BMI and healthy weight: what the NHS bands actually mean
BMI is one of the most-searched health calculations in the UK, and one of the most misunderstood — not because the sum is hard, but because the bands and their caveats rarely get explained together. Here's the short version.
How BMI is calculated
Body Mass Index is weight in kilograms divided by height in metres squared (kg/m²). In imperial units, convert height to inches and weight to pounds, then use (weight in lb ÷ height in inches²) × 703 to get the same result. It's a simple ratio, not a direct measurement of body fat — that's the source of most of its limitations.
The four standard weight bands
The bands used across the NHS and internationally split into four categories: under 18.5 is underweight, 18.5–24.9 is healthy weight, 25–29.9 is overweight, and 30 or above is obese. These thresholds come from population-level studies linking BMI to health outcomes, not from any single individual's ideal weight.
Why the NHS uses adjusted thresholds for some groups
Standard BMI bands were derived largely from studies of white European populations. Evidence since has shown that health risks — particularly type 2 diabetes — can begin at a lower BMI in some South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean groups. As a result, the NHS and NICE guidance recommend lower BMI thresholds for identifying increased risk in some of these groups.
NICE guidance has recommended using a BMI of 23 (rather than 25) as the overweight threshold for identifying increased type 2 diabetes risk in some South Asian and Chinese adults.
Where BMI genuinely falls short
BMI can't distinguish muscle from fat, so athletes and very muscular people often show a "high" BMI despite low body fat. It also doesn't account for where fat is carried, which matters for health risk, or for frame size and age-related muscle loss. Children aren't assessed on the adult bands at all — their BMI is plotted against age- and sex-specific growth percentile charts instead, because a healthy body composition changes constantly through childhood.
None of this makes BMI useless — as a fast, free, population-level screening tool it's genuinely useful as a starting point. It just isn't a diagnosis, and it works best alongside other measures and a conversation with a GP if you have concerns.
A quick note
BMI is a general screening tool, not a diagnosis, and doesn't account for muscle mass, frame size or ethnicity in a fully individualised way. See our full disclaimer.