BMI Calculator
Calculate your Body Mass Index (BMI) to check whether you are at a healthy weight for your height. Supports both metric (cm / kg) and imperial (feet-inches / stone-pounds) measurements.
What is Body Mass Index (BMI)?
Body Mass Index, commonly known as BMI, is a simple calculation used to assess whether a person is at a healthy weight for their height. It was devised by Belgian mathematician Adolphe Quetelet in the 1830s and is still widely used by healthcare professionals as a screening tool for weight-related health risks.
The formula divides a person's weight in kilograms by the square of their height in metres. The resulting number, typically between 15 and 40 for most adults, is compared against standard categories to determine whether the person is underweight, at a healthy weight, overweight, or obese.
While BMI is widely used due to its simplicity, it is not a direct measure of body fat. It does not account for muscle mass, bone density, age, sex, or ethnicity, all of which can affect the relationship between BMI and actual body fat percentage. Despite these limitations, BMI remains a useful first-line screening tool when used alongside other health assessments.
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Possible nutritional deficiency, weakened immune system |
| 18.5 to 24.9 | Healthy Weight | Lowest risk of weight-related health problems |
| 25.0 to 29.9 | Overweight | Increased risk of heart disease, diabetes |
| 30.0 to 34.9 | Obese (Class I) | High risk of weight-related conditions |
| 35.0 to 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk |
Limitations of BMI
While BMI is a useful screening tool, it has several important limitations that should be understood when interpreting your result.
Muscle vs fat: BMI cannot distinguish between muscle and fat. Athletes, bodybuilders, and people who do a lot of strength training may have a high BMI due to increased muscle mass rather than excess fat. A rugby player weighing 100kg at 1.85m would have a BMI of 29.2 (overweight), despite potentially having very low body fat.
Age: As people age, they tend to lose muscle and gain fat. An older person may have a normal BMI but a higher body fat percentage than a younger person with the same BMI.
Ethnicity: Research shows that health risks associated with body weight vary between ethnic groups. People of South Asian, Chinese, and other Asian ethnicities may have higher health risks at lower BMI levels. The NHS suggests that a BMI of 23 or above may indicate increased risk for these groups, compared to 25 for white European populations.
Fat distribution: BMI does not account for where body fat is distributed. Abdominal (visceral) fat, which sits around internal organs, is more strongly associated with health risks than fat stored on the hips and thighs. Waist circumference or waist-to-hip ratio can provide additional information about fat distribution.
Healthy Weight Range
Your healthy weight range is determined by the BMI values of 18.5 and 24.9 applied to your specific height. For a person who is 1.75m (5'9") tall, the healthy weight range is approximately 56.7 kg to 76.3 kg. For someone 1.65m (5'5") tall, it is approximately 50.4 kg to 67.8 kg.
Being within the healthy weight range does not guarantee good health, just as being outside it does not necessarily indicate poor health. However, maintaining a healthy BMI is associated with lower risk of type 2 diabetes, cardiovascular disease, certain cancers, musculoskeletal problems, and other conditions. The NHS recommends aiming for a BMI within the healthy range through a combination of balanced diet and regular physical activity.
NHS Guidance on Healthy Weight
The NHS recommends that most adults aim for a BMI between 18.5 and 24.9. If your BMI is outside the healthy range, the NHS suggests the following steps:
- Underweight (BMI below 18.5): If you are underweight, speak to your GP. Being underweight can be a sign of not eating enough or an underlying health condition. Your GP can help identify the cause and recommend an appropriate plan to reach a healthy weight.
- Overweight (BMI 25 to 29.9): Making small changes to your diet and increasing physical activity can help you reach a healthier weight. The NHS recommends losing weight gradually at a rate of 0.5 to 1 kg per week through a combination of eating fewer calories and being more active.
- Obese (BMI 30 or above): Speak to your GP about weight management support. The NHS offers structured weight management programmes and, in some cases, may recommend weight loss medication or surgery for those with a BMI of 40 or above (or 35 with weight-related health conditions).
Common Questions About BMI
Does BMI Apply to Pregnant Women?
Standard BMI categories do not apply during pregnancy, as weight gain is a normal and necessary part of pregnancy. However, your pre-pregnancy BMI is used by healthcare professionals to assess pregnancy risks and guide appropriate weight gain during pregnancy. The NHS uses pre-pregnancy BMI as part of the antenatal booking assessment.
What About Waist Circumference?
Waist circumference is increasingly used alongside BMI as a measure of health risk. Carrying excess fat around the middle (visceral fat) is associated with higher risk of type 2 diabetes, heart disease, and stroke. The NHS considers a waist circumference above 94 cm (37 inches) in men and 80 cm (31.5 inches) in women to indicate increased health risk, regardless of BMI.
How Often Should I Check My BMI?
There is no set recommendation for how often to check BMI. For most adults, checking once or twice a year is reasonable. If you are actively trying to lose or gain weight, more frequent monitoring can help track progress. Keep in mind that day-to-day weight fluctuations are normal, so it is best to track trends over weeks rather than daily readings.