BMI Calculator: Formula, Categories, and Limitations
Understand the BMI formula, learn how to calculate Body Mass Index, and explore what BMI can and cannot tell you about health.
A BMI Calculator estimates Body Mass Index (BMI) using a person's height and weight. It is a simple screening measure commonly used with adults, but it does not directly measure body fat or provide a complete picture of health.
BMI Formula
For weight in kilograms and height in metres:
BMI = Weight (kg) ÷ Height (m)²
Example: If an adult weighs 70 kg and is 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 22.9 (approximately).
If your height is entered in centimetres, convert it to metres first by dividing by 100.
Common Adult BMI Categories
For many adults, commonly used screening ranges are:
| BMI | Category | |---|---| | Below 18.5 | Underweight | | 18.5 to 24.9 | Healthy-weight range | | 25.0 to 29.9 | Overweight | | 30.0 or above | Obesity range |
These categories are general screening thresholds, not a diagnosis. Interpretation can vary by age, population, clinical context, and professional guidance. Different thresholds may be used for some populations.
How to Use a BMI Calculator
- Select the appropriate units.
- Enter your weight accurately.
- Enter your height accurately.
- Calculate BMI and read the result as a screening indicator—not a diagnosis.
What BMI Cannot Tell You
BMI does not distinguish muscle from fat, show where fat is distributed, or measure nutrition, fitness, or overall health. An athletic person with substantial muscle may have a higher BMI without having excess body fat. Older adults and people with different body compositions may also need additional assessment.
For children and teenagers, BMI is interpreted differently using age- and sex-specific growth charts. Adult categories should not be used to assess a child.
When to Seek Personal Guidance
If you are concerned about weight, growth, eating, or a health condition, discuss your situation with a qualified healthcare professional. They can consider your medical history, measurements, symptoms, and other relevant factors instead of relying on BMI alone.
Frequently Asked Questions
Is BMI a direct measurement of body fat?
No. BMI is calculated from height and weight and is used as a screening measure.
Can BMI be used for children?
Children and teenagers require age- and sex-specific interpretation. A standard adult BMI category is not appropriate for them.
Does a “normal” BMI guarantee good health?
No. BMI is only one indicator and does not rule out health conditions or replace routine care.
Final Thoughts
A BMI Calculator offers a quick, consistent calculation, but its result should be interpreted in context. Use it as one piece of information rather than a standalone judgment about health.
This article is for general education and is not medical advice or a diagnosis. Consult a qualified healthcare professional for individual guidance.