Free BMI calculator for adults. Enter your height and weight for an instant result — NHS category, your healthy weight range, and a plain-English explanation of what your BMI actually means. No sign-up.
BMI (Body Mass Index) is a measure of body weight relative to height, calculated by dividing weight in kilograms by height in metres squared: BMI = weight (kg) ÷ height (m)². The NHS uses BMI as a screening tool for weight-related health risk in adults. A healthy BMI is 18.5 to 24.9. Below 18.5 is underweight. 25 to 29.9 is overweight. 30 and above is obese. BMI is a useful starting point but does not distinguish between muscle and fat — use it alongside waist circumference and other health indicators. Updated May 2026.
BMI stands for Body Mass Index. It is a simple calculation that relates your body weight to your height, producing a single number used as a first-pass indicator of whether your weight is in a healthy range.
It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has been used by health services worldwide — including the NHS — as a quick, low-cost screening tool for weight-related health risk. It requires no equipment beyond scales and a measuring tape.
BMI is not a diagnosis and not a complete picture of health. But it is a useful starting point — particularly for identifying whether someone is significantly underweight or carrying excess weight that may affect long-term health outcomes.
BMI is calculated the same way for adults of all ages, male or female:
Enter your height and weight for an instant BMI score, NHS category, and your healthy weight range.
The NHS uses the following BMI ranges for adults aged 18 and over. These are the same ranges used by GPs in the UK for initial health screening. Unchanged for 2026.
Source: NHS UK. For South Asian, Black African and Black Caribbean adults, overweight starts at BMI 23 and obese at 27.5.
| BMI Range | Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency; discuss with GP |
| 18.5 – 24.9 | Healthy weight ✓ | Associated with lowest risk of weight-related illness |
| 25.0 – 29.9 | Overweight | Increased risk — but context matters significantly |
| 30.0 – 34.9 | Obese — Class I | Moderately increased risk of T2 diabetes, heart disease |
| 35.0 – 39.9 | Obese — Class II | Severely increased risk; GP review recommended |
| 40 and above | Obese — Class III | Very severely increased risk; medical support advised |
For adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, the NHS uses lower thresholds because research shows health risks increase at lower BMIs for these groups:
| BMI Range (Adjusted) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 22.9 | Healthy weight ✓ |
| 23.0 – 27.4 | Overweight |
| 27.5 and above | Obese |
BMI cannot distinguish between muscle and fat. A muscular athlete with very low body fat can show a BMI of 27 and be classified as overweight. Equally, someone with a BMI of 24 who has low muscle mass and high central fat may have significant metabolic risk that BMI doesn't capture.
BMI also doesn't account for where fat is stored. Central (abdominal) fat carries a higher health risk than fat stored on the hips or thighs — this isn't visible in a BMI score.
Waist circumference — a waist measurement above 94cm (men) or 80cm (women) indicates increased health risk regardless of BMI. Above 102cm (men) or 88cm (women) is high risk.
Waist-to-height ratio — divide your waist measurement by your height. A ratio below 0.5 is associated with lower cardiovascular risk. This is increasingly considered more useful than BMI alone.
Body fat percentage — measured via DEXA scan, bioelectrical impedance, or skinfold calipers. More accurate than BMI for body composition, but requires equipment and expertise.
For most adults, a BMI in the healthy range combined with a waist circumference within the low-risk threshold is a practical and meaningful target.
Source: NHS UK. You can have a healthy BMI but still be at risk if your waist measurement is above these thresholds.
| NHS category | BMI range | Associated health risks |
|---|---|---|
| Underweight | Below 18.5 | Malnutrition, weakened immune system, osteoporosis, anaemia, fertility problems |
| Healthy weight | 18.5–24.9 | Lower risk of weight-related conditions. Maintain with regular activity and balanced diet. |
| Overweight | 25.0–29.9 | Increased risk of type 2 diabetes, high blood pressure, heart disease, some cancers, sleep apnoea |
| Obese (Class 1) | 30.0–34.9 | Significantly elevated risk — type 2 diabetes, heart disease, stroke, certain cancers, joint problems |
| Obese (Class 2) | 35.0–39.9 | High risk — NHS weight management referral recommended |
| Severe obesity (Class 3) | 40+ | Very high risk — GP referral to specialist services strongly recommended |
Source: NHS UK. For South Asian, Chinese, Black African and African-Caribbean adults, overweight starts at BMI 23 and obese at BMI 27.5.
The calculator above shows your personal healthy weight range alongside your BMI score — the kg range your body would need to be within for a BMI of 18.5–24.9.
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