BMI Calculator

Body Mass Index (BMI) is a person's weight divided by the square of their height, used as a screening estimate of whether that weight falls in an underweight, healthy, overweight, or obese range. For adults, age 20 and over, this calculator returns the BMI value and its adult category. For children and teens, ages 2 to 19, it also converts BMI into a percentile against age- and sex-specific growth references, since a fixed adult cutoff does not fit a body that is still growing.

US or Metric units switch which height and weight fields are used; values convert automatically when units are changed. Age can be entered as exact years and months, or derived from a date of birth and a measurement date, which matters for children and teens since the percentile calculation is age-specific down to the month. Sex is required to compute a percentile for ages 2 to 19, since the reference parameters differ between boys and girls. For a quick adult calculation, age can be entered as the shorthand "20+".

Adult BMI applies to ages 20 and older under the standard US or Metric formula. Child and teen BMI, for ages 2 to 19, additionally estimates a percentile using age- and sex-specific parameters, and assigns a pediatric weight category from that percentile rather than the adult cutoffs. Inputs and settings are retained in the browser between visits, so returning to the calculator does not require re-entering values.

Ages for children and teens are read as exact years and months, or from a date of birth, for greater precision than a whole-year age would give. An adult age can be shortened to "20+" when only the adult calculation is needed. Height and weight accept whole numbers or decimals, and large numbers are formatted for readability. Switching units at any time recalculates the same person's values in both US and Metric measurements.

The BMI value itself is the ratio of weight to height described above. The category shows whether that BMI falls in the underweight, healthy, overweight, or obese range for adults, or in a percentile category for children and teens. The percentile compares a child's BMI with peers of the same age and sex, rather than against a fixed adult cutoff.

Adult example: A person 5 feet 6 inches (66 inches) tall weighing 150 pounds, entered at age 20+, gives BMI = (150 / 66²) × 703 ≈ 24.2, which falls in the healthy range of 18.5 to 24.9.

Child example: A 10-year-old entered at 140 cm and 38 kg gives BMI = 38 / 1.40² ≈ 19.4 kg/m²; converting that figure to a percentile additionally requires the age- and sex-specific parameters described under Explanations below.

Body Mass Index (BMI) is a screening tool used to estimate body fat based on a person's height and weight. It helps identify potential weight categories such as underweight, healthy weight, overweight, or obese. This calculator provides BMI values and categories for adults aged 20 and older, and for children and teens aged 2 to 19, it also generates BMI percentiles and categories based on age and sex. For younger individuals, BMI is plotted on a growth chart to help interpret the results in the context of normal development.

Small differences in rounding of age, height, or weight may cause results to vary slightly between calculators. BMI is only one potential indicator of health and should not be used alone to assess an individual's well-being. Additional factors, such as personal medical history, lifestyle habits, physical examination findings, and laboratory tests, should also be considered. Interpreting BMI in the context of overall health calls for a healthcare provider's input; this tool is for informational purposes only and is not a substitute for professional medical advice or diagnosis.

This BMI calculator supports general health awareness and monitoring across a variety of situations, for both adults and children:

• General health screening: Quickly assess whether an individual is underweight, at a healthy weight, overweight, or obese.
• Pediatric growth tracking: Monitor BMI percentiles in children and teens to evaluate growth patterns over time.
• Fitness planning: Support weight management goals by establishing a baseline BMI and tracking progress.
• Clinical support: Use as a supplemental tool in health consultations to discuss body composition and associated health risks.
• Educational purposes: Teach individuals and families how height, weight, age, and sex impact BMI and health status.

Body Mass Index (BMI) is a widely used tool for assessing weight categories, but it has important limitations that can lead to inaccurate or misleading results. BMI is calculated using only height and weight, so it does not measure body fat directly and cannot distinguish between fat, muscle, bone, or water weight. This can be misleading for individuals with high muscle mass, such as athletes, who may be classified as overweight or obese despite having low levels of body fat.

BMI also ignores where fat is distributed in the body. Fat stored around the abdomen poses a greater health risk than fat in other areas, but BMI cannot capture this detail. It does not account for age, sex, or ethnic differences either: older adults may have a normal BMI but still carry excess body fat due to muscle loss, and some ethnic groups face higher health risks at lower BMI levels, making general BMI cutoffs less accurate for everyone.

• Individuals with high muscle mass or athletic builds
• Older adults with reduced muscle mass
• People from ethnic groups with different health risk thresholds
• Pregnant individuals
• People with medical conditions affecting weight or body composition

BMI can be a useful screening tool, but it should not be used as the only measure of health. Additional methods such as waist circumference, body fat percentage, and clinical evaluation provide a more complete picture of an individual's health status.

For adults (20 years and older):

US Units: BMI = (weight in pounds / height in inches²) × 703

Metric Units: BMI = weight in kilograms / height in meters²

Categories: below 18.5 (Underweight), 18.5 to 24.9 (Healthy), 25 to 29.9 (Overweight), 30 and above (Obese).

Height and weight for an adult, using the "20+" age shorthand, are enough to produce a BMI value and its category.

For children and teens (2 to 19 years):

1. Calculate BMI using the formulas above.
2. Apply the LMS method: Z = [(BMI / M)^L - 1] / (L × S), where L (power), M (median), and S (coefficient of variation) are age- and sex-specific parameters.
3. Convert the Z-score to a percentile using an approximation of the normal distribution.

Categories: below 5% (Underweight), 5% to 85% (Healthy), 85% to 95% (Overweight), 95% and above (Obese).

Age (for example, 10 years), height, weight, and sex together produce a BMI value, a percentile, and a pediatric category.

This tool is intended for informational and educational purposes only. It does not provide medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for any health-related concerns or decisions.