BMI Calculator — Body Mass Index for Men, Women & Children

Body Mass Index (BMI) is the most widely used screening tool for weight category and is published by the CDC, WHO, and NIH. This BMI calculator uses the official formula and supports both imperial (pounds and inches) and metric (kilograms and centimeters) units. Beyond the BMI number, you'll see your weight category, your healthy weight range for your height, your Prime BMI, and a personalized interpretation that adapts to your lifestyle — whether you're sedentary, active, an athlete, a bodybuilder, an older adult, or pregnant. BMI for children and teens under 20 is interpreted differently using CDC growth charts; the calculator flags this automatically.

What Is BMI?

BMI, or body mass index, is a numerical value derived from a person's height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and adopted as a population-level health screening tool in the late 20th century. The CDC, WHO, and NIH all use BMI to identify whether a person's weight may be associated with elevated health risk. It's important to understand that BMI is a screening tool, not a diagnostic measure — it does not directly measure body fat and does not account for muscle mass, bone density, age, sex, lifestyle and activity level, or activity level. A high or low BMI may or may not indicate a health problem. Always consult a qualified healthcare provider for a complete assessment.

BMI Categories for Adults

The CDC uses six categories to interpret adult BMI. These apply to adults 20 years and older regardless of age or sex:

The BMI Formula

BMI is calculated identically in both unit systems — only the inputs and a conversion constant differ.

Imperial: BMI = (weight in pounds ÷ (height in inches)²) × 703
Metric:   BMI = weight in kg ÷ (height in meters)²

Worked example. A person 5'9" (69 inches) weighing 170 lbs: (170 ÷ 4,761) × 703 = BMI 25.1. The same person at 77 kg and 1.75 m: 77 ÷ 3.0625 = BMI 25.1. Both formulas always produce the same result; the 703 multiplier is purely a unit conversion factor.

BMI by Lifestyle — Does Your Activity Level Change Your Result?

The BMI number doesn't change based on lifestyle, but the interpretation absolutely should. The lifestyle selector above swaps the context card to reflect this.

Sedentary or general population. The standard CDC categories apply. For most adults at desk jobs with light activity, BMI is a reasonable first-pass health screen.

Active — 3 to 5 sessions per week. You likely carry slightly more lean muscle mass than the average sedentary person at the same BMI. Standard categories still apply, but actual health risk may be lower than BMI alone suggests. Body fat percentage is a useful complement.

Competitive athletes. Distance runners, footballers, swimmers, and team-sport athletes routinely show BMI in the overweight range due to muscle mass — without elevated cardiovascular risk. For this group, BMI is a poor screening proxy and body fat percentage is the better metric.

Bodybuilders and strength athletes. This is where BMI breaks down most visibly. A 5'10" competitive bodybuilder at 220 lbs has a BMI of 31.6 — technically Obese Class I — while potentially carrying single-digit body fat. BMI is not an appropriate health measure for this population.

Older adults (55+). Research suggests adults over 65 may have better health outcomes at BMI 25–27 than at the standard 18.5–24.9 range. Age-related muscle loss (sarcopenia) means a normal BMI can mask higher body fat than expected. Healthcare providers often consider a slightly broader acceptable range for older patients.

Pregnancy. Standard BMI categories do not apply during pregnancy. Weight gain recommendations are based on pre-pregnancy BMI and managed by an obstetric provider. Use BMI here only for general reference and consult your OB-GYN or midwife.

BMI for Women

The same BMI formula applies to women as men, and the same healthy range of 18.5–24.9 applies to adult women regardless of age. Women naturally carry a higher percentage of body fat than men at the same BMI — this is normal and healthy. BMI for women does not need to be adjusted by sex; what changes between individuals is the underlying body composition that BMI cannot see. For pregnant women, BMI is not an appropriate measure and prenatal weight is managed by a healthcare provider.

BMI for Men

Men and women use the same BMI formula and the same healthy range of 18.5–24.9. Men typically carry more lean muscle mass than women at the same BMI, which means BMI may slightly overestimate body fat in muscular men. For competitive lifters, athletes, and physically demanding occupations, BMI alone can mislead — pair it with body fat percentage or waist circumference for a fuller picture.

BMI for Children and Teens

For children and teenagers ages 2 to 19, BMI is calculated with the same formula but interpreted very differently. Instead of fixed categories, children's BMI is converted into a percentile against CDC age and sex-specific growth charts. A BMI-for-age below the 5th percentile is underweight; 5th to below 85th is healthy weight; 85th to below 95th is overweight; at or above the 95th percentile is obese. Because growth and body composition change rapidly through childhood and adolescence, the adult BMI categories do not apply. If your child's BMI puts them outside the healthy range, talk to a pediatrician — context matters far more than the number alone.

Limitations of BMI

BMI's biggest weakness is what it can't see. It cannot distinguish muscle from fat. It doesn't account for bone density, fat distribution (visceral vs subcutaneous), age, lifestyle and activity level. A 200-lb athlete with 8% body fat and a 200-lb sedentary individual with 30% body fat have the same BMI and the same height, but very different health profiles. Conversely, a person can have a normal BMI yet still have high visceral fat, poor metabolic markers, and elevated disease risk — sometimes called "metabolically obese normal weight." Treat BMI as a starting point, not a verdict. Healthcare providers combine BMI with waist circumference, blood pressure, blood glucose, lipid panels, and direct body composition measurements for a real assessment.

BMI Chart by Height and Weight

Reference table for common height and weight combinations. Use this to gut-check the calculator output or to look up a BMI without entering anything.

