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BMI Calculator — Calculate Your Body Mass Index

Use this free BMI calculator to find your Body Mass Index — a widely used screening tool that estimates whether your weight is healthy for your height. Enter your age, gender, height, and weight in US or metric units to instantly see your BMI value, weight category, healthy weight range for your height, BMI Prime, and Ponderal Index. Works for adults and children ages 2 through 120.

Modify the values and click Calculate. BMI uses height and weight; adult categories follow WHO-style bands. Children and teens need BMI-for-age percentiles — see below.

Feet and inches plus weight in pounds.

ages: 2 – 120

Your results will appear here after you calculate.

How to use

  1. Choose US, metric, or other units.
  2. Enter age (2–120), gender, height, and weight.
  3. Click Calculate to see BMI, category, gauge, healthy weight range, BMI Prime, and Ponderal Index.
  4. Use results as a screen — not a diagnosis — and discuss concerns with a healthcare provider.

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What is BMI?

Body Mass Index (BMI) is a simple measurement that uses your height and weight to estimate whether you have a healthy body weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and remains one of the most widely used screening tools in healthcare today.

While BMI is not a perfect measure of body fat — it doesn't account for muscle mass, bone density, or fat distribution — it is a useful starting point for identifying potential weight-related health risks. Doctors and health organizations including the CDC and WHO use BMI as an initial screening tool alongside other health measurements.

BMI categories (adults)

Use this table to understand what your BMI result means:

BMI rangeCategory
Below 18.5Underweight
18.5 – 24.9Normal weight
25.0 – 29.9Overweight
30.0 – 34.9Obese (Class I)
35.0 – 39.9Obese (Class II)
40.0 and aboveSeverely obese (Class III)

A healthy BMI for most adults falls between 18.5 and 24.9. If your BMI falls outside this range, consider speaking with a healthcare provider about steps you can take to reach a healthier weight.

WHO reference (extended adult table)

The World Health Organization also uses finer bands for very low or high BMI values (for adults roughly age 20+):

ClassificationBMI range (kg/m²)
Severe thinness< 16
Moderate thinness16 – 17
Mild thinness17 – 18.5
Normal18.5 – 25
Overweight25 – 30
Obese class I30 – 35
Obese class II35 – 40
Obese class III> 40

Children and teens (ages 2–20)

For children and teens, the CDC recommends BMI-for-age percentiles rather than fixed adult cutoffs. This calculator focuses on adults; for youth, use official CDC growth charts with a clinician.

CategoryPercentile range
Underweight< 5th
Healthy weight5th – 85th
At risk of overweight85th – 95th
Overweight> 95th

BMI formula

Imperial (US): BMI = 703 × weight (lbs) ÷ height² (inches). Example: 5'9" (69 in) and 160 lb → 703 × 160 ÷ 69² ≈ 23.6.

Metric: BMI = weight (kg) ÷ height² (meters). Example: 72 kg at 1.75 m → 72 ÷ 1.75² ≈ 23.5.

For daily calorie needs (BMR and TDEE), use the Calorie Calculator.

BMI Prime & Ponderal Index

BMI prime is your BMI divided by the upper end of the “normal” range (25 kg/m²). It is a quick way to see how far you are from that cutoff; your results panel shows it when you calculate.

The Ponderal Index uses height cubed instead of squared (kg/m³ in metric). It can behave differently than BMI for very tall or very short people; both are estimates, not direct body-fat measures.

Limitations of BMI

BMI does not measure body fat directly. Athletes may read “overweight” with low body fat; older adults may have more fat than BMI suggests; pregnancy and some ethnic backgrounds need different interpretations. Pair BMI with waist circumference, activity, diet, and professional advice when possible.

Health risks of being overweight

According to the CDC, excess weight is associated with higher risk of conditions such as:

  • High blood pressure and heart disease
  • Type 2 diabetes
  • Stroke
  • Sleep apnea
  • Some cancers (e.g. breast, colon, kidney)
  • Joint problems including osteoarthritis
  • Depression and anxiety

Health risks of being underweight

  • Malnutrition and vitamin deficiencies
  • Weaker immune function
  • Bone loss (osteoporosis)
  • Hormonal and fertility effects
  • Higher surgical complication risk in some cases

If your BMI is below 18.5 or you have unexplained weight loss, speak with a doctor or dietitian before making big diet changes.

Frequently asked questions

BMI categories, accuracy, children, BMI Prime, and next steps.

What is a healthy BMI?

For most adults, a healthy BMI is between 18.5 and 24.9 according to the World Health Organization. Below 18.5 is underweight; 25 and above is overweight; 30 and above is obese. Healthy ranges can vary with age, sex, ethnicity, and muscle mass — use BMI as a screen, not a final verdict.

How do I calculate my BMI?

In metric units: BMI = weight in kilograms ÷ height in meters squared. In US units: BMI = 703 × weight in pounds ÷ height in inches squared. For example, 5′9″ (69 in) and 170 lb: 703 × 170 ÷ 69² ≈ 25.1. You can also use the calculator above.

What BMI is considered obese?

A BMI of 30.0 or higher is classified as obese (WHO/CDC). Subclasses are often given as Class I (30–34.9), Class II (35–39.9), and Class III or severe obesity (40+). Risk generally rises with class.

Is BMI accurate for muscular people?

Not always. BMI cannot tell muscle from fat. Strength athletes and bodybuilders may fall in the overweight or obese range with low body fat. For them, body fat percentage or clinical measures are more informative than BMI alone.

What is a normal BMI for a woman?

The standard healthy range for adult women is 18.5–24.9, the same numeric band as for men. Women typically carry more body fat than men at the same BMI. Some evidence points to the lower-to-mid 20s as a common sweet spot for women; Asian populations often use lower overweight thresholds — discuss with your clinician.

What is a normal BMI for a man?

For adult men, 18.5–24.9 is the usual healthy range. Active men may be above 25 without excess fat. Pairing BMI with waist circumference (risk often rises above about 40 inches for men) gives a fuller picture.

How accurate is BMI for children?

The BMI formula is the same, but for ages 2–20 interpretation uses sex-specific BMI-for-age percentiles (CDC growth charts), not fixed adult cutoffs. A pediatrician can say whether a child’s trajectory is healthy.

What is BMI Prime?

BMI Prime is your BMI divided by 25 (the usual upper bound of the normal range). It is dimensionless: 1.0 means you are right at the top of normal; 1.2 means about 20% above that upper bound. Below about 0.74 often aligns with underweight categories.

What should I do if my BMI is in the overweight or obese range?

Treat it as a screening flag, not a diagnosis. A healthcare provider can review blood pressure, lipids, glucose, waist size, medications, and lifestyle. Even a 5–10% weight change can improve many risk factors; sustainable diet and activity changes beat crash diets.

How accurate is BMI overall?

For roughly 90–95% of people, BMI is a useful population-level screen. It misses muscle vs. fat, fat distribution, ethnicity-specific risk, and fitness. Use it alongside other measures and professional judgment.

What is a normal BMI for a 50-year-old woman?

The standard 18.5–24.9 range still applies at 50. Some research suggests slightly higher BMI in older adults may correlate with lower mortality in certain groups, but individual context matters — ask your doctor.

How often should I check my BMI?

Every few months is enough for most adults. If you are actively changing weight, you might check monthly along with waist measurement or how clothes fit, not BMI alone.

Who uses this calculator

Adults checking weight status before a checkup, parents tracking growth in context with pediatric guidance, people starting a fitness journey who want a baseline number, athletes comparing BMI to body composition, students learning screening tools, and anyone who wants to understand BMI categories and limits — not a diagnosis on its own.