Definition
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 = kg ÷ m². 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 — it should be used alongside waist circumference and other health indicators.
What is BMI?
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.
The BMI Formula
BMI is calculated the same way for adults of all ages, male or female:
BMI = Weight (kg) ÷ Height (m)²
Or in imperial: BMI = (Weight in lbs × 703) ÷ Height (inches)²
How to calculate BMI step by step
1
Convert your height to metres
If you know your height in feet and inches, multiply feet by 30.48 and add inches × 2.54 to get centimetres. Then divide by 100 for metres. Example: 5ft 9in = (5×30.48) + (9×2.54) = 152.4 + 22.86 = 175.26cm = 1.75m
2
Square your height
Height × Height
Multiply your height in metres by itself. Example: 1.75 × 1.75 = 3.0625
3
Divide your weight by the result
Weight (kg) ÷ Height²
Divide your weight in kilograms by the squared height. Example: 78kg ÷ 3.0625 = BMI 25.5 — just inside the overweight category.
4
Compare against NHS categories
Under 18.5 = Underweight · 18.5–24.9 = Healthy · 25–29.9 = Overweight · 30+ = Obese. Or use the free calculator below for an instant result with your personal healthy weight range.
Free BMI Calculator
Enter your height and weight for an instant BMI score, NHS category, and your healthy weight range.
Under 18.518.5–24.925–29.930+
NHS BMI Categories for Adults
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.
| 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 |
Adjusted thresholds for some ethnic backgrounds
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 |
What BMI Doesn't Tell You
⚠ BMI is a screening tool — not a diagnosis
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.
Better measures to use alongside BMI
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.
What's your healthy weight range?
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.
Calculate Now →
BMI — Frequently Asked Questions
What is a healthy BMI in the UK?
The NHS considers 18.5 to 24.9 the healthy BMI range for most adults. Below 18.5 is underweight. 25 to 29.9 is overweight. 30 and above is obese. For adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, the healthy range is 18.5 to 22.9 as health risks appear at lower BMIs in these groups.
What is the BMI formula?
BMI = weight in kilograms ÷ (height in metres × height in metres). Example: 78kg ÷ (1.75 × 1.75) = 78 ÷ 3.0625 = 25.5. In imperial units: BMI = (weight in pounds × 703) ÷ (height in inches squared). Use the calculator above for an instant result without doing the maths manually.
Is BMI 25 overweight?
Yes — 25 is the NHS threshold between healthy weight and overweight. A BMI of exactly 25 sits at the borderline. Context matters: a muscular person at 25.5 may be perfectly healthy. Someone sedentary at the same BMI with a high waist measurement has more reason to act. The NHS defines overweight as 25 to 29.9.
What BMI is considered obese?
A BMI of 30 or above is classified as obese by the NHS. This is further split into Class I (30–34.9), Class II (35–39.9), and Class III (40 and above). For South Asian and some other ethnic backgrounds, the obesity threshold is lower at BMI 27.5.
Is BMI accurate for women?
BMI uses the same formula and ranges for men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI due to hormonal differences. This means BMI can slightly underestimate metabolic risk in women. Waist circumference — under 80cm for women indicates lower risk — is a useful additional measure for women alongside BMI.
Is BMI accurate for athletes and muscular people?
No — BMI is known to overestimate health risk in muscular individuals. Muscle is denser than fat, so a lean athlete with low body fat can register a BMI of 27 or 28 and technically be classified as overweight. For athletes, body fat percentage and waist-to-height ratio are more meaningful indicators than BMI alone.
What is a healthy BMI for a woman?
The NHS healthy BMI range for women is 18.5 to 24.9 — the same as for men. For most adult women, a BMI in the 20–23 range combined with a waist measurement below 80cm is a practical health target. Women of South Asian or related backgrounds should aim for 18.5–22.9.
Can you be healthy with a high BMI?
Yes — particularly if the weight is predominantly muscle mass. BMI cannot distinguish between fat and muscle. However, a persistently high BMI of 35 or above does correlate with increased risk of type 2 diabetes, cardiovascular disease and joint problems, even in active people. Blood pressure, cholesterol, blood sugar and fitness level are more informative health indicators than BMI alone.
How much weight do I need to lose to reach a healthy BMI?
The BMI calculator above shows your personal healthy weight range — the kg range corresponding to a BMI of 18.5 to 24.9 at your height. The difference between your current weight and the top of that range is the amount to lose to reach healthy BMI. For example, someone 1.75m tall weighing 90kg has a BMI of 29.4. The top of their healthy range is 76.3kg — so they would need to lose approximately 13.7kg to reach healthy BMI.
Related Health Calculators