How to Choose a Smart Scale

A smart scale weighs you and then estimates body fat, muscle, water and bone by sending a small electrical current through your body. Models differ in what they report, how finely they weigh, who they are safe for and what they do with your data. This guide sets out what to check before you buy one, for adults. If you are pregnant, have an implanted medical device such as a pacemaker, or find that weighing yourself affects how you feel about your body, read the cautions section first.

The criteria come from named public sources: the Centers for Disease Control and Prevention (CDC), the American Heart Association, the Federal Trade Commission (FTC), manufacturers' manuals and peer-reviewed studies. We do not rank products or rate brands. Amazon links on this page open searches built from the criteria, and you check each listing against the manufacturer's specification.

Decide what you want to know

Every body-composition scale is also a weighing scale, and weight is the measurement most of them make well. In a French hospital study, three smart scales were compared with a DEXA scan, the low-dose X-ray method used as the clinical reference for body composition. The median error in body weight was 0.3 kg or less (under 1 lb), but the scales read fat mass too low by a median of 2.2 to 4.4 kg (roughly 5 to 10 lb), and the authors concluded that smart scales are not accurate for body composition.

So decide first. If you want a dependable weight to follow over weeks, a plain digital scale is enough, and digital bathroom scales on Amazon (opens in a new tab) is a search for them. If you also want body-fat, muscle and water estimates, body composition scales on Amazon (opens in a new tab) shows many; treat those numbers as that scale's own readings, not facts about your body.

Check the weighing basics

• Maximum load. Withings lists a range of 5 to 200 kg (9 to 440 lb) for the Body Smart, Garmin a maximum load of 181.4 kg (400 lb) for the Index S2, and Tanita 330 lb for the BC-401F. High-capacity bathroom scales on Amazon (opens in a new tab) narrows results by load limit.
• Step size. Withings gives a 50 g (0.1 lb) graduation for the Body Smart, and Tanita an increment of 0.2 lb (100 g) for the BC-401F.
• The floor. Garmin's manual says to use its scale on a hard, flat floor for the most accurate weight, and includes risers for carpet.

Read the extra measurements with care

Many scales list body fat percentage, muscle mass, bone mass, water percentage, visceral fat, basal metabolic rate and "metabolic age". These are estimates. The authors of the French study explain that these scales calculate fat-free mass from an equation using resistance, height, weight and age, that each scale has its own equation, and that researchers do not know the equations. Look in the manual for the definition of each number and the source of any labels. Our guide to BMI, waist and body-fat numbers explains which cutoffs come from health agencies and which do not.

Feet only, or more electrodes

Most home scales use electrodes under the feet; other designs add hand grips or more electrodes. A 2023 study in the British Journal of Nutrition compared fifteen devices, fourteen of them consumer-grade, with a laboratory model in 73 healthy adults. Each device's average error in body fat ranged from 3.5 percentage points too low to 11.7 too high, though errors in the change over 12 to 16 weeks were smaller. The authors said select eight-electrode and foot-to-foot models may be useful for tracking over time.

What an FDA mention does and does not tell you

FDA's rule for impedance plethysmographs, 21 CFR 870.2770, says a body composition analyzer "which is not intended to diagnose or treat any medical condition" is exempt from premarket notification. As we read it, that is a filing step before sale, not an FDA check of a scale's body-fat readings. If a listing mentions FDA, look at exactly what it says.

Who should not use the body-composition function

• Implanted devices. The American Heart Association says electronic body fat scales likely interfere with an implantable cardioverter-defibrillator or a pacemaker, and advises avoiding them if you have an implanted device. Withings says not to use the Body Smart if you wear a pacemaker or other internal medical device, and recommends its Weight-Only Mode, which switches off all impedance-based features, for a pacemaker or a metallic implant. Garmin's manual for the Index S2 says the device should not be used by individuals with pacemakers or other internal medical devices. Tanita says people with electronic medical implants, such as pacemakers or defibrillators, should not use BIA monitors. A 2023 Heart Rhythm study of simulations and benchtop tests found that bioimpedance scales, watches and rings could interfere with cardiac implants, and its authors do not recommend them for that group. We did not find an FDA notice for consumers about body-fat scales, so ask your cardiologist or device clinic.
• Pregnancy. Withings says the Body Smart has not been tested and/or validated for use during pregnancy and recommends disabling impedance-based measurement. Garmin's manual says to consult your physician before using the Index S2 if you are pregnant or think you may be. Tanita says pregnant individuals should use weight-only mode. The CDC advises working with your provider on weight gain goals in pregnancy.
• Children and teens. These scales are built for adults. Tanita describes a mode for ages 5 to 17 that tracks weight, BMI and fat percentage. The CDC reads children's BMI against sex-specific percentiles for age and advises discussing results with a health care provider.
• Eating disorders. The National Institute of Mental Health says being fixated or obsessed with weight loss, body weight or shape may be a sign of an eating disorder, which it says can be treated successfully. If weighing or the numbers distress you, stop and talk to a doctor.

Think about the app and your data

Weight and body measurements stored in an app are health-related data. The FTC says its Health Breach Notification Rule, amended in July 2024, covers makers of health apps and connected devices that fall outside HIPAA, and that a breach includes sharing health data without consent, not only hacking. We cannot tell you whether a given scale's app is covered, so read the maker's privacy notice. Questions worth asking:

• Does the scale need an account, and does it connect over Bluetooth or Wi-Fi? Garmin's scale uploads over Wi-Fi to a Garmin Connect account, Tanita's sends results by Bluetooth to its app.
• Can you export your data? Withings explains how to export everything as a CSV file.
• Can you hide the numbers? Withings has an Eyes-Closed Mode that hides weight and impedance values on the scale's screen while the app still holds them.

Three examples that show these criteria

These are examples, not recommendations or a ranking, described in our words from manufacturer pages that change.

• Withings Body Smart. Its support pages state a range of 9 to 440 lb with a 0.1 lb graduation, a Weight-Only Mode, and advice not to use it with a pacemaker or internal device. Search Amazon for this product (opens in a new tab).
• Garmin Index S2. The owner's manual describes foot-to-foot bioelectrical impedance, a maximum load of 400 lb and Wi-Fi upload, and warns that the device passes electric current through the user, should not be used by people with pacemakers or other internal medical devices, and that pregnant users should consult a physician first. Search Amazon for this product (opens in a new tab).
• Tanita BC-401F. Tanita's US product page lists a 330 lb capacity, a 0.2 lb increment, Bluetooth, memory for 5 people and a youth mode. Its implant warning is on Tanita's technology page, not the product page. Search Amazon for this product (opens in a new tab).

Add a tape measure

No scale measures your waist. The National Heart, Lung, and Blood Institute says a waist measurement above 35 inches for women or 40 inches for men increases health risk. Body measuring tape on Amazon (opens in a new tab) is a search for one. The method is in our guide to BMI, waist and body-fat numbers.

Where the links go

Links marked "on Amazon" or "Search Amazon" open an Amazon search for the words in the link. Amazon decides what appears, and results can include products that do not meet the criteria above, so check each listing against the manufacturer's specification.

Read next

• What bioimpedance scales can and cannot measure
• How to weigh yourself for useful trends
• BMI, waist and body-fat numbers explained

The manufacturer's manual for the scale in front of you is the authority on its use and safety warnings, ahead of any article, this one included. If you have a health condition, are pregnant or have an implanted device, ask your doctor, pharmacist or the clinic that looks after you before using the body-composition features.