BMI Calculator
Calculate your Body Mass Index from metric or imperial measurements, with the WHO adult category and the healthy weight range for your height.
Example values are shown — change any of them and the answer updates live.
BMI is a screening measure for informational purposes — it is not medical advice and not a diagnosis. For health questions, consult a qualified professional.
What is BMI?
Body Mass Index (BMI) measures weight relative to height. It is a widely used screening indicator of whether a person's weight is in a healthy range for their height. The resulting value is classified as underweight, healthy weight, overweight, or obesity. The cutoffs below are the World Health Organization (WHO) ranges for adults. BMI is a screening measure, not a diagnosis — it does not distinguish fat from muscle or bone, so it should be used together with other measurements.
Adult BMI classification (WHO)
The World Health Organization's classification for adults 20 and older — the same ranges apply to men and women.
| Classification | BMI range (kg/m²) |
|---|---|
| Severe thinness | < 16 |
| Moderate thinness | 16 – <17 |
| Mild thinness | 17 – <18.5 |
| Normal | 18.5 – <25 |
| Overweight | 25 – <30 |
| Obesity class I | 30 – <35 |
| Obesity class II | 35 – <40 |
| Obesity class III | ≥ 40 |
BMI Prime
BMI Prime is the ratio of your BMI to 25, the upper limit of the normal range. Values below 1 mean you are within or below the normal range; values above 1 mean you are above it — the number tells you directly how far from the upper limit you are.
BMI Prime = BMI ÷ 25
BMI Prime classification:
| Classification | BMI | BMI Prime |
|---|---|---|
| Severe thinness | < 16 | < 0.64 |
| Moderate thinness | 16 – <17 | 0.64 – <0.68 |
| Mild thinness | 17 – <18.5 | 0.68 – <0.74 |
| Normal | 18.5 – <25 | 0.74 – <1 |
| Overweight | 25 – <30 | 1 – <1.2 |
| Obesity class I | 30 – <35 | 1.2 – <1.4 |
| Obesity class II | 35 – <40 | 1.4 – <1.6 |
| Obesity class III | ≥ 40 | ≥ 1.6 |
Ponderal Index
The Ponderal Index is similar to BMI but divides by the cube of height instead of the square. It is more reliable than BMI for very tall or very short people, for whom BMI tends to understate or overstate body fat.
Ponderal Index = mass (kg) ÷ height³ (m)
Children and teens (ages 2–19)
For children and teens, BMI is interpreted using age- and sex-specific growth charts, not the adult cutoffs. The US Centers for Disease Control and Prevention (CDC) defines the categories below using BMI-for-age percentiles. This calculator is for adults (20 and older); for ages 2–19, use the CDC child and teen BMI calculator.
| Category | Percentile range |
|---|---|
| Underweight | < 5% |
| Healthy weight | 5% – <85% |
| At risk of overweight | 85% – <95% |
| Overweight | ≥ 95% |
Health risks associated with overweight
According to the CDC, being overweight increases the risk of:
- High blood pressure
- High LDL ("bad") cholesterol, low HDL ("good") cholesterol and high triglycerides
- Type 2 diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis (joint cartilage breakdown)
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Low quality of life, body pain and difficulty with physical functions
- An overall increased risk of mortality compared with a healthy BMI
Keeping BMI below 25 is a widely used goal, but any lifestyle change should be discussed with a doctor.
Health risks associated with underweight
- Malnutrition, vitamin deficiencies and anemia
- Osteoporosis (bone weakness and higher fracture risk)
- Decreased immune function
- Growth and development issues, especially in children and teenagers
- Possible reproductive issues in women from hormonal imbalances
- A higher chance of miscarriage in the first trimester
- Potential complications after surgery
- An overall increased risk of mortality compared with a healthy BMI
Underweight can also be a sign of an underlying condition. If the cause is not obvious, consult a health professional.
Limitations of BMI
BMI is a screening measure, not a diagnosis: it cannot see body composition. Because body types and the distribution of muscle, bone and fat vary widely, BMI should be considered together with other measurements rather than used alone.
According to the CDC:
- Older adults tend to carry more body fat than younger adults with the same BMI.
- Women tend to carry more body fat than men at the same BMI.
- Athletes and muscular people can show a high BMI from muscle mass, not excess fat.
For children and adolescents the same limits apply, plus growth and pubertal stage. BMI is a better indicator of excess fat in children with obesity than in children who are only overweight, where the extra weight may be fat or fat-free mass.
Even so, BMI reflects body fat reasonably well for 90–95% of the population and is a useful screening tool alongside other measures.
How BMI is calculated
BMI = 703 × mass (lb) ÷ height² (in)
BMI = mass (kg) ÷ height² (m)
Worked example: a 5'10" (1.78 m) person weighing 160 lb (72.6 kg) has BMI = 72.6 ÷ 1.78² = 23.0 — in the normal range.
Sources
Adult classification: World Health Organization. Children and teens percentiles and the health-risk lists: US Centers for Disease Control and Prevention (CDC).
Informational screening only — not a medical diagnosis.