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BMI Calculator

Calculate BMI for adults age 20 and older. BMI is a screening measure, not a diagnosis.

Written by Ash K
Updated October 5, 2026Editorial process
📊 BMI calculation
📋 CDC adult categories
⚖️ BMI-based weight range
📖 Limits explained
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Quick Answer

For adults age 20 and older, BMI is weight in kilograms divided by height in meters squared. CDC categories are underweight below 18.5, healthy weight 18.5 to less than 25, overweight 25 to less than 30, and obesity 30 or higher. BMI is a screening measure, not a diagnosis.

BMI (Body Mass Index) is weight in kilograms divided by height in meters squared. This page uses US CDC screening categories for adults age 20 and older, matching the calculator and its category chart. A category describes a weight-to-height relationship; it does not diagnose your health. CDC adult categories

Enter your weight and height using the selected unit system. The calculator displays your BMI, its category and the weight range corresponding to the “healthy weight” BMI band at that height. The range is derived arithmetic, not a personal weight-loss or weight-gain prescription.

CDC BMI categories for adults age 20 and older

< 18.5Underweight18.5–<25Healthy weight25Overweight30Class 135 / 40+Class 2 / 3

How BMI Is Calculated

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Key Takeaway: BMI = weight (kg) ÷ height (m)². The adult categories below apply from age 20; children and teens require a different interpretation. BMI is a screening measure to consider with other health information. CDC categories and age scope

BMI range, kg/m²CDC adult category
Below 18.5Underweight
18.5 to less than 25Healthy weight
25 to less than 30Overweight
30 or higherObesity
30 to less than 35Class 1 obesity
35 to less than 40Class 2 obesity
40 or higherClass 3 obesity

Source: CDC adult category table. “Obesity” is the broad category; the three classes subdivide it. The endpoints use “less than” so values on a boundary belong to the next category.

The calculator classifies the unrounded number and displays BMI to one decimal place. Close to a boundary, a displayed value can round to that boundary while the underlying value remains below it. Do not treat the displayed decimal as a more precise health assessment.

What BMI Cannot Tell You

BMI does not directly measure body fat. It cannot separate fat from muscle and bone or show where fat is carried. Two people can have the same BMI without the same body composition. CDC: body fat and BMI

A high BMI does not prove that extra weight is fat. The CDC notes that a person with a lot of muscle can fall in an overweight or obesity BMI category. That limitation does not establish that every athlete is healthy or that every person in another category is unhealthy. CDC: athletes and interpretation

This tool does not assign different cutoffs by ethnicity. CDC adult category labels use the same boundaries regardless of age, sex or race within the adult scope. That is the categorization used here; the tool does not estimate an individual's diabetes or cardiovascular risk from those labels. CDC adult scope

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Note: For an individual health assessment, the CDC advises considering medical history, health behaviors, physical examination and laboratory findings alongside BMI. This calculator accepts only height and weight, so it cannot make that broader assessment. CDC interpretation guidance

Our body composition estimate is also an equation-based estimate, not a direct measurement. Do not treat moving to another calculator as confirmation of a diagnosis.

BMI Formula

Metric: BMI = weight (kg) ÷ height (m)². If height is in centimeters, divide it by 100 before squaring. CDC formulas

Arithmetic example: 75 kg and 1.75 m give 75 ÷ 3.0625 = approximately 24.5 kg/m². The example demonstrates the formula; it does not represent an ideal weight for a specific person.

Imperial: The CDC hand formula is BMI = weight (lb) × 703 ÷ height (in)². At 165 lb and 69 inches, that gives approximately 24.4 kg/m². The site's implementation converts pounds and inches to metric first, so tiny differences from the rounded 703 factor can occur. CDC hand formulas

BMI-based weight range: Multiply the adult band's lower endpoint, 18.5, and upper boundary, 25, by height in meters squared. The upper boundary is exclusive. The result card rounds the corresponding weights for display; it is not a treatment target or an instruction to change your weight.

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Tip: Check the unit toggle before calculating. Enter inches, not feet, in imperial mode, and centimeters, not meters, in metric mode. Selecting another unit system clears the fields. A unit mistake can produce a mathematically valid number that does not describe the measurements you intended.

For ages 2–19, the CDC uses BMI-for-age interpretation. Read our BMI calculator for teens and its stated age limits; do not apply this adult table to a child. CDC age-specific calculator guidance

Practical use: Record the measurements, selected units and result together. The calculator helps you understand a screening category and its arithmetic. Discuss questions about your health with your healthcare provider instead of using the category as a diagnosis.

Limits of BMI as a screening measure

BMI is not a directmeasurement of body fatBMI cannot separatefat, muscle and boneBMI does not showwhere body fat is carried

Frequently Asked Questions

What is a healthy BMI?

The CDC category called “healthy weight” spans 18.5 to less than 25 for adults 20+. The name is a screening category, not a guarantee about a person's overall health or a comparison ranking two people. CDC category definitions

How accurate is BMI?

The arithmetic depends on the measurements entered. Its limits as a health indicator are a separate issue: it does not distinguish fat, muscle and bone or directly measure body fat. No universal individual health-accuracy percentage is assigned here. CDC BMI limitations

What BMI is considered obese?

The CDC adult obesity category begins at 30; classes 1, 2 and 3 begin at 30, 35 and 40 respectively. Those labels describe the category and do not by themselves diagnose a separate disease. CDC adult categories

Sources

  1. CDC: Adult BMI Categories, boundaries and adult age scope.
  2. CDC: BMI Frequently Asked Questions, formulas and interpretation limits. Sources accessed October 5, 2026.

How this page was researched

Written and maintained by Ash K, who is not a licensed medical professional. Read the method, source links and limitations on this page. A cited instrument or formula does not mean this website has independent clinical validation.

This is health information, not medical advice. It does not replace your doctor. Read our editorial process · Medical disclaimer

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Medical Disclaimer

This tool is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider with questions about your health.