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BMI explained: how it is calculated and what it does not tell you

Last updated 10 October 2026 · Written by Souren Das

This guide is general information, not medical advice. BMI is a screening number, not a diagnosis. If you are worried about your weight or health, or you are pregnant, a child, an older adult, or living with a medical condition, talk to a doctor or a registered dietitian.

Body mass index (BMI) is the number most health forms, insurance questionnaires, and fitness apps ask for. It is quick to calculate and useful for spotting trends in large groups of people. It is also widely misunderstood when applied to one person. This guide explains how BMI is calculated, what the categories mean, why the cut-offs used in India are lower, what BMI cannot tell you, and how to work it out with Health Calculator.

How BMI is calculated

BMI is your weight in kilograms divided by your height in metres, squared.

BMI = weight (kg) ÷ height (m)²

For example, a person who weighs 68 kg and is 1.70 m tall: 1.70 × 1.70 = 2.89, and 68 ÷ 2.89 = 23.5. Their BMI is 23.5 kg/m².

In pounds and inches, the formula is 703 × weight (lb) ÷ height (in)². The result is the same number.

The WHO adult categories

The World Health Organization uses these bands for adults aged 20 and over:

Below 18.5: underweight
18.5 to 24.9: healthy range
25.0 to 29.9: overweight
30.0 and above: obesity

These are the bands Health Calculator shows on its BMI card, labelled as WHO adult bands.

Why the cut-offs are lower for Indian adults

Research in South Asian populations has found that health risks linked to excess weight, such as type 2 diabetes and heart disease, tend to appear at lower BMI values than in European populations, partly because of a tendency to carry more fat around the abdomen at the same weight. A 2004 WHO expert consultation on Asian populations suggested additional action points at 23 and 27.5.

India's 2009 consensus guidelines on obesity use lower bands for Indian adults: normal from 18.0 to 22.9, overweight from 23.0 to 24.9, and obesity from 25.0. So the 68 kg, 1.70 m person above, at 23.5, is in the healthy range by WHO bands but in the overweight band by the Indian guidelines. Health Calculator mentions these lower cut-offs on the BMI card. Your doctor will know which bands apply to you.

What BMI does not tell you

Muscle versus fat. BMI uses total weight. A muscular athlete can have a BMI in the overweight range with low body fat. Someone with little muscle can have a "healthy" BMI and still carry a lot of fat.
Where fat sits. Fat around the waist carries more risk than fat on the hips. Waist measurement adds information BMI lacks. The Indian guidelines suggest attention when waist circumference is above 90 cm for men or 80 cm for women.
Age and sex. Older adults tend to have more fat at the same BMI. Women typically have more body fat than men at the same BMI.
Children and teenagers. Adult bands do not apply. Children are assessed with BMI-for-age percentile charts.
Pregnancy. BMI is not meaningful during pregnancy. Pre-pregnancy BMI is sometimes used to guide weight gain, by a doctor.
Overall health. Blood pressure, blood sugar, cholesterol, fitness, sleep, and diet matter at any weight.

Calculate it with Health Calculator

Open Health Calculator.
Choose Metric (kg/cm) or Imperial (lbs/ft).
Enter your weight and height. In imperial, height is entered in feet and inches.
Read the Body Mass Index (BMI) card. It shows the number in kg/m² and the WHO adult band. If height or weight is empty or zero, the card shows 0.0 with the prompt "Enter height and weight" instead of a category.
The other cards estimate your basal metabolic rate and daily energy needs with the Mifflin-St Jeor equation, using age, sex, and activity level. Those need all the inputs; BMI needs only height and weight.
Press Export estimate PDF if you want a one-page summary of the inputs and results to keep or take to an appointment.

Measuring accurately

Weigh yourself in the morning, after using the toilet and before eating, in light clothes.
Measure height without shoes, standing straight against a wall, heels together.
A 2 cm error in height changes BMI by about half a point for an average adult, so measure carefully rather than guessing.
Home scales vary. Use the same scale each time if you are tracking change.

Common misunderstandings

"My BMI says overweight, so I am unhealthy." Not necessarily. It is a prompt to look at other measures, not a verdict.
"My BMI is normal, so I do not need to worry." Waist size, activity, diet, and blood tests still matter.
Chasing a precise number. Day-to-day weight changes of a kilogram or more from water and food are normal. Look at trends over weeks.
Using adult bands for a teenager. Use a paediatric chart, or ask a doctor.

Privacy

Health Calculator runs in your browser tab. Your height, weight, age, and the results are not sent to FileTools Kit and are not stored. The PDF export is created on your device.

FAQ

What is a healthy BMI for Indian adults?

Under India's 2009 guidelines, 18.0 to 22.9 is the normal range. Ask your doctor what applies to you.

Is BMI different for men and women?

The formula and adult bands are the same. Body fat at a given BMI tends to differ.

How often should I check my BMI?

If you are tracking weight, monthly is plenty. Daily changes are mostly water.

Can this calculator tell me my body fat percentage?

No. That needs other methods, such as skinfold measurements or body composition scans.

Written by Souren Das, FileTools Kit, Bengaluru. Last updated 10 October 2026. Spotted a mistake or a step that does not match the tool? Email support@filetoolskit.com and I will fix it. See how tools and guides are tested.