Calculation Summary
| Metric | Result |
|---|---|
| BMI | — |
| Category | — |
| Healthy weight range | — |
| BMI Prime (BMI/25) | — |
Body Mass Index (BMI) Calculator
Use this calculator to estimate your Body Mass Index (BMI) and match it to a weight category. Where relevant, guidance accounts for age. Switch units anytime: choose “Metric Units” or “Other Units” to convert between U.S. customary and metric.
How this Body Mass Index (BMI) Calculator works
BMI estimates body fatness from height and weight and is widely used for adults (≈20+). The calculator accepts both US and Metric units and converts as needed. Your age and gender are shown for context, but adult BMI math itself uses only height and weight. For children/teens, dedicated pediatric BMI percentiles are recommended.
Formulas
- Metric:
BMI = weight(kg) / height(m)2 - US:
BMI = 703 x weight(lb) / height(in)2
BMI overview
Body mass index (BMI) is a simple height-to-weight ratio that estimates overall tissue mass and offers a broad snapshot of body size. It's widely used as a screening tool to judge whether someone's weight is appropriate for their height.
A calculated BMI falls into labeled ranges—underweight, healthy weight, overweight, or obesity—and some systems add finer classes (e.g., severely underweight, class III obesity). Cutoffs and naming can vary by region, organization, and age group.
Because both low and high body weight are linked to important health risks, BMI—though imperfect and not a direct measure of body fat—is useful for signaling when further evaluation or action may be needed. See the table below for the category ranges used by this calculator.
BMI table for adults (WHO/CDC style)
| Category | BMI range - kg/m2 |
|---|---|
| Underweight | < 18.5 |
| Normal | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obesity class I | 30.0 - 34.9 |
| Obesity class II | 35.0 - 39.9 |
| Obesity class III | ≥ 40.0 |
Examples
- Metric: 80 kg and 1.78 m → BMI = 80 / 1.78² ≈ 25.2 (overweight).
- US: 176 lb and 5'10″ (70 in) → BMI = 703x176 / 70² ≈ 25.3 (overweight).
BMI is a screening tool, not a diagnosis. Athletic builds, seniors, and people with atypical body composition may need other measures (e.g., waist-to-height ratio, body fat %).
Children & Teens (Ages 2-20)
For ages 2-20, the CDC (Centers for Disease Control and Prevention) recommends interpreting BMI by age-specific percentiles rather than adult cutoffs.
Categories (by percentile)
- Underweight: below the 5th percentile
- Healthy weight: 5th-85th percentile
- At risk for overweight: 85th-95th percentile
- Overweight: above the 95th percentile
Growth charts: use the official BMI-for-age curves for boys and girls (2-20 years) from the CDC and complementary references from the WHO.
Health Risks When BMI Is High
Excess body weight is associated with a higher likelihood of:
- High blood pressure
- Unfavorable lipids: elevated LDL, reduced HDL, high triglycerides
- Type 2 diabetes
- Coronary heart disease and stroke
- Gallbladder problems
- Osteoarthritis (degenerative joint disease)
- Sleep apnea and other breathing issues
- Certain cancers (e.g., endometrial, breast, colon, kidney, gallbladder, liver)
- Reduced quality of life
- Depression, anxiety, and other mental-health concerns
- Pain and limits in physical function
- Higher overall risk of premature death than in people with a healthy BMI
Health Risks When BMI Is Low
Being underweight can also carry health costs, including:
- Malnutrition, vitamin/mineral deficiencies, anemia
- Osteoporosis (frail bones, higher fracture risk)
- Lowered immunity
- Growth/development concerns in children and teens
- Potential reproductive issues in women due to hormonal disruption (including higher chance of first-trimester miscarriage)
- Higher surgical complication rates
- In general, increased mortality risk vs. healthy-BMI peers
Unexplained low weight can signal an underlying medical or mental-health issue (e.g., anorexia nervosa). Seek medical advice if the cause isn't obvious.
Why BMI Has Limits
BMI is a screening tool, not a direct body-fat measurement. It cannot separate fat, muscle, and bone, and should be read alongside other information (waist size, body-fat estimates, clinical evaluation). Body shapes and fat distribution vary widely.
Adults — nuances
- BMI reflects mass relative to height, not % body fat.
- Age, sex, ethnicity, muscle mass, activity change what a given BMI means.
- Two people with the same BMI can have very different body-fat levels and risks.
- Athletes and very muscular people may score “overweight” by BMI yet be metabolically healthy.
See overviews from the CDC (Adult BMI) and WHO (Obesity & Overweight).
Children & adolescents
The same caveats apply to youth. Height and puberty/maturation also influence BMI and fatness. BMI generally identifies excess adiposity better in children with obesity than in those who are merely overweight, where a higher value may reflect more fat-free mass (muscle, organs, water, etc.). In lean children, BMI differences can likewise be driven by fat-free mass.
Bottom Line
For roughly 90-95% of people, BMI correlates reasonably well with body fatness and works as a first-pass screen. Use it with other measures and professional guidance to set a healthy, individualized target.