How the BMI calculator works
Body mass index is a simple ratio of weight to height. It was designed as a quick population-level screening measure, not a personal diagnosis. You enter your weight and height in either metric or imperial units, and the calculator converts them, divides weight in kilograms by height in metres squared, and compares the result with the adult categories published by the World Health Organization.
Beyond the number itself, the calculator shows the weight range that corresponds to a BMI of 18.5 to 24.9 for your height. That range is often more useful than the single value, because it shows how far you are from the band typically associated with the lowest population-level health risk.
BMI has well-known limits. It does not distinguish muscle from fat, so a muscular athlete can be classed as overweight. It does not consider where fat is stored, age, sex, ethnicity, or pregnancy. It is intended for adults and is not suitable for children and teenagers, who are assessed using growth charts. Treat it as one data point among many, such as waist measurement, blood pressure and lab tests.
The formula and categories
BMI = weight (kg) / height (m)2
Imperial: BMI = 703 × weight (lb) / height (in)2
The adult categories are: under 18.5 underweight; 18.5 to 24.9 healthy weight; 25 to 29.9 overweight; 30 and above obesity. Some groups, including people of South Asian descent, may face higher risk at lower BMI values, and local guidelines sometimes use adjusted cut-offs.
Worked example
A person weighs 70 kg and is 175 cm tall. Height in metres is 1.75, and squared it is 3.0625. Dividing 70 by 3.0625 gives a BMI of 22.9, which falls in the healthy weight category. The weight range for BMI 18.5 to 24.9 at this height is 56.7 kg to 76.3 kg.
If the same person gained 10 kg, the BMI would rise to about 26.1, which is in the overweight category. That shows how sensitive the number is to small changes at a given height.
Using your result sensibly
- Look at trends. A change over months is more informative than a single reading.
- Combine measures. Waist circumference, fitness level, diet and family history all matter.
- Do not self-diagnose. A result outside the healthy range is a reason to talk to a doctor or dietitian, not a verdict.
- Measure consistently. Weigh yourself at the same time of day and measure height without shoes.
- Focus on habits. Regular activity, enough sleep and a balanced diet are better goals than hitting a number.
Frequently asked questions
Is BMI accurate for athletes or very muscular people?
Often not. Muscle is denser than fat, so a muscular person can have a high BMI without excess body fat. For athletes, measures such as body fat percentage or waist-to-hip ratio can be more informative.
Does this calculator work for children?
No. It uses adult categories. Children and teenagers are assessed with BMI-for-age percentiles on growth charts, which a paediatrician can provide.
What is a healthy BMI?
The WHO defines 18.5 to 24.9 as the healthy weight range for adults. Individual health depends on many factors, and some populations have different recommended cut-offs.
Why does the calculator give a weight range?
Because there is no single correct weight for a height. The range shows the weights that fall inside BMI 18.5 to 24.9, giving a realistic target band instead of one number.
How do I convert pounds and inches?
Choose imperial units and enter pounds, feet and inches. The calculator converts to kilograms and centimetres automatically, using 1 lb = 0.45359237 kg and 1 in = 2.54 cm.