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Free BMI calculator for body mass index & weight status
Calculate BMI from age, gender, height, and weight — switch cm or ft/in and kg or lb. See your category on a visual gauge, plus healthy weight range, BMI Prime, and Ponderal Index. Private and free in your browser.
Interactive BMI calculator
BMI calculator
Enter your details, then calculate BMI and weight status.
How this BMI calculator works
Enter age, gender, height, and weight — toggle cm or ft/in for height and kg or lb for weight — then press Calculate. The tool computes body mass index, maps it to WHO adult weight categories, shows a healthy weight band for your height, and also reports BMI Prime and Ponderal Index. A semicircle gauge places your result on the underweight–obesity spectrum at a glance.
Pick height and weight units
Switch height between cm and ft/in, and weight between kg and lb — the same unit toggles used on our calorie calculator.
Enter age and gender
Adult WHO cutoffs apply from age 20. For ages 2–19 the calculator still shows an adult-style category for reference and reminds you that pediatric BMI uses age- and sex-specific percentiles.
Press Calculate
You get BMI in kg/m², classification, the 18.5–25 healthy BMI window, the matching weight range for your height, BMI Prime, and Ponderal Index — plus a needle gauge for quick visual context.
Privacy note: all math runs locally in your browser. Nothing is uploaded to Small Tools Buddy servers. This tool is educational, not a diagnosis.
What BMI measures
Body mass index is a simple height–weight ratio used worldwide as a screening signal for underweight, healthy weight, overweight, and obesity. It does not measure body fat directly, but for most adults it correlates well enough to flag when a closer look — waist circumference, labs, or a clinical visit — may be worthwhile.
Categories can shift by region and age. Some Asian guidelines use lower overweight and obesity thresholds. Children and teens need growth-chart percentiles rather than fixed adult bands. Treat BMI as one data point in a broader health picture.
BMI table for adults
The World Health Organization’s adult classification (typically used from age 20) applies the same kg/m² bands for 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 |
| Obese class I | 30 – 35 |
| Obese class II | 35 – 40 |
| Obese class III | > 40 |
BMI for children and teens (ages 2–20)
The CDC recommends BMI-for-age percentiles for ages 2 through 20. Percentiles compare a child with peers of the same age and sex — a fixed adult cutoff can misclassify growing bodies.
| Category | Percentile range |
|---|---|
| Underweight | < 5th |
| Healthy weight | 5th – 85th |
| At risk of overweight | 85th – 95th |
| Overweight | > 95th |
This page’s gauge uses adult WHO bands. For pediatric interpretation, use CDC growth charts or a clinician’s assessment.
Health risks linked with higher BMI
Carrying excess weight raises the odds of several chronic conditions. Public-health sources such as the CDC associate overweight and obesity with risks including:
- High blood pressure
- Unfavorable cholesterol and triglyceride patterns
- Type 2 diabetes
- Coronary heart disease and stroke
- Gallbladder disease
- Osteoarthritis
- Sleep apnea and breathing problems
- Higher risk for several cancers
- Lower quality of life and mood disorders
- Increased overall mortality risk vs. healthy BMI
Many adults aim to stay below 25 kg/m², but the right target depends on your history. Lifestyle changes and medical follow-up belong with a qualified professional — not a web calculator alone.
Health risks linked with low BMI
Being underweight can also harm health. Possible concerns include:
- Malnutrition, vitamin gaps, and anemia
- Osteoporosis and fracture risk
- Weaker immune response
- Growth or developmental issues in youth
- Hormonal and fertility challenges
- Higher surgical complication risk
- Elevated mortality risk vs. healthy BMI
Unexplained weight loss or a persistently low BMI can signal an underlying condition (including eating disorders). Seek medical advice if low weight is unexpected or hard to reverse.
Limitations of BMI
BMI is useful at population scale and as a quick personal screen, but it cannot see muscle, bone, or fat distribution. Two people with the same BMI can have very different body composition and health risk.
In adults
Age, sex, ethnicity, training, and activity all matter. Older adults often have more fat at a given BMI; women typically have more fat than men at the same BMI; muscular athletes may look “overweight” on paper while remaining metabolically healthy.
In children and adolescents
Height velocity and puberty shift the meaning of BMI. Excess BMI tracks fat more reliably in obesity than in mild overweight, where lean mass can also raise the number. Thin children may differ in fat-free mass rather than fat alone.
For roughly 90–95% of people, BMI still offers a practical first pass when combined with other measures and clinical judgment.
BMI formulas
Example for a person who is 5'10" (178 cm) and 160 lb (72.6 kg):
US customary units
BMI = 703 × mass(lb) / height²(in) = 703 × 160 / 70² ≈ 23.0
Metric (SI) units
BMI = mass(kg) / height²(m) = 72.57 / 1.778² ≈ 23.0
BMI Prime explained
BMI Prime compares your BMI to the usual upper normal limit (25 kg/m² in WHO/CDC-style adult charts; some Asian guidelines use a lower ceiling).
BMI Prime = BMI / 25
| 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 – 1.0 |
| Overweight | 25 – 30 | 1.0 – 1.2 |
| Obese class I | 30 – 35 | 1.2 – 1.4 |
| Obese class II | 35 – 40 | 1.4 – 1.6 |
| Obese class III | > 40 | > 1.6 |
Ponderal Index
Like BMI, the Ponderal Index relates mass to stature, but it cubes height. That can reduce the extreme high/low bias BMI sometimes shows for unusually tall or short adults. Metric form used here:
PI = mass(kg) / height³(m)
For the 5'10", 160 lb example above, PI ≈ 12.9 kg/m³. Use it as a companion metric — it still cannot separate fat from muscle on its own.
Why use this free BMI calculator
- BMI with WHO adult classification
- Semicircle gauge with needle position
- cm / ft·in and kg / lb unit toggles
- Healthy BMI and weight-for-height ranges
- BMI Prime and Ponderal Index included
- Copyable result summary
- Teen/child age note for ages under 20
- Private — runs entirely in your browser
Related free tools
After checking BMI, estimate daily energy needs with our calorie calculator (BMR, TDEE, and weight-goal targets).
BMI calculator FAQ
What is a healthy BMI?
For most adults, the World Health Organization treats 18.5–24.9 kg/m² as a normal (healthy) BMI range. Values below 18.5 are underweight; 25–29.9 overweight; and 30 or higher obese, with further classes above 35 and 40. Individual health still depends on muscle mass, age, ethnicity, and medical history.
How do you calculate BMI?
In metric units, BMI = weight (kg) ÷ height² (m). In US customary units, BMI = 703 × weight (lb) ÷ height² (in). This calculator converts your inputs, then uses the metric formula so US and metric results match.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the usual upper limit of the normal adult range). A value under about 0.74 suggests underweight, 0.74–1.0 normal, above 1.0 overweight, and above 1.2 obese. Because it is a ratio, it is dimensionless and easy to compare across groups.
What is the Ponderal Index?
The Ponderal Index (also called the Corpulence Index in metric form) uses height cubed instead of squared: PI = weight (kg) ÷ height³ (m). It can be a steadier leanness estimate than BMI for very tall or very short adults, though it shares many of BMI’s limits around body composition.
Is BMI accurate for athletes or older adults?
Not always. BMI estimates excess weight, not body fat. Highly muscular people may score “overweight” while remaining lean; older adults and many women tend to carry more fat at the same BMI. Use waist size, body-fat measures, fitness, and clinical advice alongside BMI.
Does this BMI calculator store my data?
No. Age, height, weight, and gender are processed only in your browser. Small Tools Buddy does not upload or save your measurements.