Prime BMI and Ponderal Index

Two alternative metrics worth knowing. Prime BMI is your BMI divided by 24.9 (the upper boundary of the healthy range). A Prime of 1.0 means you're exactly at the upper limit of normal. Below 0.74 is underweight, 1.00–1.20 is overweight, above 1.20 is obese. Ponderal Index uses height cubed instead of squared: weight (kg) ÷ height (m)³. Normal is roughly 11–14 kg/m³. The Ponderal Index scales more reasonably for very short or very tall individuals where standard BMI tends to over- or under-estimate.

Frequently Asked Questions

What is a normal BMI for a woman?

A normal BMI for a woman is between 18.5 and 24.9, the same healthy range used by the CDC and WHO for all adults 20 years and older. Below 18.5 is underweight, 25.0–29.9 is overweight, and 30.0 or above is obese. Women naturally carry more body fat than men at the same BMI — this is normal and healthy.

What is a healthy BMI for men?

A healthy BMI for men is 18.5 to 24.9, identical to the female range. Men typically carry more muscle mass than women at the same BMI, which means BMI can slightly overestimate body fat in muscular men. Pair BMI with body fat percentage or waist circumference for a more complete picture.

How do I calculate my BMI?

Imperial: BMI = (weight in pounds ÷ height in inches²) × 703. Metric: BMI = weight in kg ÷ height in meters². A person 5'9" at 170 lbs has a BMI of 25.1 — same result as 1.75 m at 77 kg. Both formulas always agree. Use the calculator above for instant results.

What BMI is considered obese?

BMI 30.0 or higher is classified as obese. Class I is 30.0–34.9, Class II is 35.0–39.9, and Class III (sometimes called morbid obesity) is 40.0+. Each class is associated with progressively higher risk for cardiovascular disease, type 2 diabetes, hypertension, sleep apnea, and certain cancers.

Is BMI accurate for bodybuilders and strength athletes?

No. BMI cannot distinguish muscle from fat. A 5'10" competitive bodybuilder at 220 lbs has a BMI of 31.6 (Obese Class I) while potentially carrying under 10% body fat. For strength athletes, body fat percentage measured by DEXA, hydrostatic weighing, or calipers is far more meaningful than BMI.

Does BMI account for muscle mass?

No. BMI uses only height and weight — it has no way to separate lean muscle from fat. This is one of its most significant limitations. A muscular athlete and a sedentary person of the same height and weight have the same BMI despite very different body compositions.

What is a healthy BMI for active people and athletes?

The standard 18.5–24.9 range still applies but requires more context. Active people may carry more muscle and have lower actual health risk at a given BMI than sedentary individuals. For competitive athletes, an overweight BMI is common and often expected — body fat percentage is the better metric for this group.

What is a good BMI for people over 55?

Research suggests adults over 65 may have better health outcomes at BMI 25–27 compared to the standard 18.5–24.9 range. Age-related muscle loss can make a normal BMI mask higher body fat than expected. A slightly higher BMI may even be protective during illness or recovery in older adults.

What is a good BMI for my age?

For adults 20+, BMI categories don't change with age — 18.5–24.9 remains the healthy range. For older adults (65+), some research supports a slightly higher acceptable range. For anyone under 20, BMI is interpreted via CDC age and sex-specific percentile charts, not adult categories.

What is the BMI formula?

Two formulas, same result. Imperial: BMI = (weight lbs ÷ height in²) × 703. Metric: BMI = weight kg ÷ height m². Developed by Adolphe Quetelet in the 1830s, used today by the CDC, WHO, and NIH.

What should my BMI be for my height?

The healthy BMI range of 18.5–24.9 applies at every adult height, but the corresponding weight range varies. At 5'0", healthy weight is roughly 91–123 lbs. At 5'6", roughly 115–154 lbs. At 5'10", roughly 129–173 lbs. At 6'0", roughly 140–188 lbs. The calculator shows your specific range after you enter your height.

Is a BMI of 25 overweight?

Technically yes — 25.0 is the threshold for the overweight category. But it's a borderline value, just above the normal range of 18.5–24.9, and carries only a slightly elevated health risk. A BMI of 25 is a screening signal, not a diagnosis.

Can you be healthy with a high BMI?

Yes. BMI is a screening tool, not a diagnosis. Athletes and muscular individuals frequently fall in the overweight or obese BMI range despite excellent cardiovascular health. Conversely, normal BMI does not guarantee good metabolic health. Use BMI alongside waist circumference, blood pressure, blood work, and body fat percentage.

What is BMI for children?

For ages 2–19, BMI is calculated the same way but interpreted using CDC age and sex-specific percentile charts: below the 5th percentile is underweight, 5th–below 85th is healthy weight, 85th–below 95th is overweight, 95th+ is obese. Adult BMI categories do not apply to anyone under 20.

What is the difference between BMI and body fat percentage?

BMI is calculated from height and weight only. Body fat percentage measures the actual proportion of body weight that is fat tissue. BMI is a population-level proxy that can mislead at the individual level. Body fat requires direct measurement (DEXA, hydrostatic weighing, calipers) but gives a far more accurate picture of body composition.

⚠️ Medical disclaimer. BMI is a screening tool, not a diagnostic measure. It does not directly measure body fat and does not account for muscle mass, bone density, fat distribution, age, sex, lifestyle and activity level. A high or low BMI may or may not indicate a health problem. Consult a qualified healthcare provider for a complete assessment and personalized guidance. Sources: Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), National Institutes of Health (NIH).