BMI is the most widely used measure of weight status in the UK. It's on your NHS health check, your GP's screen, and most weight loss calculators. But it's also one of the most consistently misunderstood numbers in health — both over-relied on by clinicians and dismissed entirely by critics who call it useless. The honest picture sits between those two positions. BMI tells you something real and useful, within limits that are worth understanding clearly.
What BMI actually measures
Body Mass Index is calculated by dividing your weight in kilograms by your height in metres squared (kg/m²). That is all it measures. It says nothing about your body fat percentage, where fat is stored, your muscle mass, your age, your fitness level, or your metabolic health. It is a proxy — a cheap, fast, population-level screening tool developed in the 19th century that happens to correlate reasonably well with health risk at a population level, while being unreliable for many individuals.
The NHS uses four categories:
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
For people of South Asian, Chinese, Black African, Black Caribbean, and some other ethnic backgrounds, the NHS uses lower thresholds — overweight from BMI 23, and obese from BMI 27.5 — because research shows these groups carry higher health risk at lower BMI values than the thresholds derived from predominantly white European populations.
Where BMI works reasonably well
At a population level, BMI correlates meaningfully with health outcomes. Large studies consistently show that people with BMI above 30 have higher rates of type 2 diabetes, cardiovascular disease, certain cancers, and joint problems than people with BMI in the healthy range — and people with very low BMI face different but equally real health risks. This is why NHS services use it: it is free to calculate, requires no equipment, and flags enough people accurately enough to be useful as a first-pass screening tool across millions of patients.
For most people of average build and activity level, BMI gives a broadly accurate picture. If your BMI is 22, you are almost certainly not carrying a problematic amount of body fat. If it is 38, excess fat is almost certainly a real health factor regardless of other variables. The edges of the distribution are where it holds up least well.
Where BMI misleads
Muscle mass
Muscle is denser than fat. A person with significant muscle mass — a regular gym-goer, an athlete, or anyone who has done consistent strength training over years — will have a higher BMI than someone of the same height with much less muscle and more fat, despite being in better metabolic health. The most dramatic examples are elite athletes: many professional rugby players, weightlifters, and sprinters register as "obese" on BMI despite having very low body fat percentages. For most ordinary gym-goers the effect is smaller but still real — a BMI of 26 or 27 in a person who trains regularly may reflect healthy muscle rather than excess fat.
Fat distribution
Where fat is stored matters as much as how much of it there is. Visceral fat — the fat stored around the organs in the abdominal cavity — is metabolically active and strongly associated with insulin resistance, type 2 diabetes, and cardiovascular disease. Subcutaneous fat — stored under the skin, particularly on the hips, thighs, and buttocks — carries much lower risk. Two people with identical BMIs can have very different health profiles depending on their fat distribution. BMI cannot distinguish between them. Waist circumference — a direct measure of abdominal fat — is a stronger predictor of metabolic risk than BMI for many people.
Ethnicity
The standard BMI thresholds were developed using data predominantly from white European populations. Research has since established that people of South Asian descent accumulate dangerous visceral fat at lower BMI values, meaning standard thresholds underestimate their risk. The NHS has adjusted its thresholds accordingly, but many online calculators and health apps still use the original figures — which means people from these backgrounds may be receiving an inaccurate reading if they're using a calculator that hasn't been updated.
Age and sex
Body composition changes with age — older adults typically carry more fat at any given BMI than younger adults. Women generally have higher body fat percentages than men at the same BMI due to hormonal differences. Neither of these factors appears in the BMI calculation. A BMI of 24 means something different for a 25-year-old man and a 65-year-old woman, but the number itself doesn't capture this.
Height extremes
BMI was designed for people of average height and works less well at extremes. Very tall people tend to have slightly lower BMIs than their actual fat levels suggest; very short people tend to have slightly higher ones. This is a mathematical artefact of the formula rather than a reflection of their actual health status.
The practical summary: BMI is a reasonable first check for most adults of average build. It is unreliable for people with significant muscle mass, those from South Asian or East Asian backgrounds using standard thresholds, older adults, and people at height extremes. In these cases, waist circumference and waist-to-height ratio give a more accurate picture.
What to use alongside BMI
Waist circumference
The NHS recommends keeping waist circumference below 94cm (37 inches) for men and below 80cm (31.5 inches) for women, with high risk above 102cm and 88cm respectively. Measuring at the midpoint between your lowest rib and the top of your hip bone gives a direct read on abdominal fat — the type most strongly linked to metabolic disease.
Waist-to-height ratio
Dividing your waist circumference by your height gives a single number that many researchers consider a better predictor of cardiovascular risk than BMI. A ratio below 0.5 is generally considered healthy — in plain terms, your waist should be less than half your height. This measure automatically adjusts for body size and works better across different ethnicities than BMI.
Body fat percentage
DEXA scans give the most accurate body fat measurement but are expensive and not widely available. Consumer body fat scales use bioelectrical impedance, which is less accurate but directionally useful. Healthy body fat ranges are approximately 18–24% for adult women and 10–20% for adult men, though these vary by age and source.
Should you ignore your BMI?
No — but you should contextualise it. A BMI in the healthy range combined with a healthy waist circumference and no concerning metabolic markers is a genuinely reassuring picture. A BMI flagged as overweight or obese is worth taking seriously, even if you feel fine — visceral fat in particular causes metabolic changes long before they become symptomatic. Where BMI and other indicators conflict, the other indicators deserve more weight than BMI alone.
The most useful thing BMI does is give you a starting point for a conversation, not a verdict.
Sources: NHS BMI guidance, NICE guidelines on obesity, NHS ethnic group BMI adjustments, WHO BMI classification, British Heart Foundation waist measurement guidance. This article is for general educational purposes only and does not constitute medical advice.