About the BMI calculator
Body mass index (BMI) compares your weight with your height. This calculator takes height and weight in metric, US or UK units (including stone and pounds) and returns your BMI, the adult category it falls in, and the weight range that corresponds to a BMI of 18.5 to 25 for your height.
BMI is a quick screening number, not a verdict on your health. It is most useful as a starting point for a conversation with a health professional.
Who should use the BMI calculator
- Adults who want a quick, standard reference point for weight relative to height.
- People tracking changes in weight over time using a consistent measure.
- Students and teachers looking for a worked example of a simple health index.
How to use the BMI calculator: what each input means
- Units
- Metric (cm and kg), US (feet, inches and pounds) or UK (feet, inches, stone and pounds).
- Age
- Used to check the adult range. For under-18s, BMI must be read against growth charts, so this tool does not give an adult category.
- Height and weight
- Your current height and weight. Measure without shoes and in light clothing for consistency.
How the BMI calculator works: method and formula
BMI is weight in kilograms divided by height in metres squared. The result is compared with World Health Organization adult categories: below 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or more is obese, with further subdivisions for thinness and obesity classes.
The healthy weight range shown is the weight at which your height gives a BMI of 18.5 and 25. BMI Prime is your BMI divided by 25, so 1.0 is the top of the normal range. The Ponderal Index divides weight by height cubed.
- BMI
- BMI = weight (kg) ÷ height (m)²
- US units
- BMI = 703 × weight (lb) ÷ height (in)²
How to read your BMI calculator result
A BMI in the normal range does not guarantee good health, and a BMI outside it does not on its own mean poor health. BMI does not separate fat from muscle, ignores where fat is carried, and ranges may be less accurate for athletes, older adults, pregnant people and some ethnic groups.
Waist measurement, blood pressure, blood tests, diet, activity and family history give a fuller picture. A health professional can interpret all of these together.
BMI Calculator worked example
An adult who is 175 cm tall and weighs 70 kg.
- Height in metres: 1.75, so height squared is 3.0625.
- BMI = 70 ÷ 3.0625 = 22.9 kg/m².
- The healthy range for this height is 18.5 × 3.0625 = 56.7 kg to 25 × 3.0625 = 76.6 kg.
BMI 22.9: normal range.
Practical uses of the BMI calculator
- A quick check before a conversation with a doctor or dietitian.
- Keeping a simple record of weight relative to height over time.
- Converting stone and pounds to kilograms and a BMI in one step.
Assumptions and limitations of this calculator
- Adult categories only. For children and teenagers use growth charts that account for age and sex.
- Does not measure body fat, muscle, bone density or fat distribution.
- Category cut-offs are population guidelines. Some health bodies suggest different thresholds for certain groups.
This calculator is for general information and education only. It is not medical advice, a diagnosis or a treatment plan. Results are estimates based on population formulas and cannot account for your individual health. If you have a medical condition, are pregnant, are under 18, or are planning a major change to your diet or exercise, speak to a doctor or registered dietitian first.
Notes for different countries and units
- This calculator reports the standard WHO adult categories. A WHO expert consultation on Asian populations (2004) concluded that health risks can rise at lower BMI values in many Asian populations and proposed extra public-health action points at 23.0 and 27.5. These are not applied automatically. If you are of South Asian or another Asian background, read your result with that in mind and talk to your doctor.
- Units: metric (cm and kg), US (feet, inches and pounds) and UK (feet, inches, stone and pounds) are all supported, so you can enter your measurements the way you know them.