Body · lesson 5 of 10
Weight, waist & body composition
What the scale can and can't tell you, why your waist beats BMI, and how far to trust body-fat numbers.
8 min read · reviewed October 2026
Three numbers, three questions
These are three different numbers answering three different questions:
- Weight measures all of you: water, muscle, fat, bone and whatever is in your gut.
- BMI (weight ÷ height²) is a population screening tool. It can't tell muscle from fat, or show where the fat sits.
- Waist size shows where fat is stored. Fat packed around the organs (visceral fat) is the metabolically risky kind, and waist size is the cheapest proxy for it.
Keep your waist under half your height
UK NICE guidance (2022) classifies waist-to-height ratio as 0.40–0.49 healthy, 0.50–0.59 increased risk, and 0.60 or more high risk. It applies to adults with a BMI under 35, of both sexes and all ethnicities, including muscular people. A meta-analysis of studies covering more than 300,000 adults found waist-to-height ratio better than both waist size alone and BMI at flagging diabetes, high blood pressure and cardiovascular risk (Ashwell et al., Obesity Reviews 2012).
You're 175 cm (about 5'9") tall. What's the largest waist that keeps your waist-to-height ratio under 0.5?
Somewhere between 60 and 120 cm.
Reveal the answer →
Under 87.5 cm (~34.5 in)
175 × 0.5 = 87.5 cm. To measure (NICE's method): find the bottom of your ribs and the top of your hip bones, wrap the tape midway between them (usually just above the belly button), breathe out naturally, and read it with the tape snug but not pressing in. Use the same spot and the same time of day every time.
BMI: useful for populations, limited for you
BMI works for describing populations but misreads individuals. Muscular people can score 'overweight', and people with little muscle and fat around the middle can score 'normal'.
How it works →
BMI is kilograms divided by height in meters squared, so it's blind to what the weight is made of and where fat is stored. Risk also starts at lower BMI in some groups. NICE uses 23 as the start of overweight and 27.5 as the start of obesity for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.
What the studies show →
In 2025 a Lancet Diabetes & Endocrinology Commission (Rubino et al.) recommended that BMI alone not be used to diagnose obesity. Excess fat should be confirmed with at least one other measure (waist circumference, waist-to-height or waist-to-hip ratio) or a direct body-fat measurement, except at very high BMI (above 40). The Commission also separated 'clinical obesity', where organs or daily function are already affected, from 'preclinical obesity'.
The scale moves more than you do
Day-to-day swings of 1–2 kg are normal. They come from water, salt, carbohydrate (stored carbs hold water), food still in your gut, and the menstrual cycle. Weigh under the same conditions (on waking, after the bathroom, before eating, in the same clothing) and judge the weekly average, not the day. The weight a decent scale shows is accurate; its body-fat number is a different story.
How accurate is a smart scale's body-fat percentage?
Smart scales pass a tiny current through your feet and estimate body fat from how strongly your body resists it (bioimpedance). For any one person, body-fat percentage can be off by several percentage points, so treat it as a rough trend at best.
How it works →
Water conducts electricity and fat doesn't, so the scale really estimates body water. It then converts that to lean mass and fat using equations built on reference populations, plus the age, sex and height you entered. Foot-to-foot scales mostly measure the path through your legs and lower body. Hydration, a recent meal or drink, exercise, skin temperature and time of day all shift the reading, often by more than a real change of 1–2%. Hand-and-foot (8-electrode) and multi-frequency devices often do somewhat better in validation studies, but still aren't close to a scan.
What the studies show →
Studies comparing bioimpedance with DEXA commonly find a systematic bias (many devices underestimate body fat, often more so at higher body fat) and wide error for individuals. NICE advises against using bioimpedance as a substitute for BMI to measure body fat in adults. DEXA, a low-dose X-ray scan, is the practical reference: it also shows where fat sits, including visceral fat. It isn't perfect either, since results vary between machines and software. If you repeat a scan, use the same machine.
Your smart scale's body fat went from 24.1% to 22.9% overnight. What's the most likely explanation?
- You lost over a kilo of fat overnight
- Your hydration or measuring conditions changed, and bioimpedance shifts with body water
- You built muscle overnight
- The scale is broken and should be returned
Show the answer →
B.Your hydration or measuring conditions changed, and bioimpedance shifts with body water
Real fat loss doesn't happen that fast. Bioimpedance mainly responds to body water, so a drink, a salty dinner, a workout or a different time of day can move the body-fat estimate by a point or more. Compare weekly averages taken under identical conditions, and trust your waist measurement for direction.
Choose on repeatable weight first and treat body composition as a bonus trend. If you have an implanted device, look for a weight-only mode.