BMI Calculator (2026)

What BMI Measures

Body Mass Index (BMI) is a simple ratio of weight to height, used as a quick screening tool to categorize adults into weight ranges associated with different levels of health risk at a population level. It was designed to be calculated with nothing more than a scale and a measuring tape, which is why it remains the most widely used weight-screening number in medicine, insurance, and public health today.

• BMI = weight ÷ height², adjusted for the unit system used
• Normal/healthy range for most adults: 18.5 to 24.9
• Same categories apply to adult men and women in the standard WHO/CDC scale
• Does not directly measure body fat, muscle mass, or fat distribution
• Children and teens use age- and sex-specific percentiles instead of fixed categories
• Best used as one data point alongside other health measurements, not in isolation

The BMI Formula

Both unit systems produce the identical result - only the units and the constant differ:

Metric: BMI = Weight (kg) ÷ [Height (m)]²
Imperial: BMI = 703 × Weight (lb) ÷ [Height (in)]²

The imperial formula includes the 703 conversion factor so that the result matches the metric calculation exactly, since pounds and inches aren't naturally on the same scale as kilograms and meters.

BMI Categories Chart

These categories are set by the World Health Organization and adopted by the CDC for adult men and women; they are not adjusted for age within adulthood, athletic build, or ethnicity, which is a key part of BMI's known limitations covered below.

Worked Examples

Limitations of BMI

Because of these gaps, most healthcare providers use BMI as a fast first-pass screening number, then follow up with additional context - waist circumference, blood pressure, blood sugar, family history, and overall fitness - before drawing conclusions about an individual's health.

BMI for Children and Teens

For children and teens aged 2 to 19, BMI is calculated with the exact same formula, but the result is interpreted completely differently. Instead of the fixed adult categories, a child's BMI is plotted on a growth chart specific to their age and sex, and the result is expressed as a percentile compared to other children of the same age and sex - for example, a child in the 60th percentile has a higher BMI than 60% of children the same age and sex.

This distinction exists because healthy body composition changes substantially as children grow, so a single fixed cutoff (like the adult 18.5–24.9 range) would misclassify most children at different growth stages. Pediatric BMI-for-age assessment should always be done with a pediatrician, not a generic adult BMI calculator.

Better Together: Waist Size & Body Fat %

• Waist circumference. A simple tape-measure reading that flags higher health risk when it exceeds roughly 40 inches (102 cm) in men or 35 inches (88 cm) in women, since abdominal fat specifically is linked to higher cardiometabolic risk.
• Waist-to-hip ratio. Waist measurement divided by hip measurement, used to assess fat distribution pattern rather than total body size.
• Body fat percentage. Measured via skinfold calipers, bioelectrical impedance scales, or DEXA scans, giving a direct estimate of how much of total body weight is fat tissue versus lean tissue.
• Combining measures. Using BMI alongside waist circumference or body fat percentage gives a far more complete picture than any single number alone, especially for athletic or older individuals where BMI alone is least reliable.

Where BMI Came From

BMI is derived from a formula published in the 1830s by Belgian mathematician Adolphe Quetelet, originally called the Quetelet Index, developed to describe the "average man" for population statistics - not to assess an individual's health. It was renamed "Body Mass Index" and popularized for clinical and epidemiological use in the 1970s, largely because it was simple, cheap, and correlated reasonably well with more expensive body-fat measurements across large population studies, which is exactly the context where it remains most useful today.

Moving to a Healthier Range

• Set a realistic pace. A sustainable rate of weight change (commonly cited as roughly 0.5 to 1 kg, or 1 to 2 lb, per week) is more likely to stick than an aggressive crash approach.
• Focus on consistent habits. Regular physical activity, adequate protein and fiber intake, and consistent sleep tend to move both BMI and overall health markers together.
• Track more than the scale. Waist circumference, energy levels, and strength are useful complements to BMI when monitoring progress.
• Get individualized guidance. A doctor or registered dietitian can set a target range that reflects your specific age, medical history, and body composition, not just a generic chart.

Common Mistakes

• Treating BMI as a direct measurement of body fat. It's a proxy based on height and weight only, not an actual fat measurement.
• Applying adult categories to a child or teen. Pediatric BMI must be read as an age- and sex-specific percentile, not the fixed adult scale.
• Ignoring muscle mass in athletic individuals. A very muscular person can show an "overweight" or "obese" BMI while having low body fat.
• Using BMI alone to judge overall health. Blood pressure, blood sugar, cholesterol, fitness level, and family history all matter and aren't captured by BMI.
• Chasing a single "ideal" number. A healthy weight is a range, not one exact figure, and it varies by individual frame and composition.

Glossary

Body Mass Index (BMI)

A ratio of weight to height squared, used to categorize adults into weight-related risk ranges.

BMI-for-age percentile

The pediatric version of BMI interpretation, comparing a child's BMI to others of the same age and sex on a growth chart.

Waist circumference

A tape-measure reading around the waist, used to flag abdominal fat-related health risk independent of BMI.

Body fat percentage

The proportion of total body weight that is fat tissue, measured directly rather than estimated from height and weight.

Obesity classes

Subdivisions of the obese BMI category (30+) into Class 1, Class 2, and Class 3, reflecting increasing severity.

Frequently Asked Questions

Q: How do you calculate BMI?

A: BMI is calculated by dividing weight in kilograms by height in meters squared: BMI = weight (kg) / height (m)². In imperial units, the formula is BMI = 703 × weight (lb) / height (in)². The result is the same regardless of which unit system is used, since both formulas measure the same ratio of weight to height.

Q: What is a healthy BMI range?

A: For most adults, a BMI between 18.5 and 24.9 is classified as the normal or healthy weight range by the World Health Organization and the CDC. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 or above as obese, with obesity further divided into Class 1, Class 2, and Class 3 based on severity.

Q: Is BMI accurate for everyone?

A: No. BMI does not distinguish between muscle mass and fat mass, so it can overestimate body fat in muscular individuals, such as athletes and bodybuilders, and can underestimate body fat in people with low muscle mass, particularly some older adults. It also does not account for fat distribution, sex, ethnicity-specific risk differences, or frame size, which is why health professionals typically use it as a screening tool alongside other measurements rather than a standalone diagnosis.

Q: Does BMI work the same way for children as adults?

A: No. For children and teens aged 2 to 19, BMI is calculated the same way but is then compared to age- and sex-specific percentile charts, rather than the fixed adult categories, because healthy body composition changes significantly as children grow. A pediatric BMI result is reported as a percentile compared to other children of the same age and sex, not as a fixed under/normal/overweight cutoff.

Q: What is the difference between BMI and body fat percentage?

A: BMI is a simple ratio of weight to height that estimates weight status at a population level, while body fat percentage directly measures the proportion of a person's body that is fat tissue versus lean tissue, typically through methods like skinfold calipers, bioelectrical impedance, or DEXA scans. Two people can have the same BMI but very different body fat percentages depending on their muscle mass.

Q: Why do doctors still use BMI if it has limitations?

A: BMI remains widely used because it is quick, free, requires no special equipment beyond a scale and a tape measure, and correlates reasonably well with health risk at a population level across large groups of people. Most clinicians treat it as a first-pass screening tool, following up with additional measurements such as waist circumference, blood pressure, or body composition testing before making individual health assessments.