All content on this page has been medically reviewed by: Hassan Thwaini, Clinical Pharmacist and Copywriter, Master of Pharmacy (MPharm) on 21 April, 2026. GPhC Registration: 2221320View profile
Body mass index (BMI) is one of the most widely used tools in healthcare. Enter your height and weight into our BMI calculator, and you'll get an instant snapshot of where your weight sits relative to NHS guidelines.
But a number on its own only tells part of your health story. Read on to understand what your result means, where it helps to calculate other measurements alongside BMI, and what you can do next.
We ask this because health risk thresholds vary between different ethnic groups.
YOUR BMI RESULT
–
| Underweight | Normal | Overweight | Obesity |
BMI stands for body mass index. It's a simple formula that uses your height and weight to give a numerical value that broadly indicates whether you're carrying too little, too much, or an appropriate amount of weight for your size.
Despite its age,1 BMI remains the standard screening measure used by the NHS, NICE, and health organisations worldwide because it's quick, non-invasive, and costs nothing to calculate.1
Our calculator supports both metric measurements (kg and cm) and imperial measurements (stones, pounds, feet, and inches), so you can use whichever you're most comfortable with.
BMI is calculated using a simple formula:
If you weigh 80 kg and are 1.75 m tall, the calculation looks like this:
80 ÷ (1.75 × 1.75) = 26.1
A BMI of 26.1 falls into the overweight range - just above the healthy weight threshold of 24.9.
The NHS and the World Health Organization (WHO) use the same adult BMI categories:
| BMI | Category |
|---|---|
| 18.4 and below | Underweight |
| 18.5-24.9 | Healthy weight |
| 25.0-29.9 | Overweight |
| 30.0-34.9 | Obese (Class 1) |
| 35.0-39.9 | Obese (Class 2) |
| 40.0 and above | Obese (Class 3) |
These are population-level guidelines, not a diagnosis. A BMI in the overweight range isn’t automatically cause for alarm, and a BMI in the healthy range doesn’t guarantee good health. Factors including sex, ethnicity, age, and body composition all affect what a given number actually means for you. We cover each of these below.
Our adult BMI calculator isn’t designed for under-18s. In adults, the same BMI thresholds apply regardless of age. In children and teenagers, healthy BMI ranges change significantly as the body grows and develops, so a single fixed threshold would be misleading.
Instead, children's BMI is assessed using centile charts, which plot a child's BMI against reference data from thousands of children of the same age and sex. In England, the NHS uses the WHO growth reference charts.2
These define healthy weight as falling between the 2nd and 91st centile for age and sex. A child at the 91st centile has a BMI higher than 91% of children their age and sex, which falls into the overweight category. Because healthy ranges shift year by year, a child who appears overweight on a standard chart could be well within normal limits on an age-adjusted one.
The NHS uses the following categories for children:
Standard BMI thresholds were developed primarily using data from white European populations. Research now shows clearly that people from other ethnic backgrounds can develop obesity-related health complications – particularly type 2 diabetes and cardiovascular disease – at lower BMI values.3
A major population-based cohort study of 1.4 million people in England found that wide-scale research has found that people of South Asian, Asian, Chinese, and Black origins are more likely to develop type 2 diabetes than white individuals at the same BMI.3
NICE now recommends lower BMI thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African, and African-Caribbean backgrounds:4
| Category | Standard threshold | Ethnicity-adjusted threshold |
|---|---|---|
| Overweight | 25.0 kg/m² | 23.0 kg/m² |
| Obese | 30.0 kg/m² | 27.5 kg/m² |
This means people from these ethnic backgrounds should be offered health risk assessment and support at a lower BMI than white European populations, even if a standard calculator doesn't flag a concern.3,4 If you're from one of the ethnic groups listed above and your BMI is above 23.0 kg/m², it's worth speaking to your GP about your individual risk.
