Where to start
If you have one question, start with the page that answers it. For a quick screening check, read BMI and healthy weight and then waist measurement, because the two together say more than either alone. If you have seen a body fat figure and want to know whether to trust it, read the body fat percentage guide.
For a habit, learn to measure your waist the same way each month and weigh yourself at the same time of day. For a purchase, read the smart scale comparison only after the smart scale accuracy guide, so that you know what the scale can and cannot report. If you simply want the vocabulary, what body composition means comes first.
What a wellness device can and cannot do
A smart scale passes a very small electrical current through your body and estimates fat, muscle and water from how it travels. The result is an estimate that moves with hydration, meals and the time of day. It is useful for following a trend on the same scale in the same conditions, and not for a single exact figure.
It does not diagnose, treat or screen for a condition. The visceral fat guide explains why a scale’s visceral level is not the same as a scan, and the calorie guide explains why any estimate of energy use is approximate.
How to read a number from a scale or a tape
Look at the direction over weeks, not one morning. Weight can shift by a few pounds in a day with water and food, and a body fat percentage can move for the same reasons. A tape measure round the waist is simple, repeatable and cheap, which is why the NHS gives it a place beside BMI.
Remember who a figure was designed for. BMI is a screening ratio for adults, it does not see muscle or where fat sits, and the guides explain the adjustments British sources make for some ethnic groups. A number that concerns you is a reason to talk to your GP, not to change what you eat in a hurry.
How these guides are made
Every guide here starts from British sources, chiefly the NHS website, NICE guidance and the relevant British charities, and gives each figure a numbered source with the date it was read. A figure without a source is left out, and where a British source is silent the guide says so rather than borrowing a number from abroad. Weights are given in stones and pounds as well as kilograms, and lengths in inches as well as centimetres.
Health pages carry the line “Medical review in progress.” until a qualified reviewer has checked them, and no page names a reviewer before that has happened. Prices appear only on product pages and in the boxes that link to them, in pounds. Nothing here suggests that anyone other than you pays for a Chutex product, and these are wellness products, not medical devices.
When to get help
Speak to your GP if you are worried about your weight, if it changes without a reason, or if you have a history of difficulty with food. Call 999 in an emergency, such as chest pain, severe breathlessness, fainting or sudden weakness. Call NHS 111 for urgent advice that is not an emergency. For anything else that worries you about your weight, speak to your GP.
All guides in this topic
BMI and healthy weight: NHS categories, stones and pounds, and the limits of the number
How the NHS works out BMI, what the categories mean in stones and pounds, and where the number falls short.
How to measure your waist: the NHS and BHF method, waist to height ratio and when to see a GP
Where to put the tape, how to turn the number into a ratio, and what the ratio means.
Body fat percentage: what it means, how it differs from BMI and how far a scale can be trusted
What the figure means, what the NHS uses instead, and how to read a home scale estimate.
How many calories do you burn a day? Resting burn, walking and what the NHS figures mean
An estimate of your resting burn, the cost of walking, and the limits of every number.
Visceral fat: what it is, why it matters and how far you can check it at home
What deep belly fat is, which checks are meaningful, and what a scale’s visceral fat level does not show.
Smart scales in the UK: a dated comparison of nine models and what to check before you buy
Nine scales, prices read on 16 October 2026, and the accuracy findings that matter before you choose.
What is body composition? Fat, fat-free mass, and why weight alone does not tell you
What your weight is made of, how it is measured, and how to read a home scale estimate.
Are smart scales accurate? What UK and hospital tests found, and how to check your own
Weight is reliable, body fat is an estimate. The numbers behind that, and a home check you can run.
Words you will meet on these pages
- BMI
- Body mass index, a ratio of weight to height used as a screening measure.
- Waist circumference
- The distance around your waist, measured with a tape.
- Body composition
- The share of your weight that is fat, muscle, bone and water.
- Visceral fat
- Fat stored deep in the abdomen around the organs.
- Bioimpedance
- The technique a smart scale uses, passing a tiny current through the body.
- GP
- Your general practitioner, the family doctor in the NHS.
Frequently asked questions
Is BMI a diagnosis?
No. It is a screening ratio for adults and it cannot see muscle or where fat is stored. Your GP can interpret it in context.
Can I trust the body fat figure on my scale?
Treat it as an estimate that is useful for trends on the same scale. The accuracy guide explains the limits.
Why does my weight change from day to day?
Water, food and the time of day can move it by a few pounds. Look at the pattern over weeks.
Where do the figures come from?
From British sources such as the NHS and NICE, each numbered and dated. Where there is none, the guide says so.