What is BMI?
BMI stands for Body Mass Index. It is a number calculated from your height and weight that gives a rough indication of whether you have a healthy amount of body fat for your size. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s — which is worth keeping in mind, because some of its limitations trace back to that era.
Despite being over 150 years old, BMI is still widely used by the NHS, the World Health Organization, and doctors worldwide as a quick, cost-free screening tool. It takes about ten seconds to calculate, requires no special equipment, and produces a single number that is easy to track over time. That’s precisely why it has stuck around.
How is BMI calculated?
The formula is simple. You divide your weight in kilograms by the square of your height in metres:
If you prefer to think in stones and feet, you don’t need to do the conversion yourself — our BMI calculator handles imperial measurements automatically and gives you your result instantly.
BMI in pounds and inches
If you’re working in imperial units, the formula is slightly different:
The result is the same number regardless of which unit system you use — BMI is dimensionless.
BMI categories explained
Once you have your BMI number, it falls into one of several categories. The UK follows the WHO classification system for adults aged 18 and over:
| BMI range | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate insufficient nutrition or an underlying health condition |
| 18.5 – 24.9 | Healthy weight | Associated with lower risk of weight-related health problems |
| 25 – 29.9 | Overweight | Moderately increased risk of conditions such as type 2 diabetes and heart disease |
| 30 – 34.9 | Obese (Class I) | Significantly increased health risk; lifestyle changes strongly recommended |
| 35 – 39.9 | Obese (Class II) | High health risk; medical review recommended |
| 40 and above | Obese (Class III) | Very high health risk; also called ‘severe’ or ‘morbid’ obesity |
These thresholds apply to adults. Children and teenagers use different, age- and sex-adjusted BMI charts because their bodies are still developing. The NHS has a separate BMI calculator for children.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. Within this range, research consistently shows lower rates of cardiovascular disease, type 2 diabetes, certain cancers, and early mortality compared to BMI values outside the range.
That said, the “healthy” label does not mean that a BMI of 18.6 is definitively better than one of 25.1. The real-world health risks associated with BMI form a curve rather than a cliff — someone at BMI 25 is not suddenly at drastically higher risk than someone at BMI 24.9.
Adjusted thresholds for people of South Asian heritage
The NHS and WHO both recognise that the standard BMI thresholds may overestimate healthy weight in people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background. For these groups NHS guidance uses lower thresholds:
- Overweight: BMI 23 and above
- Obese: BMI 27.5 and above
This is because at the same BMI, people from these ethnic groups can carry a higher proportion of body fat and face elevated risk of conditions like type 2 diabetes at lower BMI levels. Your GP can advise on the most appropriate threshold for you.
BMI’s limitations
BMI has been criticised for decades — and many of those criticisms are valid. Here are the most important limitations to understand:
Should you use BMI?
Yes — with appropriate expectations. BMI is a reasonable starting point for most adults to get a rough sense of whether their weight is in a healthy range. It is free, instant, and gives a number you can track over time to see whether you’re moving in the right direction.
Where BMI becomes less reliable:
- If you’re very muscular — gym-goers, athletes, and people with physically demanding jobs may find BMI overestimates their body fat
- If you’re older — in your 60s and above, waist circumference becomes a more reliable indicator
- For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background — NHS guidance uses lower thresholds, 23 and 27.5
- If you’re pregnant — BMI is not used as a weight measure during pregnancy
For everyone else, BMI is a perfectly reasonable number to know — as long as you use it as one input among several, not as the final word on your health.
Beyond BMI: other measures to know
If you want a fuller picture of your weight and body composition, these complementary measures are worth looking at alongside BMI:
Waist circumference
Fat carried around the middle — the abdomen — is more metabolically active and associated with higher risk of heart disease and type 2 diabetes than fat carried on the hips and thighs. The measure the NHS now publishes is not a fixed number of centimetres but a ratio: keep your waist size to less than half your height. So for someone 5’10” (177 cm), that is a waist below 88 cm (35 inches). The NHS has a waist-to-height tool that does the arithmetic.
Body fat percentage
Measured by DEXA scan, underwater weighing, or consumer smart scales (with varying accuracy), body fat percentage directly measures what BMI can only estimate. There is no single agreed healthy range: published reference ranges differ between men and women and shift with age, and the NHS does not publish one.
How to read the dates above. Checked means we opened that page and confirmed it still carries the wording quoted beside it. Record checked means we confirmed the citation against the PubMed record — authors, journal and year — but the article itself sits behind a publisher paywall and we have not re-read the full text. The dates are when a person actually opened the source; they are not bumped when a release is packaged. One entry carries no link: WHO Technical Report Series 894. The WHO library serves the same empty page for documents that exist and documents that do not, so a working link cannot be demonstrated from here. The report is a printed and PDF publication and is cited in full above.
Sources and further reading
- National Health Service. Overweight and obesity. NHS, 2025. nhs.uk/conditions/overweight-and-obesity — the adult BMI categories, the lower thresholds of 23 and 27.5 for Black, Asian and other minority ethnic groups, and “keep your waist size to less than half your height”. Checked 18 October 2026.
- National Health Service. Calculate your waist-to-height ratio. NHS, 2025. nhs.uk/health-assessment-tools/calculate-your-waist-to-height-ratio — how to take the measurement, and who the ratio does not apply to. Checked 18 October 2026.
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva, 2000. — the international BMI classification the UK follows.
- Eknoyan G. Adolphe Quetelet (1796–1874) pubmed.ncbi.nlm.nih.gov/17890752 — the average man and indices of obesity. Nephrology Dialysis Transplantation, 2008;23(1):47–51. — the origin of the index. Record checked 19 October 2026.
- 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. Obesity Reviews, 2012;13(3):275–286. pubmed.ncbi.nlm.nih.gov/22106927 Record checked 19 October 2026.
- Gallagher D, Heymsfield SB, Heo M, et al. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition, 2000;72(3):694–701. pubmed.ncbi.nlm.nih.gov/10966886 — why published body-fat reference ranges differ by age as well as sex. Record checked 19 October 2026.