What BMI actually measures
Body mass index is a simple ratio of weight to height, used since the 19th century as a quick screen for whether someone's weight sits in a range associated with good health. It is not a body fat measurement and it knows nothing about you beyond two numbers, which is both its strength (anyone can work it out) and its weakness (see the limitations below).
If you think in feet and stones, the tool converts for you: pounds × 0.453592 gives kilograms, inches × 0.0254 gives metres, and the formula runs on the metric values. For most adults the NHS groups the result like this: below 18.5 is underweight, 18.5 to 24.9 is the healthy weight range, 25 to 29.9 is overweight, and 30 or above is obese.
Worked examples
That second example shows how fine the boundaries can be: a pound or two either way moves the label, while the underlying health picture barely changes. The categories are bands drawn across a smooth scale, not cliff edges.
The limitations of BMI
BMI cannot tell muscle from fat. Rugby players and weightlifters routinely score as overweight while carrying very little fat, and an older adult who has lost muscle can score in the healthy range while carrying more fat than the number suggests. It also says nothing about where weight sits, and fat around the middle carries more risk than fat elsewhere, which is why the NHS often pairs BMI with a waist measurement.
Thresholds are not one size fits all either. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, health risks rise at lower BMIs, so the NHS flags increased risk from 23 and high risk from 27.5 rather than 25 and 30. BMI is also not designed for pregnancy, and children and young people are assessed with age and sex specific centile charts rather than the adult ranges used here.
None of this makes BMI useless. Across whole populations it tracks health outcomes well, and for most adults it is a reasonable first look. It is simply one signal, best read alongside everything else you and your GP know about you.
Where to go from here
Whatever your number says, it is a prompt for a conversation, not a judgement. The NHS BMI calculator asks a few extra questions (age, sex, ethnicity, activity) and gives tailored follow-up, and your GP or practice nurse can put the number in the context of your health as a whole. Your inputs here never leave your browser; nothing is uploaded or stored.
This tool is guidance, not medical advice. For a proper assessment of your weight and health, use the NHS service above or speak to a health professional.
Frequently asked questions
What is a healthy BMI?
For most adults the NHS treats 18.5 to 24.9 as the healthy weight range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight and 30 or above as obese. Lower thresholds apply for people from some ethnic backgrounds.
How is BMI calculated?
Weight in kilograms divided by height in metres squared. If you use imperial units the tool converts first: pounds multiplied by 0.453592 gives kilograms, and inches multiplied by 0.0254 gives metres. The result is the same either way.
Is BMI accurate for everyone?
No. It cannot tell muscle from fat, so muscular people can score high while carrying little fat, and older adults can score in range while carrying more. It is not designed for pregnancy, and children are assessed with centile charts instead. Treat it as a starting point, not a verdict.
Why does the NHS use different thresholds for some ethnic groups?
Research shows that people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds face higher health risks at lower BMIs. For these groups the NHS flags increased risk from a BMI of 23 and high risk from 27.5.
What should I do if my BMI is outside the healthy range?
A single number is not a diagnosis, so there is no need to panic either way. The NHS BMI service on nhs.uk takes more factors into account, and your GP or practice nurse can look at the fuller picture, including waist measurement and lifestyle.