Body fat percentage: what it means, how it differs from BMI and how far a scale can be trusted

What is body fat percentage?

The BHF describes body fat percentage as the amount of your total body weight that is made up of fat.¹ The rest of your weight is everything else: bones, muscle, blood and organs. A bathroom scale cannot separate the two, so the percentage has to come from a different kind of measurement or from an estimate.¹

Why does the BHF care? It says a higher percentage of body fat can increase the chance of heart and circulatory diseases, which means a person with a healthy BMI can still be at risk if more of their weight is fat.¹ The NHS puts the same idea in plainer terms: overweight and obesity mean having more body fat than your body needs.²

Not all fat is a problem. Which? explains that some “essential fat” is needed to stay healthy, because it helps protect the internal organs, stores fuel for energy and regulates important hormones.⁴ It also says women typically need a higher body fat percentage than men.⁴ None of the British pages checked for this guide gives a percentage for essential fat, so this guide does not either.

Weight, shape and fat percentage are different questions. This page covers the percentage. For the NHS measures, see the guides to BMI and healthy weight and waist measurement.

What do the sources say about healthy body fat ranges?

Less than you might expect. The NHS gives bands for BMI and a target for waist-to-height ratio, but the NHS pages checked for this guide do not publish a healthy body fat range.²¹³ Which? says there is no universally agreed ideal body fat percentage.⁴ A reader who has seen neat age-by-age charts online should know that this guide could not read a chart of that kind from an official or peer-reviewed table, so it does not print one.

What can be said is shown below. Each row is a reference point that appears in a source, with its limits stated, and none of them is an NHS recommendation.

Where Gallagher fits in

Gallagher and colleagues built healthy body fat ranges by linking the BMI limits of 18.5, 25 and 30 to predicted body fat in 1,626 adults with a BMI up to 35.⁷ Their model used 1/BMI, sex, age and ethnic group, and the standard errors of the estimate ran from 2.8 to 5.4% fat.⁷ This guide draws on the abstract only.⁷ A later letter in Mayo Clinic Proceedings reports that the approach allows a higher body fat with advancing age and in Asian people, which its authors call debatable.⁸

So the honest summary is that ranges depend on the method, the population and the model, and they come with a margin of several percentage points even in the research that produced them.

Sex, age and the same BMI

The Heritage Family Study, covering 665 adults aged 17 to 65, found that at the same BMI women’s body fat percentage was 10.4% higher than men’s, and that age also changed the relationship.¹⁰ That is one reason a single number for everyone would be misleading.

BMI or body fat: which question does each answer?

BMI answers a weight-for-height question. Body fat percentage answers a what-is-the-weight-made-of question. Neither is a verdict on health by itself.

For most adults the NHS calls a BMI of 25 to 29.9 overweight and 30 or more obese. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds it uses 23 to 27.4 for overweight and 27.5 or more for obese.² The NHS is open about the limit: the BMI calculator cannot tell muscle from fat, so a very muscular person may land in the overweight or obese band without carrying much fat.²

The BHF adds that BMI cannot show where fat sits in the body and may be misleading for people who are elderly, have a lot of muscle or are pregnant.¹ It also says waist-to-height ratio may be more accurate.¹¹

Research points the same way. In a study of 13,601 US adults, a BMI of 30 or more missed more than half of the people with excess body fat, and BMI performed worse as age increased.⁹ That study estimated body fat by bio-impedance, so it is a measure of how BMI compares with another estimate, not with an X-ray scan.

What NICE tells clinicians to do

NICE guideline NG246 says that in adults with a BMI below 35, professionals should measure waist-to-height ratio as well as BMI as a practical estimate of central adiposity.³ It also says not to use bioimpedance as a substitute for BMI as a measure of general adiposity in adults.³ In other words, the national approach pairs BMI with a tape measure, not with a body fat scale.

The BHF’s worked example of the ratio uses inches: a 38 inch waist divided by a 67 inch height gives 0.57, and a ratio from 0.5 to 0.59 suggests increased risk.¹ The NHS ratio tool is for adults with a BMI under 35 and should not be used by anyone with a diagnosed or suspected eating disorder.¹³

How is body fat measured?

The BHF lists five ways to estimate body fat percentage: skinfold callipers and smart scales, which you can use yourself, and DEXA, underwater weighing and BOD POD air displacement, which are specialist settings.¹ The prices below come from a BHF page updated on 21 May 2024 and may have moved since.

