BMI Calculator
Calculate your Body Mass Index and understand your weight category
BMI Categories Reference
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Malnutrition risk |
| Normal weight | 18.5 – 24.9 | Low risk |
| Overweight | 25.0 – 29.9 | Moderate risk |
| Obese Class I | 30.0 – 34.9 | High risk |
| Obese Class II | 35.0 – 39.9 | Very high risk |
| Obese Class III | 40.0 and above | Extremely high risk |
About this tool
BMI is weight in kilograms divided by height in metres squared - a formula devised in the 1830s by a Belgian statistician studying populations, not patients. That origin explains both its usefulness and its limits. Across a large group it correlates reasonably with body fat and health outcomes, which is why health services still use it for screening. Applied to one individual it can be misleading, because the formula has no way to distinguish muscle from fat. A rugby player and a sedentary person of identical height and weight get identical BMIs, describing very different bodies. It also takes no account of where fat is carried, and abdominal fat carries considerably more health risk than the same amount elsewhere - which is why waist circumference is often the more informative measurement. Treat BMI as one rough indicator among several, worth knowing and not worth agonising over. This calculates in your browser; nothing is recorded.
How to use this tool
- Choose metric or imperialKilograms and centimetres, or pounds and feet and inches - the result is identical either way.
- Enter your height and weightAccuracy matters more for height than weight, since height is squared in the formula.
- Read the BMI figure and rangeThe number and the category it falls in are shown together.
- Read it in contextConsider your build, muscle mass and waist measurement alongside it rather than treating the number as a verdict.
How to calculate BMI
BMI is your weight in kilograms divided by your height in metres squared. For someone who weighs 70 kg and is 1.75 m tall: 1.75 x 1.75 = 3.06, and 70 / 3.06 = 22.9. In pounds and inches the formula is 703 x weight / height squared, so 154 lb at 5 ft 9 in (69 in) gives 703 x 154 / 4,761 = 22.7.
The adult WHO categories are: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or more obese. Some health services use lower cut-offs for people of South Asian, Chinese and Black African origin, and BMI does not apply to children, who are measured against age-and-sex growth charts instead.
Key features
- Instant calculation in both metric and imperial units.
- Shows the category alongside the number, so the figure means something.
- Runs in your browser - your height and weight are never transmitted or stored.
- No account, and nothing tracked between visits.
- A reasonable starting point for a conversation with a doctor.
Common uses
- A general check of whether your weight sits in the usual healthy range.
- Tracking a change over time during a weight loss or gain programme.
- Filling in a health questionnaire or insurance form that asks for BMI.
- Understanding a BMI figure a doctor or app has quoted.
- Comparing metric and imperial figures when a form specifies one.
Tips for better results
- Measure height accurately. Because it is squared, a two-centimetre error shifts BMI more than a kilogram of weight does.
- Weigh yourself at a consistent time - first thing in the morning is the usual convention, since weight fluctuates through the day.
- If you carry significant muscle, expect BMI to overstate your category. It cannot tell muscle from fat.
- Waist circumference is often the more useful number, since abdominal fat carries more risk than the same weight elsewhere.
- For a metabolic figure rather than a weight category, the BMR Calculator estimates daily calorie needs.
Mistakes to avoid
- Treating BMI as a diagnosis. It is a population screening tool, not an individual medical assessment.
- Applying adult BMI ranges to children and teenagers, who need age and sex-specific percentile charts instead.
- Ignoring build and ethnicity - risk thresholds differ across populations, and several health bodies use lower cut-offs for South Asian groups.
- Mixing units, such as entering height in centimetres while the calculator expects metres.
- Chasing a BMI number rather than health markers like fitness, blood pressure and blood sugar, which matter more.
Available options
- Height
- Weight
- BMI category
Frequently asked questions
No - BMI doesn't distinguish between muscle and fat mass, so very muscular individuals can show a "high" BMI despite low body fat, and it doesn't account for body composition, age, or sex differences in detail. It's a useful general screening tool, not a complete health assessment.
Generally: under 18.5 is underweight, 18.5-24.9 is healthy weight, 25-29.9 is overweight, and 30+ is obese - though these ranges are general guidelines, not individual medical diagnoses.
No. BMI is a screening indicator, not a diagnosis, and it cannot tell muscle from fat or account for where weight is carried. Use it as one input alongside waist measurement, fitness and blood markers, and discuss anything concerning with a doctor who can assess your individual situation.
Not with adult ranges. Children and teenagers are assessed against age and sex-specific percentile charts, because normal body composition changes substantially during growth. A doctor or paediatric chart is the right reference.
Because the formula only knows height and weight, and muscle is denser than fat. Athletes and people who lift regularly frequently register as overweight by BMI while carrying low body fat - this is the best-known limitation of the measure.
No. The calculation runs in your browser and nothing is transmitted or saved between visits.
For most adults the WHO classes 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity. Some health bodies use lower cut-offs for people of South Asian, Chinese and other Asian backgrounds, because health risks rise at a lower BMI.
Waist circumference is the most useful addition, because fat around the middle carries more health risk. A common guide is keeping your waist below half your height. Body fat percentage gives a further view.
It depends on height: for someone 1.75 m tall, 5 kg is about 1.6 BMI points; for someone 1.60 m tall it is nearly 2 points. Enter your target weight to see the exact figure.