Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. The World Health Organization classes an adult BMI under 18.5 as underweight, 25 or more as overweight and 30 or more as obesity[1]. The NHS uses lower thresholds of 23 and 27.5 for people from Asian, Middle Eastern, Black African and African-Caribbean backgrounds[2].
How the BMI calculator works
The calculator divides your weight by the square of your height. A person who is 1.70 m tall and weighs 70 kg has a BMI of 70 divided by 2.89, which is 24.2. If you enter feet, inches and pounds, the calculator converts them to metric first, so the answer is the same either way.
It then compares the result with the adult cut-offs used in the World Health Organization's global data: under 18.5 is underweight, 25 or more is overweight and 30 or more is obesity[1]. Those are the thresholds behind the WHO estimate that 2.6 billion adults were living with overweight in 2024[3].
Lower thresholds for some ethnic backgrounds
People from some backgrounds develop type 2 diabetes and heart disease at a lower BMI. For that reason the NHS uses lower cut-offs for adults from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds: a BMI of 23 to 27.4 is overweight and 27.5 or more is obese[2]. Tick the box in the calculator and it applies those thresholds and adjusts your healthy weight range.
| BMI | Most adults (WHO) | Asian, Middle Eastern, Black African or African-Caribbean background (NHS) |
|---|---|---|
| Under 18.5 | Underweight | Underweight |
| 18.5 to 22.9 | Healthy weight | Healthy weight |
| 23 to 24.9 | Healthy weight | Overweight |
| 25 to 27.4 | Overweight | Overweight |
| 27.5 to 29.9 | Overweight | Obese |
| 30 or more | Obesity | Obese |
Your healthy weight range
The calculator also turns the healthy BMI band back into kilograms or pounds for your height. For most adults that is a BMI from 18.5 up to just under 25; with the lower thresholds it runs to just under 23. The range is a guide to where health risks are lowest across large populations. It is not a target you must hit, and a dietitian will look at much more than one number.
What BMI does not tell you
BMI is quick and free, which is why every health system uses it, but it measures size rather than health. It cannot tell muscle from fat, so strong, muscular people can score as overweight. It says nothing about where fat is stored, and fat around the middle carries more risk than fat on the hips. It is not used on its own in pregnancy, and children and teenagers are assessed on growth charts for their age and sex instead of adult cut-offs.
That is why we suggest pairing your BMI with our waist-to-height ratio check. The NHS advises keeping your waist to less than half your height[2].
When to talk to a dietitian
A BMI outside the healthy range is a reason to look closer, not a diagnosis. Book a session if your BMI is 25 or more and you would like a realistic plan, if it is under 18.5 or you have lost weight without trying, or if you are starting a weight-loss medicine and want to protect muscle and nutrition while you lose weight. Our weight management guide explains how nutrition care works alongside the new obesity medicines.
Book a video, phone or in-person session with a professional from our network. Your result can be attached so you do not start from scratch.
More free tools
- Calorie calculator
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- Fibre target
- Waist-to-height ratio
- Which professional do I need?
- Healthy eating check
- Compare two countries
Frequently asked questions
What is a healthy BMI for adults?
For most adults, a BMI of 18.5 to 24.9 falls between the WHO underweight and overweight cut-offs[1]. For people from Asian, Middle Eastern, Black African or African-Caribbean backgrounds the NHS treats 23 or more as overweight[2].
Is BMI accurate for athletes?
Not always. BMI cannot separate muscle from fat, so muscular people can score as overweight while carrying little body fat. A waist measurement or a body-composition assessment gives a better picture.
Can I use this BMI calculator for children?
No. Children and teenagers are assessed against growth charts for their age and sex. Ask your doctor, health visitor or a paediatric dietitian.
Why are the thresholds lower for some backgrounds?
Because the risk of type 2 diabetes and heart disease starts rising at a lower BMI in those groups, the NHS uses 23 and 27.5 instead of 25 and 30[2].
Does the calculator store my data?
No. The calculation runs in your browser. Nothing is sent to us unless you choose to book a session and share the result.
Sources and citations
Every figure on this page was read on the publisher's own page. Where a country or body does not publish a figure we say so instead of estimating.
- World Health Organization, Body mass index (BMI): Global Health Observatory indicators (accessed 16 Oct 2026)
- NHS (UK National Health Service), Overweight and obesity in adults (reviewed 29 Apr 2026)
- World Health Organization, Obesity and overweight: fact sheet (7 Oct 2026)
