Body Mass Index (BMI) is the world's most widely used screening measure for weight categories. It takes your height and weight and turns them into a single number that places you in one of four bands: underweight, healthy weight, overweight, or obesity. Use the calculator above for an instant result, then read on to understand what your number actually means — and where BMI falls short.
How BMI is calculated
The formula is simple division. In metric units:
Example: a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
In imperial units:
Example: 154 lbs at 5’9″ (69 in) gives 703 × 154 ÷ (69 × 69) = 22.7.
BMI categories
The standard adult cut-offs, used by the CDC and WHO, are:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
What your result means
Underweight (below 18.5): may indicate undernutrition or an underlying health issue. If your BMI is consistently in this range, speak with a healthcare professional.
Healthy weight (18.5–24.9): associated with the lowest risk of weight-related disease. Maintaining it comes down to balanced eating and regular activity.
Overweight (25–29.9): above the healthy range. Even modest, steady weight loss — 5–10% of body weight — measurably improves blood pressure, cholesterol and blood sugar.
Obesity (30+): linked with higher risk of heart disease, type 2 diabetes and joint problems. A clinician can help build a safe, sustainable plan.
Limitations of BMI
BMI is a screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so athletes and very muscular people can score as "overweight" while being perfectly healthy. It also doesn't account for age, sex, bone density, or fat distribution — two people with the same BMI can have very different body compositions. For children and teens, BMI is interpreted against age-and-sex percentiles, not adult cut-offs. Treat your number as a starting point for a conversation, not a verdict.
Frequently asked questions
What is a good BMI?
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range and is associated with the lowest risk of weight-related health problems.
Is BMI accurate for muscular people?
Not always. Muscle is denser than fat, so muscular individuals can have a high BMI without excess body fat. In that case, waist circumference or body-fat percentage are better guides.
Does BMI work for children?
Children's BMI is calculated the same way but interpreted differently — against percentiles for their age and sex, not the adult cut-offs above.
What is the difference between BMI and body fat percentage?
BMI estimates weight category from height and weight alone. Body fat percentage measures actual fat mass, usually via calipers, scales or DEXA scans, and gives a more precise picture of composition.
Can my BMI change quickly?
BMI moves with your weight, so it can shift within weeks. But healthy, sustainable change is gradual — rapid swings usually reflect water weight, not fat loss.
Should I see a doctor about my BMI?
If your BMI is outside the healthy range, or if you're unsure what it means for you, a healthcare professional can interpret it alongside blood pressure, cholesterol, lifestyle and family history.
References
Formulas and categories follow the CDC's BMI guidance and the WHO fact sheet on overweight and obesity. Background: Body mass index — Wikipedia.