BMI Calculator for Children and Adults

What BMI measures

Body mass index relates weight to height.

The equation is weight in kilograms divided by height in meters squared, producing a value in kg/m². It does not measure fat, muscle, bone, fitness, diet quality, or metabolic health. Its value comes from offering a quick, inexpensive way to identify weight ranges that are associated with health risk across large groups. For one adult, BMI is a screening result. A category can prompt a closer assessment, but it cannot diagnose obesity-related disease, malnutrition, an eating disorder, or good health. Clinicians interpret it with weight history, waist size, blood pressure, laboratory results, medications, physical function, and symptoms. The same BMI can represent different body composition and risk in two people.

Researchers and public health agencies can calculate BMI consistently from height and weight, which permits comparisons over time. That strength does not make the number a complete personal health score. Use the calculator to place a measurement in context, then decide whether other information changes the interpretation.

[1][2]

BMI formulas and worked examples

Metric formula: BMI = weight (kg) ÷ height (m)². A person who weighs 77 kg and is 1.78 m tall has BMI = 77 ÷ (1.78 × 1.78) = 24.3 kg/m².

If height is recorded in centimeters, use BMI = weight (kg) ÷ height (cm)² × 10,000. US customary formula: BMI = weight (lb) ÷ height (in)² × 703. For 170 lb and 70 in, BMI = 170 ÷ (70 × 70) × 703 = 24.4 kg/m². The factor 703 converts pounds and inches into the metric relationship. Keep full precision during the calculation and round the final BMI to one decimal place.

Small input errors can move a result near a category boundary. At 170 lb, entering 69 in instead of 70 in changes BMI from 24.4 to 25.1. Recheck height, weight, units, and age before interpreting an unexpected result. For a child or teen, the calculated BMI is only the first step because age and sex are needed to determine the percentile.

[3]

How to use the BMI calculator

Choose the adult calculation only for someone age 20 or older.

Enter measured weight and height in one unit system, then confirm that pounds were not entered as kilograms or centimeters as inches. Read the BMI and category together. A result close to a cutoff should be treated as approximate because ordinary measurement and rounding error can change the displayed category. For ages 2 through 19, use the child and teen mode and enter exact age and sex as required by the growth-chart calculation. The result needs a BMI-for-age percentile, not an adult category. A pediatric percentile describes position within the CDC reference population; it is not the percentage of the child's body that is fat.

Save the raw date, weight, height, calculated BMI, and any relevant circumstance instead of saving only the category. If the result will guide medical care, a clinician should repeat the measurements and review growth, health history, examination findings, and laboratory data. Do not use this calculator to set a child's calorie target or to self-diagnose a disease.

[1][4][5]

Measure height and weight accurately

Use a calibrated digital scale on a hard, level surface. Weigh with shoes removed and in light clothing.

Empty pockets and stand still with weight distributed across both feet. For trend tracking, use the same scale at a similar time of day, preferably under similar clothing and meal conditions. Day-to-day shifts often reflect water, food, and bowel contents rather than tissue change. Measure standing height against a wall-mounted stadiometer when possible. Remove shoes, bulky hair accessories, and hats. Stand with heels together, legs straight, shoulders relaxed, and head level so the line from the lower eye socket to the ear canal is horizontal. Lower the headpiece firmly onto the crown and record the measurement without rounding to a favored number.

Repeat an implausible measurement. Adults can lose measured height with aging, vertebral compression, or posture, so an old driver-license height may overstate current stature and understate BMI. For children, accurate age, height, and weight are essential because modest errors can shift the percentile. Clinical growth monitoring should use trained measurement technique and serial values.

[3][6]

Adult BMI categories

CDC categories for adults age 20 and older use fixed thresholds. Underweight is below 18.5. Healthy weight is 18.5 to less than 25. Overweight is 25 to less than 30. Obesity begins at 30 and is divided into classes.

These labels describe statistical ranges and should not be used as judgments about a person. A boundary does not create an abrupt biological change. An adult with BMI 24.9 and one with BMI 25.0 have nearly the same weight relative to height. Risk generally changes along a continuum and depends on factors that BMI omits. A category is most useful when it leads to an appropriate next step, such as checking waist circumference, blood pressure, glucose, lipids, nutrition, or unintentional weight change.

[1][2]

BMI for children and teens

Children change body proportions as they grow, so adult cutoffs do not apply from ages 2 through 19.

CDC interpretation compares BMI with children of the same age and sex in a reference population. Underweight is below the 5th percentile, healthy weight is the 5th to less than the 85th, overweight is the 85th to less than the 95th, and obesity is at or above the 95th percentile. Severe obesity is at or above 120% of the 95th percentile or BMI 35 kg/m², whichever threshold is lower. CDC extended BMI-for-age growth charts support plotting very high BMIs that exceed the traditional charts. AAP guidance recommends at least annual BMI assessment from age 2, followed by evaluation appropriate to the child rather than watchful labeling from one number.

A percentile crossing can reflect a growth spurt, measurement error, medication effect, illness, or sustained change in weight gain. Clinicians review height velocity, developmental stage, family history, sleep, food access, mental health, and signs of weight-related conditions. Children younger than 2 are assessed with WHO weight-for-length standards rather than BMI categories.

