Body Fat Percentage: How to Measure It and Which Numbers Actually Matter
Smart scales, gym machines and fitness apps all promise your body fat percentage, and they rarely agree. Here is how accurate each method really is, why BMI can hide high body fat in Pakistanis, which number to track instead, and how to measure in a way that shows real progress.
By Taha Raza
Published
Two people can weigh 80 kg and look completely different — one muscular, one carrying a lot of fat around the middle. That is why body fat percentage, the share of your weight that is fat, has become a favourite number in fitness. The trouble is measuring it well.
How accurate are smart scales for body fat?
Good for weight, rough for fat. A 2021 French study weighed hospital patients on one of three popular smart scales straight after a DEXA scan, the reference method. Weight was within about 0.3 kg. Fat mass, though, was off by a median of 2.2 kg, 4.4 kg and 3.7 kg for the three scales, with individual errors much larger. For a 70 kg person, 3–4 kg of fat is about 5 percentage points of body fat. Smart scales and handheld gym machines use bioelectrical impedance: they send a small current through the body and estimate fat from how easily it passes. Water conducts well, so hydration changes the result — a 2004 expert review stressed that the method needs normal hydration and standard conditions to be reliable.
| Method | Accuracy | Practical notes |
|---|---|---|
| DEXA scan | The reference used in research | Hospital or diagnostic centre; costs money; good for a yearly check |
| Smart scale / gym impedance machine | Weight accurate; fat mass off by 2–4 kg on average vs DEXA | Useful for trends only, under the same conditions each time |
| Skinfold callipers | Reasonable with a skilled, consistent tester | Results vary a lot between testers |
| Waist-to-height ratio | Better predictor of health risk than BMI | Free — a tape measure; aim for under 0.5 |
| BMI | Misses body fat, especially in South Asians | Fine for population screening, poor for individuals |
| Progress photos and clothes fit | Not a number, but sensitive to change | Same light, same pose, every 2–4 weeks |
Why is BMI misleading for Pakistanis?
Because Asians tend to carry more body fat at the same BMI. A 1998 meta-analysis found that for the same body fat, age and sex, Chinese, Thai and Indonesian adults had BMIs 1.9 to 3.2 units lower than white adults. A 2000 study in the American Journal of Clinical Nutrition, which measured white, African American and Asian adults to build body-fat ranges from BMI, found ethnic group changed how much fat a given BMI meant. South Asians often have the most pronounced version of this — the 'thin-fat' or skinny-fat build covered in our body recomposition guide — which is why lower BMI cut-offs are used for Asian populations. A Pakistani man with a 'normal' BMI of 23 can have a body fat percentage and waist that put him at real risk of diabetes.
What is a healthy body fat percentage?
There is no single agreed healthy range, but obesity by body fat is commonly defined as more than 25 percent for men and more than 35 percent for women. Women naturally carry more essential fat than men, and body fat tends to rise with age. Very low body fat — the 'shredded' look on social media — is not a health target: it is hard to maintain, and in women very low body fat can stop periods, which is a warning sign. For most people trying to get healthier, moving out of the obese range and shrinking the waist matters far more than chasing a specific percentage.
What is waist-to-height ratio, and how do I measure it?
Divide your waist by your height, in the same units. A 2012 meta-analysis of 31 studies with more than 300,000 adults from several ethnic groups found waist-to-height ratio was better than both BMI and waist circumference at identifying diabetes, high blood pressure, abnormal cholesterol and heart disease risk. The rule of thumb, now used in UK guidance: keep your waist to less than half your height — a ratio under 0.5. A man who is 175 cm tall should aim for a waist under 87.5 cm. Measure around the middle, roughly level with the belly button, after breathing out normally, without pulling the tape tight.
How to track body fat changes properly
- 1Pick one main measure — waist and waist-to-height ratio are the cheapest and most useful.
- 2Measure first thing in the morning, after using the toilet, before eating or drinking, and not after exercise.
- 3If you use a smart scale, use the same one under those same conditions and look at the 2–4 week trend, not daily readings.
- 4Take progress photos every 2–4 weeks in the same light and pose.
- 5Expect the scale and the body fat reading to bounce with water, salt and food — especially after a dawat or a salty restaurant meal.
- 6If you want an accurate number, get a DEXA scan once, then track waist and photos between scans.
Losing fat while building muscle can leave the scale weight almost unchanged while the waist shrinks — another reason weight alone misleads. Our guide to calories for weight loss and our protein guide cover how to make that happen.
This article is general information, not medical advice. If your waist-to-height ratio is 0.5 or more, ask your doctor about checks for blood sugar, blood pressure and cholesterol. Smart scales using electrical current are generally not recommended for people with pacemakers or implanted defibrillators — check the manufacturer's advice.
Frequently Asked Questions
Are smart scales accurate for body fat?
Not very. In a 2021 study, three smart scales measured weight within about 0.3 kg but misjudged fat mass by a median of 2.2–4.4 kg compared with a DEXA scan. Use them for trends under the same conditions, not exact numbers.
What is a healthy body fat percentage?
There is no single agreed range, but obesity is commonly defined as more than 25 percent body fat for men and 35 percent for women. Waist-to-height ratio under 0.5 is a simpler, well-tested health target.
Is BMI accurate for South Asians?
Not very. Asian groups carry more body fat at the same BMI than white people, which is why lower Asian BMI cut-offs are used. A normal BMI can hide high body fat and a large waist.
What is the most accurate way to measure body fat?
A DEXA scan is the reference used in research. At home, waist-to-height ratio and progress photos are more useful than smart-scale body fat readings.
Why does my body fat reading change every day?
Smart scales use electrical impedance, which depends on how much water you hold. Food, salt, drinks, exercise and time of day all change the reading. Measure first thing in the morning and follow the 2–4 week trend.
Sources
- Frija-Masson J et al. Accuracy of smart scales on weight and body composition: observational study. JMIR mHealth and uHealth, 2021
- Kyle UG et al. Bioelectrical impedance analysis — part I: review of principles and methods. Clinical Nutrition, 2004
- Deurenberg P et al. Body mass index and percent body fat: a meta analysis among different ethnic groups. International Journal of Obesity, 1998
- Gallagher D et al. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition, 2000
- Ashwell M et al. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obesity Reviews, 2012
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