BMI Calculator
Body mass index with your category and healthy weight range.
Body Mass Index
- Category
- Normal
- Healthy weight range
- 59.9 kg – 80.7 kg
BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat or account for age, sex, or build. Discuss your results with a clinician.
How the BMI calculator works
Body Mass Index is a single number that relates your weight to your height. The formula is your weight in kilograms divided by the square of your height in metres:
BMI = weight(kg) / height(m)^2
The calculator accepts either metric (kilograms and centimetres) or imperial (pounds and inches) input. Imperial values are converted first, using the exact factors 1 lb = 0.45359237 kg and 1 in = 2.54 cm, so both unit systems produce the same result. Height in centimetres is divided by 100 to get metres before squaring.
The result is sorted into the World Health Organization adult categories:
- Underweight below 18.5
- Normal 18.5 to 24.9
- Overweight 25 to 29.9
- Obese 30 and above
The boundaries are inclusive at the bottom of each higher band, so a BMI of exactly 25.0 reads as overweight, not normal. The tool also reports a healthy weight range for your height by inverting the normal band: the lowest healthy weight is 18.5 * height(m)^2 and the highest is 24.9 * height(m)^2.
Worked example
Take someone who is 180 cm tall and weighs 80 kg.
- Height in metres:
180 / 100 = 1.8 - Height squared:
1.8 * 1.8 = 3.24 - BMI:
80 / 3.24 = 24.69
That sits inside the normal band, so the category is Normal. The healthy weight range for 1.8 m is 18.5 * 3.24 = 59.94 kg at the low end and 24.9 * 3.24 = 80.68 kg at the high end, which means 80 kg is near the top of healthy.
If the same person weighed 100 kg instead, the BMI would be 100 / 3.24 = 30.86, which lands in the Obese band.
The same answers come out in imperial. Someone 71 in tall weighing 154 lb converts to about 180.34 cm and 69.85 kg, giving a BMI of 21.48, comfortably Normal.
How to use it
- Pick your unit system, then enter your weight and height.
- Read your BMI, its category, and the healthy weight range for your height.
- Switch units at any time. The calculator reconverts your figures, so you do not need to do the arithmetic yourself.
Limitations
BMI is a screening tool, not a diagnosis. It uses only height and weight, so it cannot separate muscle from fat or tell where fat is stored, and it ignores age, sex, and build. The adult cut points do not apply to children, and they fit athletes, pregnant people, and some ethnic groups poorly. Use the number as one quick signal, pair it with measures like waist circumference, and talk to a clinician before drawing any conclusion about your health.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization treats a BMI from 18.5 up to but not including 25 as the healthy range. Below 18.5 is underweight, 25 to just under 30 is overweight, and 30 or above is obese. These cut points are the same for men and women. They are population guides, not a personal diagnosis, so treat the band as a starting point rather than a target you must hit exactly.
How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres. If you enter pounds and inches, the calculator first converts them to kilograms and centimetres using the exact factors 1 lb = 0.45359237 kg and 1 in = 2.54 cm, then applies the same formula. Working in one consistent unit system is what makes the number comparable across people.
Why does the same BMI mean different things for different people?
BMI only uses height and weight. It cannot tell muscle from fat, so a very muscular person can read as overweight while carrying little fat. It also does not account for age, sex, frame size, or where fat is stored. Two people with the same BMI can have very different health profiles, which is why it is a screening signal, not a verdict.
Is BMI accurate for children or athletes?
No. The adult cut points in this tool do not apply to anyone under 18, who should be assessed with age and sex specific percentile charts instead. Highly trained athletes and pregnant or breastfeeding people are also poorly served by BMI. For these groups, ask a clinician for a more appropriate measure.
What can I use alongside BMI?
Waist circumference and waist to height ratio capture where fat sits, which matters more for cardiovascular and metabolic risk than total weight alone. Body fat percentage, blood pressure, blood glucose, and lipid panels give a fuller picture. BMI is best read as one quick input into that wider assessment.