How BMI is calculated
Body mass index (BMI) compares your weight to your height with a single formula. It's the same idea in either unit system:
Metric: BMI = weight (kg) ÷ height (m)2
Imperial: BMI = 703 × weight (lb) ÷ height (in)2
The 703 factor in the imperial version simply converts pounds and inches into the metric result, so both formulas give the same BMI for the same person.
What the categories mean
- Under 18.5 — underweight
- 18.5 – 24.9 — healthy weight
- 25.0 – 29.9 — overweight
- 30.0 and above — obesity
These adult ranges are a screening guide, not a verdict. Where you land can flag whether a closer look at your health is worthwhile.
What BMI doesn't capture
BMI is fast and useful for populations, but it has real limits for individuals. It can't tell muscle from fat, doesn't account for where you carry weight, and isn't tuned for children, pregnant people, or elite athletes. Measures like waist circumference, body-fat percentage, and your doctor's assessment fill in the gaps BMI leaves.
A quick example
Someone 175 cm tall weighing 75 kg has a BMI of about 24.5 — near the top of the healthy range. The same person in imperial units (about 5'9" and 165 lb) lands on the same number, since the two formulas are equivalent.
Frequently asked questions
How is BMI calculated?
BMI is your weight divided by the square of your height. In metric units it is kg ÷ m². In imperial units it is (pounds ÷ inches²) × 703. The result is a single number that places you into a weight category.
What are the BMI categories?
For adults: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is in the obesity range. These ranges are the same regardless of sex but apply to adults aged 20 and over.
Is BMI an accurate measure of health?
BMI is a quick screening tool, not a diagnosis. It does not distinguish muscle from fat, so very muscular people can read as "overweight" while others within a healthy BMI may carry excess body fat. Use it as one signal alongside measures like waist circumference and a doctor’s assessment.
Does BMI work for children and athletes?
Not directly. Children and teens use age- and sex-specific BMI percentiles rather than the fixed adult ranges, and highly muscular athletes often fall outside the categories the standard formula assumes. For these groups, consult a clinician for context.
Source: BMI weight categories follow the World Health Organization classification (overweight 25.0–29.9, obesity 30.0+) — see the WHO obesity and overweight fact sheet. Last reviewed June 2026 by the CalcQuill editorial team.
Disclaimer: BMI is a general screening tool, not medical advice or a diagnosis. Consult a healthcare professional for guidance about your weight and health.