BMI Calculator (Body Mass Index)
Calculate your BMI using metric or imperial units, see your weight category, and understand what your result means. Includes adult and child BMI interpretation.
BMI Calculator
Age helps interpret BMI, especially for children and teens.
Your BMI result
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kg/m²BMI is a screening tool only. It does not directly measure body fat or health.
For your height, a healthy BMI (18.5–24.9) corresponds to a weight range of —.
For children and teens, BMI is interpreted using age- and sex-specific percentiles.
What is BMI?
BMI (Body Mass Index) is a simple number that relates your weight to your height. It is widely used as a screening tool to classify underweight, healthy weight, overweight, and obesity in adults.
BMI does not directly measure body fat, but it correlates reasonably well with more direct measures in most people and is easy to calculate.
How to calculate BMI
Metric formula
\(\text{BMI} = \dfrac{\text{weight (kg)}}{\left[\text{height (m)}\right]^2}\)
Imperial formula
\(\text{BMI} = \dfrac{\text{weight (lb)} \times 703}{\left[\text{height (in)}\right]^2}\)
Example (metric):
- Weight = 70 kg
- Height = 175 cm = 1.75 m
- \(\text{BMI} = 70 / 1.75^2 \approx 22.9\)
BMI categories for adults
For adults aged 18 and over, the World Health Organization (WHO) defines these BMI categories:
| BMI | Category | Possible health implications |
|---|---|---|
| < 18.5 | Underweight | Higher risk of nutritional deficiencies, weakened immunity, and osteoporosis. |
| 18.5 – 24.9 | Healthy weight | Lowest overall risk for many weight-related conditions for most people. |
| 25.0 – 29.9 | Overweight | Increased risk of heart disease, type 2 diabetes, and some cancers. |
| 30.0 – 34.9 | Obesity class I | Moderately increased risk of weight-related health problems. |
| 35.0 – 39.9 | Obesity class II | High risk of serious health conditions. |
| ≥ 40.0 | Obesity class III | Very high risk; medical evaluation and support strongly recommended. |
BMI for children and teens
For children and adolescents (typically ages 2–17), BMI is interpreted differently. Instead of fixed cut-offs, BMI is compared with a reference population of the same age and sex to calculate a BMI percentile.
Typical categories (using percentiles) are:
- Underweight: < 5th percentile
- Healthy weight: 5th to < 85th percentile
- Overweight: 85th to < 95th percentile
- Obesity: ≥ 95th percentile
This calculator does not compute official percentiles, but entering age and sex helps you and your clinician interpret the BMI result in context.
Limitations of BMI
BMI is useful at the population level, but it has important limitations for individuals:
- It does not distinguish between fat mass and muscle mass.
- It does not show where fat is distributed (e.g., around the waist vs. hips).
- Cut-offs may not be equally appropriate for all ethnic groups.
- It is less accurate for very muscular people, older adults with low muscle mass, and pregnant people.
For a more complete picture of health, BMI should be considered alongside waist circumference, blood pressure, blood lipids, blood sugar, lifestyle, and medical history.
How to improve your BMI safely
If your BMI is outside the healthy range, small, sustainable changes are more effective than extreme diets:
- Focus on whole foods: vegetables, fruits, whole grains, lean proteins, and healthy fats.
- Limit sugary drinks, ultra-processed snacks, and large portion sizes.
- Build regular physical activity into your week (e.g., brisk walking, cycling, strength training).
- Prioritize sleep and stress management, which influence appetite and weight regulation.
- Work with a healthcare professional or registered dietitian for personalized guidance.
Never start a drastic diet, intense exercise program, or weight-loss medication without discussing it with a qualified professional, especially if you have existing health conditions.
Medical disclaimer
This BMI calculator is for general information and education only. It does not provide a medical diagnosis and is not a substitute for professional medical advice, diagnosis, or treatment.
Always consult your doctor or another qualified health provider with any questions you have about your weight, BMI, or health before making major changes to your diet, exercise, or medications.
BMI Calculator – Frequently Asked Questions
What is BMI and why does it matter?
BMI (Body Mass Index) is a quick way to relate your weight to your height. It helps identify whether you may be underweight, a healthy weight, overweight, or obese. While it does not measure body fat directly, it is correlated with health risks such as heart disease, type 2 diabetes, and some cancers.
What is a healthy BMI range for adults?
For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight. Below 18.5 is underweight, 25.0–29.9 is overweight, and 30 or higher is classified as obesity. However, these ranges are only one part of assessing health and may not apply equally to every ethnic group or individual.
Is BMI accurate for athletes or very muscular people?
BMI can overestimate body fat in people with high muscle mass, such as athletes or bodybuilders, because muscle is denser than fat. In these cases, BMI may label someone as overweight or obese even when their body fat is low. Additional measures like body fat percentage and waist circumference are more informative.
How is BMI different for children and teens?
Children and teens are still growing, so their BMI is interpreted using age- and sex-specific percentiles rather than fixed cut-offs. A pediatric BMI chart shows how a child compares with others of the same age and sex. Your child’s doctor can use BMI percentiles along with growth patterns and overall health to assess weight status.
Can BMI be used during pregnancy?
BMI is useful before pregnancy to classify pre-pregnancy weight status, but it is not used to monitor weight during pregnancy. Recommended weight gain in pregnancy depends on your pre-pregnancy BMI and other factors. Always discuss weight and nutrition in pregnancy with your obstetric provider or midwife.