What it is
A smart scale measures your weight and syncs it to an app. Most also send a tiny electrical current through your feet (or hands and feet) to estimate body composition: fat, muscle, water, and sometimes 'visceral fat', 'bone mass' or 'metabolic age'. This is bioimpedance. Water conducts electricity and fat doesn't, so the scale estimates your body water and converts it using equations built on reference populations.
The weight is the trustworthy part. Composition figures move with hydration, meals, exercise and time of day, often by more than any real change.
How strong is the evidencemoderate
Compared with DEXA scans, three consumer smart scales weighed people to within about 0.3 kg, but underestimated fat mass by a median of about 2–4 kg, and the authors concluded they aren't accurate for body composition. A review of 17 longitudinal studies found regular self-weighing went with more weight loss and wasn't linked to worse psychological outcomes in people trying to lose weight. Nothing shows that tracking bioimpedance numbers improves health.
The claims you’ll meet5 checked
Tracks your weight accuratelyHolds upWeight errors against a reference were a few hundred grams at most.
Accurately measures body fat and muscleFalseEstimates can be kilograms off for one person and swing with hydration. At best, a rough trend under identical conditions.
Weighing often helps you lose weightHolds upIn people trying to lose weight, regular self-weighing goes with more weight loss. It isn't for everyone.
'Metabolic age', 'visceral fat' and 'bone mass' readingsUnprovenThese are derived from the same bioimpedance estimate plus your age and height, not measured directly.
Hand-and-foot (8-electrode) models are accurateMixedThey often do somewhat better than foot-only scales in validation studies, but they still aren't close to a DEXA scan.
What to look for5
- ✓Repeatable weight: step on three times in a row and the readings should agree closely.
- ✓A weight capacity that suits you, and a platform big enough to stand on naturally.
- ✓A weight-only mode if you have an implanted device, are pregnant, or simply don't want composition numbers.
- ✓Multi-user recognition if your household shares it, and data export or syncing to your phone's health app.
- ✓Privacy settings you control; body data is sensitive.
What to skip3
- ×Metabolic age, visceral fat ratings, protein percentage and similar derived numbers.
- ×Paying extra for composition features expecting clinical accuracy. If you need body composition, a DEXA scan on the same machine each time is the reference.
- ×Treating day-to-day swings of 1–2 kg as fat gained or lost.
How to use it4
- →Place it on a hard, flat floor; carpet and uneven tiles skew readings.
- →Weigh at the same time (on waking, after the bathroom, before eating) in the same clothing.
- →Judge the weekly average, not the day.
- →If you use composition figures at all, compare only readings taken under identical conditions, over months.
Take care3
- !Don't use bioimpedance if you have a pacemaker, implanted defibrillator or other implanted electronic device; use weight-only mode. Most makers also advise against it in pregnancy.
- !If weighing triggers restriction, bingeing or distress, or you have a history of an eating disorder, skip the scale and talk to a clinician.
- !Losing more than 5% of your body weight over 6–12 months without trying is worth a medical check.
The studies2 cited · checked against PubMed
Accuracy of Smart Scales on Weight and Body Composition: Observational Study ↗Frija-Masson J, Mullaert J, Vidal-Petiot E, Pons-Kerjean N, Flamant M, d'Ortho MP · JMIR mHealth and uHealth · 2021Validation study
Three commercial smart scales were tested against DEXA in about 50 hospital patients; each scale measured weight to within a median of 0–0.3 kg, but fat-mass errors had medians of 2.2–4.4 kg; the authors concluded smart scales are not accurate for body composition.
Self-weighing in weight management: a systematic literature review ↗Zheng Y, Klem ML, Sereika SM, Danford CA, Ewing LJ, Burke LE · Obesity (Silver Spring) · 2015Systematic review
Across 17 longitudinal studies of adults seeking weight loss, regular self-weighing was associated with more weight loss and not with adverse psychological outcomes such as depression or anxiety.