The reference methods do not agree exactly with one another either. In the Saga test, a professional Tanita body composition scale and skinfold callipers differed by about 2.75% on average.⁵ That is worth remembering whenever a home scale is described as “wrong” or “right”: it is being compared with something that has its own margin.

What can a smart scale estimate, and how far can it be out?

A smart scale does not see fat. It sends a weak current, measures how easily it passes and turns the result into a percentage using the maker’s equations. That is why the figure is an estimate. The maker of the scale you buy decides the equation, and different makers get different answers for the same person.

The Saga and University of Bath test

Saga compared eight smart scales with a professional Tanita scale and skinfold callipers, using four volunteers in their 40s and 50s.⁵ Two of the eight, the Garmin Index S2 and the Fitbit Aria Air, gave no body composition figures, so the body fat results come from the others. On body fat, those scales differed from the callipers by 6.75% on average and from the Tanita by 7.12%. The best scale, the Withings Body Scan, was at 4.3%. The worst were the Eufy Smart Scale P2 Pro at 11.74% and the Xiaomi Mi Body Composition Scale 2 at 10.68%.⁵

One volunteer’s numbers show what that looks like. The callipers gave 10.7% body fat and the Tanita 10.5%. The Withings Body Scan said 17.2%, and the Xiaomi and Eufy scales said 26.4% and 30.5%.⁵ Saga also noticed that volunteers with lower body fat seemed to have slightly larger discrepancies, and it cautions that its sample was small.⁵

The French hospital study

A study in JMIR mHealth and uHealth compared three smart scales with DEXA in about 50 hospital patients each. Median errors in fat mass were 2.2 kg, 4.4 kg and 3.7 kg, and the authors concluded that smart scales are not accurate for body composition and should not replace DEXA.⁶ Both the study and the Saga test are of older models. They tell you about the method’s limits, not about any one scale on sale now.

Chutex says the same about its own scale on the Vita product page: its bio-impedance estimates are indicative and vary with hydration, meals, recent exercise and time of day, and it does not claim clinical-grade accuracy like DEXA.¹⁶

A worked example in stones and pounds

The numbers here are invented to show the arithmetic. Take an adult who weighs 13 st 5 lb. A stone is 14 lb, so 13 x 14 + 5 = 187 lb. One pound is 0.45359237 kg, so 187 lb = 84.8 kg.

Suppose their body fat is 20%. Fat mass is 84.8 x 0.20 = 17.0 kg. The rest, 84.8 x 0.80 = 67.9 kg, is everything that is not fat.

Now suppose 3 kg of fat disappears and nothing else changes. Weight becomes 84.8 minus 3, which is 81.8 kg, which is about 12 st 12 lb. Fat mass becomes 17.0 minus 3, which is 14.0 kg. The new percentage is 14.0 divided by 81.8, which is 17.1%.

Two things stand out. First, losing 3 kg moved the percentage by only 2.9 points, from 20.0% to 17.1%. Second, Saga’s reported gaps between scales and reference methods, from 4.3% to 11.74%, are not small next to a 2.9 point change.⁵ This is why Saga advises looking at trends over time rather than any one-off reading.⁵

The example also assumes lean mass stays exactly the same, which real life rarely does. The BHF notes that body weight may not fall even while fat is being lost, because you may be gaining muscle at the same time.¹

How to measure at home so readings are comparable

Whichever method you use, the aim is the same: change as little as possible between readings, so that a difference means something. The BHF recommends weighing at the same time of day, under the same conditions and on the same scales, for example in the morning after emptying your bladder.¹ It also warns that dehydration can make smart scales overestimate body fat.¹

An NHS weight-loss guide advises weighing once a week rather than daily, because weight fluctuates from day to day.¹⁴ The same logic applies to a body fat figure, which moves with hydration as well as with fat. If you want to smooth out noise, you can take three readings in a row and note the middle one. That is a suggestion for tidiness, not a method from a source.

Who should not use a body fat scale

The BHF says that anyone with a pacemaker should not use a smart scale for body fat, and its pacemaker advice lists electronic body fat scales among the devices to avoid completely.¹¹⁵ If you have another implanted electronic device, check with the team that looks after it before standing on one.

A note on body image

Numbers about the body can become a source of worry. There is no target in this guide, and no figure here is something to aim for. The NHS waist-to-height tool should not be used by anyone with a diagnosed eating disorder or who thinks they may have one.¹³ If you are in that position, talk to your GP instead of tracking measurements.

Frequently asked questions