Body Mass Index (BMI) is one of the most searched health-screening numbers on the web. People look for the BMI formula, BMI categories, and whether their height and weight produce a “normal” range. This guide explains the math in plain language, lists the adult bands most often cited alongside World Health Organization (WHO) thresholds and CDC educational materials, and makes clear what BMI is not designed to do. For an instant result with metric or US inputs, use our free BMI calculator on CodeOpx.
What is BMI?
BMI is a simple ratio of weight to height. It is used in population health and primary care as a screening tool—not a diagnosis of body fat, muscle, or disease risk by itself. Think of it as a starting point for conversation with a qualified clinician, not a final verdict on your health.
The BMI formula (metric)
When height is in meters and weight is in kilograms:
BMI = weight (kg) ÷ height (m)²
Example: 70 kg and 1.75 m → 70 ÷ (1.75 × 1.75) ≈ 22.9.
The BMI formula (US customary units)
When you measure height in inches and weight in pounds, an algebraically equivalent form is:
BMI = 703 × weight (lb) ÷ height (in)²
The constant 703 comes from converting inches to meters and pounds to kilograms in one step. Our online BMI calculator accepts either unit system and updates the category as you type.
Standard adult BMI categories (WHO-style bands)
For adults, the same broad bands appear across many public health pages:
- Underweight: BMI below 18.5
- Normal weight: BMI from 18.5 up to 25
- Overweight: BMI from 25 up to 30
- Obesity class I: BMI from 30 up to 35
- Obesity class II: BMI from 35 up to 40
- Obesity class III: BMI 40 or higher
Exact wording can vary slightly by source; always follow guidance from your healthcare provider and official agency pages when decisions matter.
Why BMI can mislead (even when the math is right)
BMI does not measure body fat percentage. People with high muscle mass may register a high BMI with low body fat. Older adults, some chronic conditions, pregnancy, and differences in body composition across populations can all change how BMI relates to health risk. That is why major agencies describe BMI as a screening measure—not a stand-alone test.
Children and teens need a different approach
Pediatric BMI uses age- and sex-specific percentiles on growth charts. The adult cutoffs in this article do not apply to minors. If you are calculating BMI for a child, use pediatric growth-chart guidance from a clinician or official health resources—not adult thresholds.
Quick FAQ
Is “normal BMI” the same as “optimal health”? No. It is one screening band. Many factors shape health.
Does this page give medical advice? No. It is educational. For personal decisions, talk to a licensed professional.
Where can I compute my BMI now? Use the CodeOpx BMI calculator (adult) with kg/cm or lb/ft-in, shareable URL parameters, and built-in disclaimers.
Conclusion
Understanding the BMI formula and adult categories helps you read headlines and health content more critically. Pair that knowledge with professional advice when you need individualized recommendations—and use tools that show their math and limits, like our free BMI calculator.
