BMI calculator

How BMI is calculated

Body Mass Index compares your weight against your height to produce a single number. The formula is deliberately simple — it needs only two measurements, which is exactly why it became the standard screening tool in general practice.

Take someone who is 5 ft 9 in tall and weighs 11 st 6 lb. That converts to 1.75 m and 72.6 kg. Squaring the height gives 3.06, and 72.6 divided by 3.06 comes to a BMI of 23.7 — comfortably inside the healthy range. The calculator above handles the stones-to-kilograms conversion for you, so you can enter your measurements however you normally think about them.

What each BMI category means

The thresholds below are the ranges the NHS and the World Health Organization use for most adults aged 18 and over. The NHS publishes lower boundaries for some ethnic backgrounds — those are covered in the next section, and the calculator above will show them.

It is worth treating these boundaries as gentle gradients rather than cliff edges. A BMI of 25.1 does not represent a materially different level of health risk from 24.9 — the categories exist because population health needs round numbers, not because your body changes state when you cross one.

BMI thresholds are lower for some ethnic groups

This is the single most commonly missed detail about BMI, and it matters a great deal in the UK.

People of South Asian, Chinese, other Asian, Black African, African-Caribbean and Middle Eastern family background tend to develop weight-related health risks — particularly type 2 diabetes — at a lower BMI than white European populations. This appears to relate to how and where the body stores fat rather than total fat alone.

In practice this means a BMI of 24 might be read as unremarkable for one person and as a prompt for a conversation about diabetes risk for another. If this applies to you, it is worth raising with your GP rather than reading your category from the standard table.

Where BMI falls short

BMI knows your height and your weight. It knows nothing else about you — and several of the things it cannot see matter.

It cannot distinguish muscle from fat

Muscle is denser than fat, so a well-trained rugby player or weightlifter can register as obese while carrying very little body fat. This is the most frequently cited criticism of BMI, though it affects a fairly small share of the population.

It cannot see where fat is stored

Fat carried around the abdomen carries a higher metabolic risk than fat carried on the hips and thighs, but BMI treats both identically. Two people with the same BMI can have quite different risk profiles.

It is not designed for everyone

• Children and teenagers under 18 — BMI is interpreted using age and sex-specific centile charts, not the adult thresholds above.
• During pregnancy — BMI is not a meaningful measure; your midwife will use your pre-pregnancy weight as the reference point.
• Older adults — muscle mass declines with age, so BMI can understate body fat in people over about 65.

A useful second measurement

Because BMI misses fat distribution, waist size is a valuable companion measure. Measure around your middle at the level of your belly button, without pulling the tape tight. NHS guidance is to keep your waist size to less than half your height — so someone 5 ft 9 in (69 in) is looking at a waist under about 34.5 in. The NHS describes the ratio as a measure to read alongside BMI, and has a tool that does the arithmetic and explains how to take the measurement.

What to do with your result

A BMI figure is a starting point for a question, not an answer in itself. What it is genuinely good at is flagging when something might be worth a closer look.

If your result sits in the healthy range and you feel well, there is little to act on. If it sits outside that range, the useful next step is context: how is your waist measurement, has your weight changed noticeably in the past year, and how active are you? The NHS recommends at least 150 minutes of moderate activity a week for adults, alongside strengthening exercise on two days — and improvements in fitness produce real health benefits even when the number on the scale barely moves.

For a persistent result well outside the healthy range, an unexplained change in weight, or a result you are unsure how to read, a GP can interpret it alongside blood pressure, blood glucose and your family history. That combined picture is far more informative than BMI alone.

Frequently asked questions

Sources and further reading

• World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva, 2000. — the international BMI classification used in the table above.
• 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, and why it was built to describe populations. Record checked 19 October 2026.
• Keys A, Fidanza F, Karvonen MJ, et al. Indices of relative weight and obesity. Journal of Chronic Diseases, 1972;25(6):329–343. pubmed.ncbi.nlm.nih.gov/4650929 — the paper that named it “body mass index” and brought it into health screening. Record checked 19 October 2026.
• National Health Service. Overweight and obesity in adults. NHS, page last reviewed 29 April 2026. nhs.uk/conditions/overweight-and-obesity — the source of both threshold sets the calculator applies: overweight from 25 and obese from 30 for most adults, and overweight from 23 and obese from 27.5 for people from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background. Checked 18 October 2026.
• National Health Service. Calculate your body mass index (BMI) for adults. NHS, page last reviewed 28 November 2023. nhs.uk/health-assessment-tools/calculate-your-body-mass-index — why the NHS asks for ethnic background before showing a BMI result, and the formula used above. Checked 18 October 2026.
• National Health Service. Calculate your waist to height ratio. NHS, page last reviewed 25 February 2025. nhs.uk/health-assessment-tools/calculate-your-waist-to-height-ratio — the waist rule quoted above, verbatim: “You should try to keep your waist size to less than half your height.” 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: systematic review and meta-analysis. Obesity Reviews, 2012;13(3):275–286. pubmed.ncbi.nlm.nih.gov/22106927 — the evidence behind the “waist under half your height” rule. Record checked 19 October 2026.