What Is BMI and How Is It Calculated? (Formula + Examples)
What BMI actually measures
BMI was developed in the 1830s by Belgian statistician Adolphe Quetelet and became the standard adult weight screening tool in the 1990s. It's popular because it's free, fast, and needs only height and weight. Doctors use it to flag people who may be at higher risk of conditions such as type 2 diabetes, high blood pressure, and heart disease.
What BMI doesn't do is tell muscle from fat, or show where fat is stored. We cover that in is BMI accurate?
The BMI formula
kg = weight in kilograms, m = height in metres; lb = weight in pounds, in = height in inches.
Worked example (metric)
- Height 170 cm → 1.70 m
- Square it: 1.70 × 1.70 = 2.89
- Weight 68 kg ÷ 2.89 = BMI 23.5 → healthy weight ✅
Worked example (imperial)
- Height 5 ft 9 in → 69 inches
- Square it: 69 × 69 = 4,761
- Weight 180 lb: 703 × 180 ÷ 4,761 = BMI 26.6 → overweight range
Skip the maths: calculate your BMI instantly
Metric or imperial, with your category and healthy weight range for your height.
What your BMI number means (adults)
Underweight 18.5–24.9
Healthy 25–29.9
Overweight 30+
Obesity
| BMI | WHO / CDC category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class 1 |
| 35.0 – 39.9 | Obesity class 2 |
| 40.0 and above | Obesity class 3 (severe) |
See what these categories mean for you in what is a healthy BMI range?
Who BMI categories don't apply to
- Children and teens (2–19): use BMI-for-age percentiles, not adult categories. See our Calorie Calculator for Kids.
- Pregnant people: BMI before pregnancy is used to guide healthy weight gain instead.
- Very muscular people and athletes: may show "overweight" while having low body fat.
- Some ethnic groups: people of South Asian, Chinese, and some other Asian backgrounds may have higher health risk at lower BMIs. Some guidelines (for example NICE in the UK) use a lower overweight cut-off of 23.
Frequently asked questions
What is BMI in simple terms?
BMI (Body Mass Index) is a number that compares your weight to your height. It is a quick screening tool used to place adults into weight categories (underweight, healthy weight, overweight, or obesity). It does not measure body fat directly.
What is the formula for BMI?
In metric units, BMI = weight (kg) ÷ height (m)². In imperial units, BMI = 703 × weight (lb) ÷ height (in)². For example, 68 kg at 1.70 m gives a BMI of 23.5.
What is a normal BMI?
For adults, a BMI of 18.5 to 24.9 is classed as a healthy weight by the WHO and CDC. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obesity.
Is BMI the same for men and women?
Yes. Adult BMI uses the same formula and categories for men and women. However, at the same BMI women typically carry more body fat than men, which is one of BMI's known limitations. Children and teens use age- and sex-specific BMI percentiles instead.
Sources
- CDC: Adult BMI Categories
- WHO: Body mass index (BMI) classification
- NIH / NHLBI: Calculate Your Body Mass Index
- NICE: Overweight and obesity management (ethnicity-adjusted BMI thresholds)
- NHS: Obesity (BMI and waist measurement)
This article is for general education and is not medical advice. BMI is a screening tool and does not diagnose health conditions. Talk to a healthcare professional about your individual weight and health. See our medical review policy and disclaimer.