How BMI is calculated and what the categories mean
BMI (body mass index) uses your height and weight to put your body weight into a category. Adolphe Quetelet developed it in the 19th century as a statistical tool for whole populations, and the World Health Organization later adopted it to classify weight in adults.
The formula: BMI = weight (kg) ÷ height (m)².
Example: 80 kg and 175 cm gives 80 ÷ (1.75 × 1.75) = 80 ÷ 3.0625 = 26.1, which falls in the overweight category.
The adult categories are a screening measure, not a diagnosis. They are not meant for children or teenagers, and pregnancy or a lot of muscle changes how you should read the number. NICE also advises caution with BMI for people aged 65 and over. The calculator places you in a category using the unrounded result, so the number it shows is rounded but the category is exact. The CDC’s guide to adult BMI categories explains the same boundaries and limits.
| BMI | Category | What it usually means |
|---|---|---|
| Below 18.5 | Underweight | May point to too little energy intake or muscle. Talk to a healthcare professional. |
| 18.5 to below 25 | Healthy weight | The standard range for most adults, though body composition still varies widely within it. |
| 25 to below 30 | Overweight | Above the healthy range. Usually extra body fat, but sometimes extra muscle in people who train. |
| 30 and above | Obesity: class 1 (30 to below 35), class 2 (35 to below 40), class 3 (40 or more) | Linked with higher health risks. A body composition check and a medical opinion are worth getting. |
Lower BMI cut-offs for Asian, Middle Eastern and African backgrounds
A WHO expert consultation found that many Asian populations carry a higher risk of type 2 diabetes and heart disease at a lower BMI than European populations, and suggested extra action points at 23 and 27.5.2 UK NICE guidance now applies lower thresholds to people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, who are more prone to fat around the middle.3
Many people living in Dubai and Abu Dhabi fall into these groups, so the calculator shows your result on both scales.
| Category | Standard thresholds | Lower thresholds (NICE) |
|---|---|---|
| Healthy weight | 18.5 to below 25 | 18.5 to below 23 |
| Overweight | 25 to below 30 | 23 to below 27.5 |
| Obesity | 30 or above | 27.5 or above, with classes 2 and 3 starting at 32.5 and 37.5 |
Why BMI has limits
BMI was designed for whole populations, not to judge one person. It treats every kilogram the same, so a kilogram of muscle and a kilogram of fat count equally. That causes four well-known problems:
- Muscular people are misclassified. Someone with a lot of muscle and little fat can land in the overweight or even obesity range while being metabolically healthy.
- Thin-fat people are missed. Someone with little muscle and a high body fat percentage can have a healthy BMI while carrying harmful visceral fat around the organs.
- It says nothing about where fat sits. Fat around the organs carries far more risk than fat under the skin, and BMI cannot tell them apart.
- The thresholds do not fit everyone. South Asian and Arab populations, among others, can face higher metabolic risk at lower BMI values than the standard cut-offs suggest.
BMI ignores muscle and body composition. Two people with the same BMI can have completely different health. PTD Fitness coaches do not use BMI as the main progress measure. They track body fat, lean mass and body measurements, which tell you what is actually changing.
Waist-to-height ratio and waist size
Your waist-to-height ratio is your waist divided by your height, both in centimetres. The simple rule is to keep your waist to less than half your height, a ratio below 0.5. A large review of studies found it a better screening tool for heart and metabolic risk than waist size or BMI alone,4 and NICE recommends measuring it alongside BMI for adults with a BMI under 35.3
| Waist-to-height ratio | Central fat | Health risk |
|---|---|---|
| 0.4 to 0.49 | Healthy | No increased risk |
| 0.5 to 0.59 | Increased | Increased risk |
| 0.6 or more | High | Further increased risk |
NICE says these bands work for men and women of every ethnicity, including people with a lot of muscle. Waist size on its own is also useful: WHO links a waist above 94 cm in men or 80 cm in women with an increased risk, and above 102 cm in men or 88 cm in women with a substantially increased risk, whatever your BMI.5
To measure, find the bottom of your ribs and the top of your hips, wrap the tape around your waist midway between them, which is just above the belly button, and breathe out naturally before you read it.
Estimating body fat with a tape measure
The body fat estimate in the calculator uses the US Navy circumference method, which Hodgdon and Beckett developed in 1984 from height, neck and waist measurements, plus hips for women.6 It needs nothing more than a tape measure, which makes it a practical way to track change at home. It is an estimate and can be several percentage points away from a lab scan, especially at very high or very low body fat, so use it to follow your own trend rather than to compare yourself with others.
| Range | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletic | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| High | 32% or more | 25% or more |
For most people a body fat percentage is a more useful goal than a BMI number: for example, 15–20% for men or 22–28% for women. When you strength train, muscle gain can keep your BMI the same even while you lose a lot of fat.
Better ways to track body composition in the UAE
These methods give a fuller picture of your health and progress than BMI:
InBody scan
InBody machines use bioelectrical impedance, a weak electrical current, to estimate body fat, lean muscle and body water in a few minutes, with no radiation. Many fitness centres and private clinics in Dubai and Abu Dhabi have one. The printout also shows muscle in each arm, leg and the trunk, which helps you spot imbalances.
DEXA scan
Dual-energy X-ray absorptiometry (DEXA) is the reference standard for body composition. It separates bone, lean tissue and fat precisely, and medical imaging centres in Dubai offer it. One scan gives you a complete baseline to compare against later in a programme.
STYKU 3D body scan
PTD Fitness clients have STYKU 3D body scans at the PTD office in Detroit House, Motor City, to track body shape, measurements and body fat during their programme. The scan is done at the office, while your training happens at your home, building gym or office.
Tape measurements
Waist, hip and limb measurements taken regularly by a coach are a free, practical way to see body composition change. Your waist in particular says more about metabolic risk than BMI.
Progress photos and your weight trend
Scale weight moves every day with water, food and hormones. A 7-day average of your morning weight plus monthly progress photos shows the real trend without the daily noise. PTD Fitness coaches use this alongside body measurements in every programme.
Sources
This page is general education, not medical advice. The main references behind the numbers above:
- Centers for Disease Control and Prevention. Adult BMI categories.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004.
- National Institute for Health and Care Excellence. Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity. 2025.
- Ashwell M, Gunn P, Gibson S. 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.
- World Health Organization. Waist circumference and waist-hip ratio: report of a WHO expert consultation. 2011.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for US Navy men and women from body circumferences and height. Naval Health Research Center, 1984. Background in Peterson DD, History of the U.S. Navy body composition program. Military Medicine, 2015.
Before
After
Before
After
Before
After
Before
After