What is BMI?
Body mass index (BMI) is a measure that uses your height and weight to work out if your weight is in a healthy range. It was developed in the 19th century by Belgian statistician Adolphe Quetelet, and it’s the same formula the NHS, the WHO, and most UK GPs use today as a first screening check. It is calculated as your weight in kilograms divided by the square of your height in metres (kg / m²).
For most adults aged 18 to 65, BMI is a reasonable first signal of whether your weight is in the healthy range. It’s not a diagnosis — it’s a screening tool.
The NHS BMI categories
The NHS uses four categories for adults. For people of Asian, Middle Eastern, Black African and African-Caribbean family background, the healthy and overweight thresholds are lower — these groups have a higher risk of type 2 diabetes and cardiovascular disease at lower BMIs.
| Category | White / Other BMI | Asian / Black / Middle Eastern BMI |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obese | 30 and above | 27.5 and above |
Underweight (BMI below 18.5)
A BMI below 18.5 is classed as underweight by the NHS. It can signal undernutrition, an underlying condition, or simply a naturally small frame. If your BMI is below 18.5 and you haven’t been trying to lose weight, speak to your GP — sustained underweight is associated with reduced immunity, weaker bones, and fertility issues.
Healthy weight (BMI 18.5 – 24.9)
The NHS considers 18.5 to 24.9 the healthy BMI range for most White adults. For adults of Asian, Middle Eastern, Black African or African-Caribbean family background, the upper end of the healthy range is lower, at 22.9, reflecting higher cardiovascular and type 2 diabetes risk at equivalent BMIs.
Overweight (BMI 25 – 29.9)
A BMI of 25 to 29.9 (or 23 to 27.4 for the ethnic groups above) is classed as overweight. At this level the NHS recommends lifestyle changes — a modest calorie deficit, more physical activity, and tools such as the NHS Weight Loss Plan — before any clinical intervention.
Obese (BMI 30 and above)
A BMI of 30 or more (27.5 or more for the ethnic groups above) is classed as obese. Your GP may refer you to a tier-2 weight management service, and at BMI 35+ with a health condition, or BMI 40+, specialist NHS services including pharmacotherapy (Wegovy, Mounjaro) and bariatric surgery may apply.
When BMI doesn’t tell the whole story
BMI is a blunt instrument. It can be misleading for several groups, and it should always be read alongside your waist measurement and overall health picture.
Athletes and muscular people
Muscle is denser than fat, so a very fit person can land in the overweight or even obese range on BMI alone while being in excellent health. Rugby players, weightlifters and long-term gym-goers routinely score above 25 without any excess fat. Waist circumference or a body-fat estimate gives a much better picture for this group.
Pregnant women
BMI is not used during pregnancy. Healthy weight gain depends on your starting BMI and whether you’re carrying one baby or more. Speak to your midwife or GP — they use NHS-specific gestational weight-gain ranges, not BMI.
Children and young people under 18
Adult BMI categories don’t apply to under-18s. Children use a different, age-and-sex-adjusted scale called BMI centile, which compares a child’s BMI to others of the same age and sex. Ask your GP or use the NHS child BMI tool.
Adults over 65
In older adults, a BMI slightly above the official healthy range (up to around 27) is associated with better outcomes, particularly around frailty and recovery from illness. The NHS healthy range still applies as a guide, but small changes are less urgent at this stage than they are for younger adults.
Some ethnic groups
As noted in the categories table, NICE and the NHS use lower BMI thresholds for adults of Asian, Middle Eastern, Black African and African-Caribbean family background, because cardiovascular and type 2 diabetes risk rises at lower BMIs in these groups.
Pair BMI with waist measurement
A better picture of your weight-related health risk comes from combining BMI with your waist measurement — the fat that sits around your organs (visceral fat) is the fat that most affects your health. A waist-to-height ratio above 0.5 is a simple warning signal regardless of BMI.
How to lower your BMI (if you need to)
If your BMI is in the overweight or obese range and you’d like to bring it down, the NHS recommends small, sustainable changes rather than crash diets. The evidence is consistent: slow losers keep more weight off.
Start with small, sustainable changes
Aim for a loss of around 0.5–1 kg (1–2 lb) per week — the pace most UK adults can keep up. A loss of 5–10% of your starting weight meaningfully improves blood pressure, cholesterol and type 2 diabetes risk, even if your BMI is still above the healthy range at the end. The biggest early wins usually come from dropping liquid calories, halving starchy portions, and moving more.
Use free NHS-backed resources
The NHS Weight Loss Plan is a free 12-week app (no paywall) combining daily calorie and activity goals with weekly lessons. The NHS healthy weight hub has recipes, activity plans and the NHS Fitness Studio videos. For a longer walkthrough, see our own step-by-step guide to lowering your BMI.
When to see your GP
If your BMI is 30 or above, ask your GP about tier-2 weight management services — free, structured 12-week programmes available in most UK areas. At BMI 35+ with a health condition, or BMI 40+, you may be eligible for specialist NHS services including pharmacotherapy (Wegovy, Mounjaro) and, in some cases, bariatric surgery.
Frequently asked questions
Is this the official NHS BMI calculator?
How do I calculate BMI manually?
What is a healthy BMI for a woman / for a man in the UK?
Where does the percentile data come from?
Is BMI accurate for very muscular or very athletic people?
Do you store my data?
Disclaimer: This calculator is for general information only and is not medical advice. If you have questions about your weight or health, speak to your GP. Not affiliated with or endorsed by the NHS. Data source: Health Survey for England (NHS Digital).