Calculate your Body Mass Index and see where you fall on the healthy weight scale.
| Category | BMI Range | Weight Status |
|---|---|---|
| ⬤ Underweight | < 18.5 | Below healthy range |
| ⬤ Normal weight | 18.5 – 24.9 | Healthy range |
| ⬤ Overweight | 25.0 – 29.9 | Above healthy range |
| ⬤ Obese | ≥ 30.0 | Significantly above healthy range |
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How BMI Is Calculated and What It Measures
How BMI is calculated
Body Mass Index divides your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². For example, someone weighing 70 kg at 1.75 m has a BMI of 22.9. In imperial units, the equivalent formula is (weight in lbs × 703) ÷ (height in inches)². The index was devised by Belgian mathematician Adolphe Quetelet in the 1830s and adopted as a clinical screening tool from the 1970s onwards.
Limitations of BMI
BMI doesn't directly measure body fat and doesn't account for muscle mass, bone density, age, sex, or ethnicity. Athletes and bodybuilders often have a high BMI due to muscle rather than fat, while older adults can have a "normal" BMI masking unhealthy levels of body fat due to age-related muscle loss. For people of South and East Asian ethnicity, health risks often begin at lower BMI thresholds than the standard cut-offs suggest.
BMI for children & teenagers
Adult BMI categories do not apply to children and teenagers under 18. For younger people, BMI is plotted against age- and sex-specific growth chart percentiles — a BMI-for-age at or above the 95th percentile is classified as obese, and between the 85th and 95th percentile as overweight. Paediatric BMI should always be interpreted by a GP or paediatrician.
Alternative measures
Waist circumference is a stronger predictor of cardiovascular risk — thresholds of 88 cm (35 in) for women and 102 cm (40 in) for men indicate significantly elevated risk. Waist-to-height ratio (waist ÷ height below 0.5 is the target) works across different body sizes and ethnicities. Body fat percentage measured via DEXA scan or hydrostatic weighing gives the most accurate assessment of body composition. Use the WHR calculator to check your waist-to-hip ratio alongside BMI.
Worked example
A person weighing 75 kg at a height of 1.78 m: BMI = 75 ÷ (1.78²) = 75 ÷ 3.1684 = 23.7 — falling in the Healthy weight range (18.5–24.9). At the same height, a weight of 80 kg gives BMI 25.2 (Overweight), while 68 kg gives BMI 21.5 (comfortably Healthy).
Related calculations
Working in different units? The height converter and weight converter switch between metric and imperial. For a target rather than a category, the ideal weight calculator gives a figure and the healthy weight calculator gives the range your height supports.
BMI Categories & What They Mean
Underweight — BMI below 18.5
Being underweight may indicate undernutrition, an eating disorder, or an underlying medical condition. It is associated with weakened immune function, reduced bone density, and fertility issues. If your BMI is below 18.5 and you haven't intentionally lost weight, it is worth discussing with a GP to rule out an underlying cause.
Healthy weight — BMI 18.5–24.9
A BMI in the healthy range is associated with the lowest risk of weight-related conditions including type 2 diabetes, heart disease, and certain cancers. Maintaining a healthy weight through regular physical activity and a balanced diet is one of the most effective long-term health investments. Annual health checks are still worthwhile within this range.
Overweight — BMI 25.0–29.9
Being overweight increases the risk of high blood pressure, raised cholesterol, type 2 diabetes, and sleep apnoea. Research consistently shows that losing even 5–10% of body weight produces meaningful improvements in blood pressure, blood sugar, and cholesterol levels. Sustainable dietary changes and increased physical activity are the most effective starting points.
Obese — BMI 30 and above
Obesity is classified into three classes: Class I (30–34.9), Class II (35–39.9), and Class III (40 and above). It is associated with significantly elevated risks of cardiovascular disease, type 2 diabetes, several cancers, and obstructive sleep apnoea. A GP can assess individual risk and refer to weight management services, which may include dietary support, structured activity programmes, or medical treatment.
Frequently Asked Questions
What is BMI and how is it calculated?
Body Mass Index is calculated as weight in kilograms divided by the square of height in metres (kg/m²). For example, 70 kg ÷ (1.75 m)² = BMI 22.9. In imperial units, the formula is (weight in lbs × 703) ÷ height in inches². Enter your measurements in the calculator above to get your result instantly.
What is a healthy BMI range for adults?
For most adults, a BMI between 18.5 and 24.9 is considered healthy, 25.0–29.9 is overweight, and 30 or above is obese. Some guidelines recommend lower thresholds for people of South and East Asian ethnicity, where health risks can begin at a BMI of 23 or above.
What are the limitations of BMI?
BMI does not directly measure body fat and does not account for muscle mass, age, sex, or ethnicity. A rugby player and a sedentary person can have the same BMI but very different body compositions. It is a useful population-level screening tool but should not be used as the sole measure of an individual's health.
What should I do if my BMI is outside the healthy range?
Use your result as a starting point for a conversation with a GP or practice nurse, who can assess additional risk factors — waist measurement, blood pressure, and blood glucose — alongside BMI. Small, sustainable changes to diet and activity levels tend to produce better long-term outcomes than rapid weight loss.
Evidence & Methodology
How This Page Is Grounded
Method
Calculates BMI as weight in kilograms divided by height in metres squared and applies the adult category thresholds shown on the page.
Important limitation: BMI is a population screening measure, not a diagnosis, and does not directly measure body fat or account for every individual difference.
Primary Sources
- Obesity and overweight World Health Organization
- Calculate your body mass index NHS
Quality Checks
Metric and imperial conversions and category boundaries are checked against published examples.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.