
Eufy Smart Scale P3
Many metrics at a low price. Trustworthy for trends.
What to skip
- Any scale promising medical-grade accuracy. Not a thing in this category.
- Any scale advertising metabolic age or biological age. These are entirely model outputs, not measurements.
Pregnancy and BIA
Most BIA scales advise against use during pregnancy because the small current involved poses a theoretical risk. Standard weight measurement remains fine. Read the manual.
The DEXA alternative
If you genuinely want accurate body composition data, a DEXA scan at a clinic gives you the reference standard for around $50-150 depending on location. Two scans a year will tell you more than a smart scale every morning.
Bottom line
Smart scales are good at weight, mediocre at body composition, and useful for trends. Buy one if you want an automatic daily weight log. Do not take the body-fat readings as gospel — they are approximations dressed up as decimals.
