Smart scales are accurate for weight but unreliable for body fat, with an error margin of roughly plus or minus 5-8%. They use bioelectrical impedance (BIA), sending a current up through your feet, and the conversion from impedance to body-fat percentage is where the inaccuracy creeps in. They are useful for tracking weight trends, not for trusting any single body-fat number.
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Try FitCommit FreeA smart scale measures one real thing well, your weight, and estimates everything else. For body fat it uses foot-to-foot BIA: a tiny current travels up one leg and down the other, and the scale measures resistance. Because the current mostly takes the lower-body path, foot-only models effectively estimate your whole body fat from your legs, then fill in the rest with an algorithm.
Those algorithms differ wildly between brands, so a Withings, a Renpho, and a Eufy can give you three different body-fat numbers on the same body the same morning. None of them is measuring fat directly; they are all guessing from leg impedance.
On top of that, BIA is highly sensitive to hydration, recent food, alcohol, and exercise. A glass of water, a salty dinner, or a workout can move the reading several points with zero change in actual fat.
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Use a smart scale for what it does well: tracking weight trends and syncing them automatically to your phone. The weight reading is accurate, and a daily weigh-in averaged over a week is genuinely useful for managing a cut or bulk.
Treat the body-fat number as a loose direction at best. If you weigh in the same way every morning, the body-fat reading can hint at whether you are trending leaner over months, but do not make nutrition decisions off any single reading or off small week-to-week moves.
If you specifically want body fat rather than just weight, a smart scale is the weakest option. A gym InBody (plus or minus 3-5%) is better but requires a trip. An AI body scan lands at similar accuracy to InBody and works from your phone.
FitCommit analyzes three photos to estimate body fat and lean mass at around plus or minus 3-5%, and because it reads your visible body rather than leg impedance, it is far less sensitive to the hydration swings that make smart-scale body-fat readings jump. At $3.99/month for unlimited scans, it is a more trustworthy body-fat signal than a scale, while you can keep the scale for daily weight.
Accuracy ranges and error margins on this page are stated against DEXA as the reference method and are drawn from peer-reviewed body-composition research. Every method estimates body fat through a different physical proxy (X-ray absorption, electrical impedance, skinfold thickness, or visual analysis), so absolute numbers differ between methods even on the same person, same day. Treat any single reading as an estimate with a few points of error, and track the trend under consistent conditions rather than chasing a perfect number. This is educational content, not medical advice.
About plus or minus 5-8%, the least accurate common method. They estimate body fat from foot-to-foot electrical impedance, which is heavily affected by hydration and varies by brand. The weight reading is accurate; the body-fat reading is a rough estimate.
BIA is sensitive to hydration, food, alcohol, exercise, and time of day. Water and food shift your body water, which changes impedance and therefore the body-fat estimate, even when your actual fat has not changed. Weigh in under consistent conditions to reduce the noise.
Not meaningfully. Hand-to-foot models that add hand electrodes are somewhat better than foot-only ones, but all consumer BIA scales share the same core limitation and land in a similar plus or minus 5-8% range. Price mostly buys features, not body-fat accuracy.
Almost everything: DEXA (plus or minus 1-2%), Bod Pod (plus or minus 2-3%), gym InBody and AI body scans (plus or minus 3-5%). For convenient home tracking, an AI body scan is the most accurate option that does not require a clinic visit.
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