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.
Want to track body fat between visits? FitCommit uses three phone photos to provide an estimate you can follow under similar conditions each week.
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.
Use DEXA, InBody, or calipers for occasional anchors, then use FitCommit AI body scan to track body fat, lean mass, TDEE, and macros from your phone between tests.
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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 want body-fat context rather than weight alone, InBody provides segmental bioimpedance data in a gym or clinic while FitCommit provides a photo-based estimate at home. They use different proxies and their absolute numbers should not be treated as interchangeable.
FitCommit analyzes three photos to estimate body fat and lean mass. Lighting, clothing, posture, hydration, and normal day-to-day variation can move the result, so scan under similar conditions and follow the trend.
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.
DEXA provides a clinical baseline, InBody provides segmental bioimpedance data, calipers depend on tester skill, and photo-based tools provide at-home estimates. Choose based on the decision you need to make and the setting you can repeat.
Deficit levels with timelines and calorie targets for fat loss
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Visual reference for every body fat percentage for men and women
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Take three photos to estimate body fat and lean mass, then connect the result to TDEE, macro targets, After Photo, and your weekly trend.
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