BMI is a useful screening tool, but it does have some limitations when applied to individuals.1,5
Muscle mass
BMI can't distinguish between fat and muscle. Because muscle is denser than fat, people with high lean muscle mass - athletes, manual workers, or those who train regularly - can score a high BMI while carrying comparatively little body fat. Equally, someone with low muscle mass (known as sarcopenia) and a normal BMI could still be carrying an unhealthy proportion of body fat.1,5
Age
The BMI formula doesn't change with age, but what a given BMI means does. As we get older, muscle mass typically declines and body fat tends to increase, even if weight stays the same. An older person with a healthy BMI may therefore be carrying more metabolically harmful fat than a younger person with the same score.5
Fat distribution
Where fat is stored in the body matters as much as how much there is. Visceral fat – the fat stored deep in the abdomen around your organs – is strongly associated with metabolic disease, insulin resistance, and cardiovascular risk.6 Subcutaneous fat, which sits just beneath the skin, is generally less metabolically harmful.6 BMI gives no information about how your body stores fat, or where.1,5
Since 2022, NICE has recommended measuring waist-to-height ratio (WHtR) alongside BMI in adults with a BMI below 35 kg/m², as a practical way to assess fat stored around the abdomen (central adiposity).4
As a working guide, a waist circumference of less than half your height (0.5 or under) falls within the healthy range.7 NICE uses the following thresholds:
WHtR below 0.5: healthy central adiposity
WHtR 0.5–0.59: increased health risk
WHtR 0.6 or above: high health risk
A 2016 study found WHtR may predict heart health risks better than BMI or waist size.7 Since then, multiple studies have found that waist-to-height ratio is actually a better predictor of heart disease and premature death than BMI alone.8
Use a tape measure to find the midpoint between your lowest rib and the top of your hip bone.
Record that measurement. This usually falls around your belly button.
Divide that number by your height (both in the same unit: cm or inches).
GLP-1 receptor agonists Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two medications currently licensed in the UK for weight management treatment. Both are recommended by NICE and available through specialist weight management services, but eligibility is based on specific BMI criteria set by NICE.
This information is intended for educational purposes only. Prescription treatment is subject to clinical assessment and may not be suitable for everyone.
For the purposes of NICE guidance, qualifying conditions include type 2 diabetes, hypertension (high blood pressure), obstructive sleep apnoea, and cardiovascular disease, among others.4,9 Both medications are designed to be used as part of a clinically managed weight management programme, and not as standalone solutions.
Wu Y, Li D, Vermund SH. Advantages and limitations of the body mass index (BMI) to assess adult obesity. Int J Environ Res Public Health. 2024;21(6):757.
Royal College of Paediatrics and Child Health. UK-WHO growth charts: 2–18 years [Internet]. London: RCPCH; [cited 2026]. Available from: https://www.rcpch.ac.uk/resources/uk-who-growth-charts-2-18-years
Caleyachetty R, Barber TM, Mohammed NI, Cappuccio FP, Hardy R, Mathur R, et al. Ethnicity-specific BMI cutoffs for obesity based on type 2 diabetes risk in England: a population-based cohort study. Lancet Diabetes Endocrinol. 2021;9(7):419–426.
National Institute for Health and Care Excellence. Semaglutide for managing overweight and obesity [TA875]. London: NICE; 2023.
Sweatt K, Garvey WT, Martins C. Strengths and limitations of BMI in the diagnosis of obesity: what is the path forward? Curr Obes Rep. 2024;13(3):584–595.
Neeland IJ, Ross R, Desprès JP, Matsuzawa Y, Yamashita S, Shai I, et al. Visceral and ectopic fat, atherosclerosis, and cardiometabolic disease: a position statement. Lancet Diabetes Endocrinol. 2019;7(9):715–725.
Ashwell M, Mayhew L, Richardson J, Rickayzen B. Waist-to-height ratio as an indicator of ‘early health risk’: simpler and more predictive than using a ‘matrix’ based on BMI and waist circumference. PLoS ONE. 2016;11(3):e0149351.
Abdi Dezfouli R, et al. Waist to height ratio as a simple tool for predicting mortality: a systematic review and meta-analysis. Int J Obes (Lond). 2023;47(12):1286–1301.
National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity [TA1026]. London: NICE; 2024.
National Institute for Health and Care Excellence. Semaglutide for reducing the risk of major adverse cardiovascular events in people with cardiovascular disease and overweight or obesity [ID6441]. London: NICE; 2026.
Muscogiuri G, Verde L, Vetrani C, et al. Obesity: a gender-view. J Endocrinol Invest. 2024;47:299–306.
National Institute for Health and Care Excellence. Overweight and obesity management [NG246]. London: NICE; 2023.
Medically reviewed:
