How the BMI calculator works
Body mass index (BMI) is a quick screening number that compares your weight with your height. Doctors and health organisations use it to spot whether a person may be underweight, at a healthy weight, overweight or obese. This calculator works in metric units (centimetres and kilograms) or imperial units (feet, inches and pounds), converts everything to metres and kilograms, and places your result on a colour-coded scale.
The BMI formula in plain words
BMI = weight in kilograms ÷ (height in metres × height in metres)
If you use imperial units, height is converted at 2.54 cm per inch and weight at 0.4536 kg per pound before the same formula is applied. The healthy weight range shown is the weight that would give a BMI between 18.5 and 24.9 at your height.
Worked example
A person who is 170 cm tall and weighs 68 kg:
- Height in metres: 1.70, squared = 2.89
- BMI = 68 ÷ 2.89 = 23.5, which is in the normal range
- Healthy weight at this height: about 53.5 kg to 72.0 kg
BMI categories for adults
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25 to 29.9: overweight
- 30 and above: obese
These are the World Health Organization adult categories. Research shows that many South Asian people face higher health risks at lower BMIs, so Indian guidelines often treat 23 to 24.9 as overweight and 25 or more as obese. If you are of Indian origin, keep this lower threshold in mind.
Important limits of BMI
BMI is a screening measure only, not a diagnosis. It does not tell muscle from fat, so athletes may show a high BMI while being very fit, and older adults may have a normal BMI while carrying excess fat. It also says nothing about where fat is stored; waist size is a useful extra check (health risk rises above about 90 cm for Indian men and 80 cm for Indian women). BMI categories are not meant for children, teenagers or pregnant women. Talk to a doctor before making major changes to your diet or exercise.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is considered healthy. Many Indian guidelines suggest staying below 23 because health risks start rising at lower BMIs in South Asian populations.
How is BMI calculated?
Divide your weight in kilograms by the square of your height in metres. For 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 22.9.
Is BMI accurate for everyone?
No. BMI does not separate muscle from fat or show where fat is stored, so it can mislead for athletes, older adults and some ethnic groups. It is a screening tool, not a diagnosis.
Can children use this BMI calculator?
No. Children and teenagers need BMI-for-age percentile charts because their body composition changes as they grow. This calculator is for adults aged 18 and over.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the normal range. A value below 1.0 means you are under that limit; 1.2 means 20% above it.