What Is a BMI Calculator and How Does It Work?
A BMI calculator estimates Body Mass Index, a screening number based on an adult’s weight relative to height. It does not directly measure body fat, muscle mass, bone density, or where body fat is stored. Instead, it converts height and weight into one standardized value that can be compared with adult BMI categories.
This calculator supports metric measurements and U.S. customary measurements. After you enter your values, it displays your BMI to one decimal place, the corresponding adult category, the measurements used, and an estimated weight range associated with BMI values from 18.5 through 24.9 at the same height.
The calculation runs in your browser and does not require an account. BMI should be treated as an initial screening measure—not a diagnosis or a complete assessment of health. The CDC’s overview of BMI provides additional context.
How to Use This BMI Calculator
- Select Metric or U.S. customary units.
- Enter your current weight in kilograms or pounds.
- For metric units, enter height in centimeters. For U.S. units, enter feet and inches in the separate fields.
- Select Calculate BMI.
- Review your BMI, adult category, and estimated healthy-weight range in the live result panel.
Use measurements taken at roughly the same time and avoid mixing units. The calculator is for adults age 20 and older. People ages 2 through 19 require age- and sex-specific BMI percentiles; use the CDC Child and Teen BMI Calculator instead.
The BMI Formula: How Your Score Is Calculated
With metric units, divide weight in kilograms by height in meters squared:
With U.S. customary units, multiply weight in pounds by 703 and divide by total height in inches squared:
The factor 703 converts pounds and inches to the same BMI scale used by the metric formula. This calculator converts feet and inches into total inches automatically.
Understanding Adult BMI Categories
For adults age 20 and older, the standard categories below are used as screening ranges. These categories match the CDC adult BMI categories.
| Adult BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy Weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity (Class 1) |
| 35.0–39.9 | Obesity (Class 2) |
| 40.0 or higher | Obesity (Class 3) |
What Is a Healthy BMI for Adults?
A BMI from 18.5 through 24.9 is categorized as Healthy Weight for adults. This category is a population-level screening range, not a promise that every person inside it has the same health risk or body composition.
What BMI Range Is Considered Overweight?
A BMI from 25.0 through 29.9 is categorized as Overweight for adults. A result in this range should be interpreted with other information such as waist circumference, medical history, activity level, and body composition.
What BMI Range Is Considered Obesity?
A BMI of 30.0 or higher is categorized as Obesity. The calculator also identifies Class 1, Class 2, and Class 3 ranges so the displayed result matches the detailed adult category breakdown.
Is BMI Accurate for Everyone? Muscle, Age, and Body Composition
BMI is useful because it is quick, consistent, and inexpensive, but it cannot distinguish fat from muscle or show fat distribution. The NHLBI explains that BMI is a screening tool and may be considered alongside measures such as waist circumference.
- Muscular adults and athletes: Greater muscle mass can produce a higher BMI even when body-fat levels are not high.
- Older adults: Age-related loss of muscle can make BMI underestimate body fat in some people.
- Body-fat distribution: BMI does not show whether fat is concentrated around the waist.
- Children and teens: Adult categories are inappropriate because BMI must be compared with age- and sex-specific growth percentiles.
- Pregnancy: Current pregnancy weight should not be interpreted with ordinary adult BMI categories; healthcare guidance commonly considers pre-pregnancy BMI.
For additional context, compare BMI with other relevant measurements rather than replacing one screening number with another. The Body Fat Calculator, Waist-to-Hip Ratio Calculator, and Ideal Weight Calculator provide different estimates, but none substitutes for a medical assessment.
BMI Calculation Examples
Metric BMI Example
An adult who weighs 72 kilograms and is 1.75 meters tall has a BMI of approximately 23.5:
A BMI of 23.5 falls within the Healthy Weight category for adults.
U.S. BMI Example
An adult who weighs 160 pounds and is 5 feet 10 inches tall is 70 inches tall in total and has a BMI of approximately 23.0:
Small differences between calculators can occur because of rounding, particularly near a category boundary.