What Is a BMI Calculator?
A BMI calculator estimates Body Mass Index, a simple screening measure that relates your weight to your height. It's widely used by clinicians as a quick, population-level indicator of whether a person's weight falls in a range associated with lower or higher health risk.
How to Use the BMI Calculator
- Choose US or metric units.
- Enter your weight.
- Enter your height.
- Your BMI and category are calculated instantly.
BMI Formula
Worked Example
Height: 5 ft 8 in = 68 inches
Weight: 160 lb
BMI = 703 × 160 ÷ 68² = 24.3
Result: 24.3 falls in the "normal weight" range (18.5–24.9).
Understanding Your Results
BMI places you into one of four standard categories: underweight, normal weight, overweight, or obesity. It's a useful, fast screening tool, but it doesn't distinguish between muscle and fat — a muscular athlete can have a "high" BMI without excess body fat, and it doesn't account for age, sex, or where fat is distributed on the body.
Limitations of BMI
- Doesn't distinguish muscle mass from fat mass.
- Doesn't account for bone density, body frame, or fat distribution.
- Standard ranges may not translate directly across all ethnic groups or age ranges.
- Not designed for pregnant women, children, or highly muscular athletes.
Common Mistakes to Avoid
- Treating BMI as a diagnosis rather than a screening tool.
- Ignoring other health indicators like waist circumference, body fat percentage, and fitness level.
- Comparing your BMI directly to someone with a very different body composition or athletic background.
Frequently Asked Questions
For most adults, a BMI between 18.5 and 24.9 is classified as the "normal weight" range, though individual health depends on many other factors too.
Not always — BMI doesn't distinguish muscle from fat, so muscular or highly athletic individuals may show a higher BMI without having excess body fat.
BMI is calculated as weight in kilograms divided by height in meters squared (or, in US units, 703 times weight in pounds divided by height in inches squared).
No — children and teens use age- and sex-specific BMI percentile charts rather than the adult BMI categories used here.
Consider discussing your weight, body composition, and overall health with a doctor or registered dietitian, who can look at the full picture beyond a single number.
It's a useful quick screening tool but not a complete picture — waist circumference, body fat percentage, blood pressure, and other markers all matter for a fuller assessment.