Where to start
Weight alone says little about health, because two people at the same weight can carry very different amounts of fat and muscle. That is the reason body composition has its own set of measures, and the reason a lot of questions people type are really about one of them.
If your question is about a percentage, begin with body fat percentage. If it is about the scale in your bathroom, read the best smart scales for body composition and the guide to how bioimpedance scales work. If it is about energy, start with the pages on BMR and calories burned. Each page links onward to the others.
Home estimates and clinical measures
Most smart scales send a very small electrical current through the body and estimate fat, muscle and water from how the current travels. That method is called bioelectrical impedance analysis. It is convenient, and it is also sensitive to hydration, recent meals and exercise. Clinics use other methods, such as a DEXA scan, that have their own strengths and costs.
The practical rule is simple. Use a home scale to follow your own trend under the same conditions, and do not compare its number with a different device or a clinical report as if they were the same measurement.
People with a pacemaker or another implanted electronic device should not use an impedance scale without asking their doctor.
How to read a body composition number
A percentage without context is hard to judge. Reference ranges depend on age and sex and on the method used to measure them, so the guides give ranges only where a named source gives them, and they say which method that source used. A number from your scale and a number from a clinical scan are not interchangeable.
Conditions change the reading. Hydration, a large meal, a hard workout, the time of day and even the state of your feet on the electrodes can move an impedance estimate. The guide on the best time to weigh yourself and the page on smart scale accuracy explain how to keep conditions comparable.
Use the trend, not the day. A single weigh-in is noise. A few weeks of readings under the same conditions show a direction. If you are making changes to diet or exercise for a health reason, a doctor or a registered dietitian can set goals that fit you.
What a home reading can help with
A body composition scale can help you notice whether a routine is working over weeks, for example whether lean mass holds steady while weight changes. It cannot tell you whether a number is healthy for you, diagnose a condition or replace a conversation with a doctor.
BMI, waist measures and body fat percentage each answer a slightly different question, and the guides explain what each one was designed to describe. Where a source says a measure has limits for certain people, such as older adults or very muscular people, the guide repeats that limit.
All guides in this topic
How to calculate BMR by hand: the Mifflin-St Jeor equation in pounds and inches
The published equation, three worked examples and the error to expect.
Visceral fat: what it is, waist thresholds, and what a scale can and cannot tell you
Where it sits, the waist numbers to know, and which measurements to trust.
Muscle mass: how it is measured, the skeletal muscle index, and what impedance cannot see
The index doctors use, the methods behind the number, and how far to trust a scale.
DEXA scan vs smart scale: how each works, how far they agree, and how to weigh in consistently
Two ways to split weight into fat and lean mass, compared on method, evidence and limits.
Withings vs Renpho: smart scales compared by price, metrics, app and subscription
A dated table from the official pages, and how to choose without a made-up ranking.
BMI chart for adults: categories, a height and weight table in pounds, and the limits of BMI
The CDC categories, the formula in pounds and inches, and a table you can read in ten seconds.
Body water percentage: typical ranges, why scales vary, and how to measure it comparably
What the number is, what sources say is typical, and why your reading moves.
Metabolic age: what a smart scale means by it, what research says, and how to read it
A number some scales show next to your real age, and what it can and cannot tell you.
Best time to weigh yourself: what the research says and a routine you can repeat
Why the same conditions beat any single hour, and how to read the number.
How to measure body fat at home: tape, calipers and scales compared
Three home methods, what each one gets wrong, and how to track a trend.
How to lower body fat percentage: what the CDC and NIH say about food, activity and pace
A reasonable pace, what to do about food and movement, and what does not work.
Waist-to-hip ratio: how to measure it, the WHO reference values and what they leave out
The formula, the tape protocol and the numbers institutions use.
Are smart scales accurate? What validation studies show about body fat readings
Why a scale can repeat itself and still be wrong, and how to use it anyway.
How many calories do I burn a day? Total daily energy expenditure (TDEE) explained
The parts of daily energy use, the multipliers, and a calculation you can check.
Muscle loss after 50: what sarcopenia is, what you can check at home, and when to get help
How fast muscle fades with age, the checks that matter, and the activity guidelines.
What is body composition? Compartments, bioelectrical impedance and how the methods compare
What the body is made of, how it is measured, and why weight alone is not enough.
Words you will meet on these pages
- Body fat percentage
- The share of your total weight that is fat.
- Visceral fat
- Fat stored deep in the abdomen around the organs.
- Lean mass
- Everything that is not fat, including muscle, water, organs and bone.
- BMR
- Basal metabolic rate, the energy your body uses at rest.
- BIA
- Bioelectrical impedance analysis, the method most smart scales use.
- DEXA
- A low-dose X-ray scan that estimates fat, lean tissue and bone.
How these guides are made
Each guide is written for readers in the United States. It uses American units and sources such as the National Institutes of Health, the Centers for Disease Control and Prevention and other named institutions. Every figure carries a numbered source with the date it was read. Prices of devices are read from the maker's own page on the day written and dated in the table.
Health topics carry the line “Medical review in progress.” until a qualified reviewer has checked the page. The pages do not replace advice from a health professional. In a medical emergency in the United States, call 911.
Frequently asked questions
Is a smart scale accurate?
It is useful for trends under the same conditions. Its readings are estimates that move with hydration, meals and exercise, and they differ from a clinical scan.
When should I weigh myself?
The best-time guide explains how to keep conditions comparable. The general idea is the same time, the same state and the same scale.
Do these guides tell me what weight or body fat I should have?
They present reference ranges from named institutions where such ranges exist. They do not set a target for you. A health professional can interpret a number in the context of your age, health and goals.
Can I use a body composition scale with a pacemaker?
No guide here says it is safe. Impedance scales are not for people with a pacemaker or another implanted electronic device unless a doctor says otherwise.