BMI Calculator

How this BMI calculator works

Body Mass Index (BMI) is a screening number used worldwide to group adults into underweight, normal weight, overweight, and obesity categories based on height and weight. It is quick to calculate and useful for population health and first-pass personal context — but it does not measure body fat directly and should not be treated as a full health assessment.

Formula

BMI = weight (kg) ÷ [height (m)]². If you enter imperial units, the tool converts pounds and inches to metric before applying the same formula. Categories commonly used for adults are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obesity.

Worked example

Someone who is 1.75 m tall and weighs 70 kg has BMI = 70 ÷ (1.75²) ≈ 22.9, which falls in the normal range. The same person at 90 kg would have BMI ≈ 29.4 (overweight). Athletes with high muscle mass can land in a higher category without excess fat — one reason BMI is only a screen.

How to use it

1. Choose metric or imperial units.
2. Enter height and weight carefully (standing height without shoes; a consistent scale).
3. Read the BMI value and category label.
4. Use the result as one data point alongside waist size, fitness, labs, and clinician guidance.

Frequently asked questions

Is BMI the same as body fat percentage?

No. BMI uses only height and weight. Body fat estimates need additional measurements or devices. See our BMI limitations guide and the body fat calculator for related context.

Should children use these adult categories?

No. Pediatric BMI is interpreted with age- and sex-specific percentiles. This page uses common adult bands only.

Why might an athlete show as “overweight”?

High muscle mass raises weight without meaning excess fat. BMI cannot tell muscle from fat — another reason it is a screen, not a diagnosis.

What to look at besides BMI

Waist circumference, fitness capacity, blood pressure, lipids, and clinical history often matter more for personal decisions than a single BMI band. If you want a composition-oriented estimate, try the body fat calculator (U.S. Navy method) and still treat it as an approximation. For calorie planning after you understand body-size context, use the calorie calculator and read TDEE and calorie targets.

Older adults, pregnant people, and highly muscular athletes are classic cases where BMI categories mislead — details in the BMI limitations guide.

Limitations

BMI cutoffs differ for children and some ethnic groups. Pregnancy, edema, and amputation change interpretation. This page is for general education only.

Not a substitute for professional medical advice.