What is BMI?
Body Mass Index (BMI) is a simple number that relates your weight to your height. It is one of the most widely used screening tools in the world for estimating whether an adult's weight sits in a range associated with good health. Doctors, insurers, and public-health bodies such as the World Health Organization use it because it is quick to calculate and needs only two measurements.
How is BMI calculated?
The formula divides your weight by the square of your height:
For example, a person who is 170 cm tall and weighs 65 kg has a BMI of 65 ÷ (1.70 × 1.70) = 22.5, which falls in the healthy range.
BMI categories for adults
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These cut-offs apply to adults aged 20 and over. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts instead of fixed ranges.
What are the limitations of BMI?
BMI does not distinguish between muscle and fat. A muscular athlete can register as "overweight" while carrying very little body fat, and an older adult can have a "healthy" BMI while carrying more fat than the number suggests. It also does not show where fat is stored — abdominal fat carries different health implications than fat stored elsewhere. That is why BMI works best as a starting point, alongside measures like waist circumference, body-fat percentage, and advice from a healthcare professional.
Why BMI matters for health
A BMI well above the healthy range is associated with an increased risk of several conditions, which is why doctors use it as a first-line screen:
- High blood pressure and heart disease
- Type 2 diabetes and insulin resistance
- High cholesterol and stroke
- Certain cancers, arthritis, and breathing problems
Risk rises further if you also smoke, are sedentary, have a family history of heart disease, or already have high blood pressure or abnormal blood lipids. Even a modest 5% weight reduction measurably improves most of these risk factors.
Measuring yourself accurately
- Height: stand against a wall without shoes, heels and back touching the wall, look straight ahead, and have someone mark the top of your head with a flat object.
- Weight: weigh first thing in the morning, after using the bathroom and before eating, in minimal clothing.
- Consistency beats precision: weight fluctuates day to day (especially across the menstrual cycle) — weigh 2–3 mornings and average them.
What should I do with my result?
If your BMI falls outside the healthy range, treat it as a prompt to look deeper rather than a verdict. Small, sustainable changes to eating patterns and daily activity tend to work better than drastic ones, and a doctor or registered dietitian can help you interpret your number in the context of your age, build, and health history. You can also explore our calorie calculator and TDEE calculator to understand your daily energy needs.
Practical guide for using this result
How to read the number
The category is a broad adult screening range, not a grade. A stable or improving waist measurement, strength and daily function can matter as much as a small BMI change.
A practical four-week plan
Choose one meal pattern, one movement habit and one measurement routine. Review the trend after several weeks instead of reacting to a single weigh-in.
When another measure is better
Use waist circumference, body-fat estimates, blood pressure, glucose and lipid results when risk assessment needs more detail.
BMI with body fat: reading both numbers together
The Google Search Console phrases “BMI fat,” “BMI with body fat” and “BMI vs body fat” describe a real source of confusion: BMI and body-fat percentage answer different questions.
BMI is a size-for-height screen
BMI compares weight with height. It is useful for broad adult screening, but it cannot identify whether weight comes from muscle, fat, bone or fluid.
Body fat estimates composition
A body-fat estimate tries to describe the share of weight that is fat. Tape, scale and skinfold methods all have error, so trends under consistent conditions are more useful than one exact-looking value.
Use context, not one cutoff
Waist size, blood pressure, fitness, blood tests, age, ethnicity, pregnancy and muscularity can change the meaning of a BMI result. Use the number as a starting point for discussion, not a diagnosis.
Sources & references
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.
- CDC. About Adult BMI. Centers for Disease Control and Prevention, Division of Nutrition, Physical Activity, and Obesity.