Complete Guide to BMI: Formula, Categories, Reliability & More

Body Mass Index (BMI) is a widely used screening tool to estimate body fat and assess health risks. This guide explains everything you need to know — without any calculator.

1. What is BMI?

BMI stands for Body Mass Index. It is a simple numerical value calculated from a person's height and weight. BMI helps categorize individuals into weight status groups that may indicate potential health risks.

Created in the 1830s by Belgian mathematician Adolphe Quetelet, it was originally known as the "Quetelet Index." Today, it's a standard tool used by healthcare professionals worldwide.

2. BMI Formula

The result is a single number that helps determine if a person is underweight, normal weight, overweight, or obese.

3. How to Calculate Your Body Mass Index?

Step-by-step manual calculation:

1. Measure your height in meters (or convert feet/inches to inches first).
2. Measure your weight in kilograms (or pounds).
3. Square your height in meters (height × height).
4. Divide weight by height squared (for metric) or use the 703 multiplier (for imperial).
5. Round to one decimal place for standard reporting.

4. BMI Categories (WHO Standards)

BMI Range	Category	Health Implication
Below 18.5	Underweight	May indicate malnutrition, eating disorders, or other health issues
18.5 – 24.9	Normal Weight	Lowest risk for weight-related diseases
25.0 – 29.9	Overweight	Increased risk of heart disease, diabetes, and hypertension
30.0 and above	Obese	High risk of serious conditions like stroke, cancer, and joint problems

5. What is BMI Prime?

BMI Prime is a derived index that compares your BMI to the upper limit of the normal range (25).

• Less than 0.74: Underweight
• 0.74 to 1.00: Normal weight
• Greater than 1.00: Overweight or obese

It’s mainly used in research to standardize comparisons across populations.

6. How Reliable Are BMI Calculations?

Strengths of BMI:

• Quick, inexpensive, and easy to compute.
• Strong correlation with body fat in large population studies.
• Useful for tracking trends over time.

Limitations of BMI:

• Does not measure body fat directly – muscular athletes often have high BMI but low fat.
• Ignores fat distribution – abdominal fat is more dangerous than fat on hips/thighs.
• Not suitable for all groups: children, pregnant women, elderly, or very muscular individuals.
• Ethnic variations: Asians have higher risk at lower BMI; Black individuals may have lower risk at higher BMI.

7. BMI Calculator for Women

The BMI formula is identical for men and women. However, women naturally carry higher body fat percentage due to reproductive needs (pregnancy, breastfeeding).

Healthy BMI range for women: Still 18.5–24.9, but some experts recommend 20–22 as optimal for fertility, hormonal balance, and bone health.

During pregnancy, BMI is not used — weight gain guidelines are based on pre-pregnancy BMI instead.