Most people have had their BMI calculated at some point. A number appears, and nobody quite explains what to do with it. You are told it is "a bit high" or "fine" and that is usually where the conversation ends.
This guide covers what that number actually means, what the NHS categories are, and what counts as a healthy BMI for your height and background.
Table of Contents
- What BMI is and how it is calculated
- The NHS BMI categories
- What each category means for your health
- Why BMI is not the whole picture
- BMI thresholds for different ethnic backgrounds
- Your healthy weight range, not just a number
- Check your own BMI and healthy weight range
What BMI is and how it is calculated
BMI stands for Body Mass Index. It is a simple ratio of your weight to your height, calculated as:
BMI = weight in kilograms divided by height in metres squared
So for someone who weighs 75 kg and is 1.75 m tall, the calculation is: 75 divided by (1.75 × 1.75) = 24.5.
That is all it is. It does not measure body fat directly, and it does not know anything about your age, muscle mass, or where you carry weight. What it does is give clinicians and researchers a consistent, quick way to group people by weight relative to height at a population level.
The NHS BMI categories
The NHS uses six BMI ranges for adults aged 18 and over. These apply to both men and women.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 to 24.9 |
| Overweight | 25 to 29.9 |
| Obese class I | 30 to 34.9 |
| Obese class II | 35 to 39.9 |
| Obese class III | 40 and above |
The healthy weight range of 18.5 to 24.9 is the range where research consistently shows the lowest risk of weight-related health conditions. The categories above and below carry progressively higher risk the further from that range you fall.
What each category means for your health
Underweight (below 18.5)
A BMI below 18.5 suggests your weight may be too low for your height. This can be associated with nutritional deficiencies, a weakened immune system, and in some cases, bone density loss.
If you fall into this range, it is worth speaking to your GP, particularly if your weight has dropped unintentionally.
Healthy weight (18.5 to 24.9)
A BMI in this range suggests your weight is proportionate to your height. Research consistently links this range to lower rates of type 2 diabetes, cardiovascular disease, and certain cancers compared with the ranges above or below it.
Staying within this range is not a guarantee of good health, and someone at 24.8 is not meaningfully different from someone at 25.1. It is a guide, not a diagnosis.
Overweight (25 to 29.9)
Around 64% of UK adults fall into the overweight or obese categories. An overweight BMI does not mean you are unwell, but it does indicate an elevated risk of high blood pressure, type 2 diabetes, and joint problems over time.
For many people in this range, modest weight loss of 5 to 10% of body weight produces measurable improvements in blood pressure, cholesterol, and blood glucose.
Obese class I (30 to 34.9)
Class I obesity significantly increases the risk of cardiovascular disease, sleep apnoea, type 2 diabetes, and several cancers. The NHS typically considers weight management support from this point onwards.
Clinical interventions such as structured weight loss programmes, medication, and specialist referral are available at this category.
Obese class II (35 to 39.9)
At class II, the health risks are more significant and more likely to already be present. Most patients in this range will have at least one related condition such as hypertension, high cholesterol, or prediabetes.
NHS bariatric surgery criteria typically begin at class II obesity, particularly when combined with a weight-related health condition.
Obese class III (40 and above)
Class III obesity is associated with substantially reduced life expectancy and a high burden of related conditions. It is sometimes referred to as severe or morbid obesity.
Specialist input is almost always recommended at this level, and surgical options are considered appropriate depending on the individual's clinical picture.
Why BMI is not the whole picture
BMI has real limitations, and clinicians know this. The two most important are body composition and fat distribution.
A highly muscular person can have a BMI in the overweight range despite carrying very little body fat. Muscle is denser than fat, so it pushes the number up. This is why BMI alone is not used as a diagnostic tool; it is a screening indicator.
Fat distribution also matters. Carrying excess weight around the abdomen, what clinicians call central adiposity, is more strongly linked to heart disease and diabetes than carrying the same weight on your hips or thighs. A person with a BMI of 26 and a large waist measurement may carry more health risk than someone at 28 with most of their weight carried elsewhere.
For a more complete picture, waist circumference is usually assessed alongside BMI. For most adults, a waist of more than 88 cm for women or 102 cm for men indicates a higher risk of weight-related conditions, regardless of BMI category.
BMI thresholds for different ethnic backgrounds
The standard NHS BMI categories were developed primarily from research on White European populations. For several ethnic groups, health risks associated with excess weight appear at a lower BMI than the standard cutoffs suggest.
The NHS and World Health Organisation recommend lower thresholds for people from:
- South Asian backgrounds (including Indian, Pakistani, Bangladeshi, and Sri Lankan)
- Chinese and East Asian backgrounds
- Black African and Black Caribbean backgrounds
For these groups, the adjusted thresholds are:
| Category | Standard BMI | Adjusted BMI |
|---|---|---|
| Overweight | 25.0 | 23.0 |
| Obese | 30.0 | 27.5 |
This reflects evidence that people from these backgrounds tend to carry a higher proportion of body fat at the same BMI as White European adults, and that the associated metabolic risks appear earlier.
If you belong to any of these groups, your clinician should be using the adjusted thresholds when interpreting your BMI. The difference is clinically meaningful. A South Asian adult with a BMI of 24.5 is within the healthy range by the standard scale but above the overweight threshold by the adjusted one.
Your healthy weight range, not just a number
Most BMI conversations focus on the single number. A more useful question is: what weight range would put me in the healthy category for my height?
This gives you a target range to work within rather than a number to hit. For a person who is 170 cm (5 ft 7 in) tall, the healthy weight range is approximately 53 to 72 kg. For someone who is 180 cm (5 ft 11 in) tall, it is roughly 60 to 81 kg.
That range is what "healthy BMI" means in practice. It is not a single point on the scale. It is a band of roughly 18 to 20 kg within which your weight, for your height, corresponds to the lowest long-term health risk.
Knowing your range is more useful than knowing your number, because it tells you how far you are from the boundary in either direction and gives you a realistic sense of what movement would actually change your category.
Check your own BMI and healthy weight range
You can check your own BMI, see which NHS category you fall into, and get your personal healthy weight range with our free calculator. It also applies the ethnicity-adjusted thresholds for South Asian, East Asian, and Black African backgrounds, so the reading you get reflects the clinical guidance relevant to you.