[4][5][7]

Screening is not diagnosis

BMI can identify people who may benefit from further assessment.

It cannot show whether someone has high blood pressure, dyslipidemia, diabetes, fatty liver disease, sleep apnea, osteoarthritis, nutrient deficiency, or another condition. Conversely, a BMI in the healthy range does not exclude those conditions. Diagnosis depends on history, examination, and condition-specific testing. An adult result outside the healthy range should be interpreted with the direction and pace of weight change. Unintentional loss, rapid gain, edema, pregnancy, medication changes, and chronic illness can make a routine category misleading or medically urgent. A clinician can determine whether the priority is nutrition support, weight management, investigation of a cause, or no intervention.

For children, screening should lead to respectful, family-centered assessment. The AAP guideline treats obesity as a chronic disease and recommends evaluation for comorbidities and evidence-based treatment when indicated. A percentile alone does not establish the cause, severity of health effects, or appropriate treatment intensity.

[1][7][8]

Add waist circumference and clinical risk

BMI does not identify fat distribution.

Visceral and abdominal fat are associated with cardiometabolic risk, so waist circumference can add information, particularly for adults with BMI below 35. Measure at the top of the hip bones after a normal exhalation, with the tape horizontal and snug without compressing skin. Use the same anatomical site every time because protocols differ. Traditional US clinical thresholds flag increased risk above 35 in for women and 40 in for men, but those thresholds do not fit every population or body size. An international consensus statement concluded that waist circumference improves risk classification and should be treated as a clinical vital sign, while also noting that sex- and ethnicity-specific cut points require care.

Pair body size with direct risk measures. Blood pressure, fasting glucose or A1c, lipids, smoking, sleep, physical activity, family history, and existing disease often matter more to a decision than a small BMI difference. Waist measurement is also a screening measure, not a direct scan of visceral fat.

[8][9]

Where BMI can misclassify people

BMI cannot separate fat mass from lean mass.

Strength athletes, some tactical personnel, and people with substantial muscle may enter an overweight or obesity category despite lower body fat. The reverse can occur after muscle loss: an older adult can have a BMI in the healthy range while carrying a higher fat percentage and low muscle reserves. Body composition, strength, and function clarify these cases. Risk at a given BMI also differs across populations. A WHO expert consultation found that many Asian populations experience substantial type 2 diabetes and cardiovascular risk below the standard BMI 25 cutoff. It retained international categories for reporting but identified 23, 27.5, 32.5, and 37.5 kg/m² as additional action points for public health and clinical decisions. Ethnicity should inform assessment, not be used to infer an individual's body composition.

Pregnancy, edema, ascites, limb loss, marked spinal curvature, and conditions that change hydration or body proportions weaken the equation's meaning. BMI was not designed to set athletic weight classes or diagnose sarcopenia. When the estimate conflicts with examination, performance, waist size, or laboratory findings, investigate the disagreement instead of assuming the formula is correct.

[2][10][11][12]

Track trends without chasing daily noise

For personal tracking, measure weight under comparable conditions and update height when it may have changed. Weekly or monthly summaries show direction more clearly than isolated daily values.

Record relevant changes such as a new medication, illness, resistance-training block, pregnancy, or fluid retention. A category change caused by rounding is not proof of a meaningful health change. Use outcomes that match the goal. Someone managing blood pressure should follow blood pressure. Someone improving glucose should follow the laboratory plan set with a clinician. During strength training, waist, performance, and body composition may explain stable weight. During aging or recovery from illness, maintaining muscle and function may matter more than lowering BMI.

Seek clinical review for unexplained weight loss or gain, a child crossing growth percentiles, symptoms of an eating disorder, pregnancy-related concerns, or a result that may affect medication or surgery. The calculator supports a conversation and a trend record. It should not determine treatment by itself.

[2][5][7]

How it works

BMI = weight (kg) / height (m)². WHO categories: underweight (<18.5), normal (18.5 to 24.9), overweight (25 to 29.9), obese (≥30). For children, compare to CDC age-and-sex percentiles.

Frequently asked questions

• Is BMI accurate for athletes?BMI overestimates body fat in muscular people because it does not distinguish muscle from fat. Use a body fat calculator if you carry significant muscle mass.
• What is a healthy BMI?For adults, 18.5 to 24.9 is considered normal weight by WHO standards.
• Does BMI work for children?BMI for children must be interpreted using age and sex percentile charts, not adult cutoffs.

Related calculators

References

1. CDC. Adult BMI Categories
2. CDC. BMI Frequently Asked Questions
3. CDC. Calculating BMI
4. CDC. Child and Teen BMI Categories
5. CDC. Summary and References: Using BMI-for-Age Growth Charts
6. CDC. Anthropometric Indices: Definitions and Categories
7. American Academy of Pediatrics. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity
8. National Heart, Lung, and Blood Institute. The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults
9. Ross et al.. Waist Circumference as a Vital Sign in Clinical Practice
10. WHO Expert Consultation. Appropriate Body-Mass Index for Asian Populations and Its Implications for Policy and Intervention Strategies
11. Rothman. BMI-Related Errors in the Measurement of Obesity
12. Heymsfield et al.. Body Mass Index and Body Composition Scaling to Height in Children and Adults