Build the adult index from unrounded metric inputs
The calculation is weight in kilograms divided by height in meters squared. A person who weighs 81 kilograms and is 1.80 meters tall has a raw BMI of 81 / 1.80² = 25. The squared height is the denominator, so a unit mistake has an amplified effect: 180 centimeters must become 1.80 meters before it is squared. Keep the raw quotient for classification and round only the displayed answer.
This page deliberately asks for an age of at least 20 because CDC interpretation changes before that birthday. For anyone ages 2 through 19, use the Child and Teen BMI Calculator instead; it combines BMI with age and a sex-specific reference rather than applying fixed adult cutoffs. That distinction is part of the method, not a cosmetic label.
Read a boundary before reading the category name
CDC's adult bands use the raw value, with each upper edge excluded until the next category begins.
Below 18.5
The screening label is underweight. A low result does not explain its cause and cannot distinguish a stable body size from illness, inadequate intake, or another clinical concern.
18.5 through below 30
The healthy-weight band runs from 18.5 to below 25; overweight begins at 25 and ends below 30. Exactly 25 therefore belongs to the latter band even if a rounded display conceals a nearby raw value.
30 and above
The obesity category begins at 30, with class boundaries at 35 and 40. These are population screening categories, not a finding about an individual's metabolic health or a prescription to change weight.
Pair BMI with measurements that answer different questions
BMI cannot tell fat mass from muscle, bone, organs, or water. The Body Fat Calculator addresses a different question with a circumference regression, while the Waist-to-Height Ratio Calculator looks at central adiposity in a bounded adult screening context. Neither automatically corrects or replaces BMI; disagreement is a reason to examine the measurements and context.
NIDDK notes that muscularity, aging-related changes in muscle and bone, and where fat is stored can make BMI less informative for some people. A clinician may combine the index with history, examination, laboratory findings, and health behaviors. The online result alone cannot diagnose obesity, malnutrition, an eating disorder, or any other condition.
Audit an adult BMI result near a cutoff
A short reconstruction prevents the most common category errors.
- Confirm the measurement date
Use height and weight that describe the same person at roughly the same time. Remove shoes for height and avoid substituting a stated driver's-license height when a current measurement is available.
- Normalize before dividing
Convert centimeters to meters and then square height. Do not round height, squared height, or the quotient during the intermediate steps.
- Classify the hidden raw value
A displayed 25.0 might originate just below or just above 25. The calculator assigns the band from full precision so the category and arithmetic remain consistent.
Decide what should happen after the screen
The useful next step depends on the question that prompted the calculation.
- If the goal is energy planning rather than classification, start a separate estimate with the Calorie Calculator; a BMI category is not a calorie prescription.
- Bring unexpected results, large recent changes, symptoms, pregnancy, edema, major muscle loss, or concerns about eating to a qualified health professional rather than trying to diagnose the cause from the quotient.
- Record the source measurements and date. A series collected consistently can be more informative than an isolated value, but even a trend remains one piece of a broader health assessment.
Adult BMI equationBMI (kg/m2) = weight (kg) / [height (m)]^2Require an adult age and metric inputs, convert centimetres to metres, calculate raw BMI, and classify the unrounded result at 18.5, 25, 30, 35, and 40. Round only the displayed BMI.
CDC adult BMI screening categories applied to the unrounded result| Adult screening category | Unrounded BMI (kg/m²) |
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| Underweight | Below 18.5 |
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| Healthy weight | 18.5 to below 25 |
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| Overweight | 25 to below 30 |
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| Obesity, class 1 | 30 to below 35 |
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| Obesity, class 2 | 35 to below 40 |
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| Obesity, class 3 | 40 or above |
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Method questions
Adult BMI calculator FAQ
- Which inputs does the adult BMI calculator use?
- Enter an age of at least 20, standing height, and weight. U.S.-unit entries are converted before the same metric equation runs.
- Why can the displayed BMI and category look different at a cutoff?
- The calculator keeps the raw quotient for classification and rounds only the displayed BMI. A displayed value at a boundary can therefore belong to the category selected from the value before rounding.
- Can adult BMI categories be used for children or teens?
- No. CDC adult categories begin at age 20. Ages 2 through 19 require an age- and sex-specific BMI-for-age reference rather than fixed adult cutoffs.
- Does BMI measure body fat or diagnose a condition?
- No. BMI is a screening ratio, not a direct body-fat measurement, diagnosis, or individualized assessment of health risk.
This is an adult screening reference, not a target weight, body-fat estimate, diagnosis, or treatment recommendation.