What Does Your BMI Mean? Categories, Limits and What to Do Next
BMI, or Body Mass Index, is a quick number that compares your weight with your height. Doctors use it as a first screening step, and it is easy to work out at home. It is useful, but it is also widely misunderstood, so here is what it does and does not tell you.
How BMI is calculated
BMI = weight in kilograms ÷ (height in metres)2
Take a person who weighs 70 kg and is 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. BMI = 70 ÷ 3.0625 = 22.9.
If you know your height in centimetres, divide by 100 first. If you use pounds and inches, the formula is weight × 703 ÷ height in inches squared.
The standard categories
The World Health Organization groups adult BMI like this:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy range |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
On this scale, the person in our example, with a BMI of 22.9, is in the healthy range.
Cut-offs often used for Indians and other Asian populations
Studies show that many South Asians carry more body fat, and a higher risk of diabetes and heart disease, at a lower BMI than people of European descent. Because of this, WHO expert advice and several Indian guidelines suggest tighter cut-offs for Asian adults, with overweight beginning near 23 and obesity near 25. If you are of Indian origin, treat a BMI of 23 to 25 as a prompt to look at your waist size, diet and activity, not as a sign that all is well.
What BMI cannot tell you
- It cannot separate muscle from fat. A muscular athlete can have a BMI in the overweight range with very little body fat.
- It ignores where fat is stored. Fat around the abdomen carries more risk than fat on the hips. Waist circumference gives extra information.
- It changes meaning with age. Older adults can lose muscle and still have a normal BMI. Children and teenagers need age-specific charts, and a simple adult BMI is not suitable for them.
- It does not account for pregnancy or certain medical conditions.
Finding your healthy weight range
You can reverse the formula. For a height of 1.75 m, the healthy range of 18.5 to 24.9 corresponds to weights from 18.5 × 3.0625 = about 56.7 kg to 24.9 × 3.0625 = about 76.3 kg. Our BMI Calculator works this out for you and supports both metric and imperial units.
What to do with your number
- If you are in the healthy range, the best step is to keep up regular activity and balanced eating.
- If you are above it, small and steady changes matter more than crash diets: more walking, fewer sugary drinks, regular sleep, and smaller portions of fried and processed food.
- If you are underweight, think about whether you are eating enough, and have unexplained weight loss checked by a doctor.
- Measure your waist as well. Many guidelines treat a waist above about 90 cm for men and 80 cm for women (South Asian cut-offs) as a warning sign.
The bottom line
BMI is a screening tool, not a diagnosis. Use it as one data point, alongside your waist size, blood pressure, blood sugar, family history and how you feel. Anyone with a health concern should talk to a doctor or a registered dietitian. This article is for general information and does not replace medical advice.