BMI (Body Mass Index) is a numerical value calculated from your weight and height. It provides a simple screening measure for weight categories: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (30 and above).

How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight (kg) / height (m)². In imperial units: BMI = (weight in pounds × 703) / (height in inches)².

Is BMI accurate for everyone?

BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat mass, so very muscular people may have a high BMI without being overweight. It also doesn't account for age, sex, bone density, or fat distribution. Consult a healthcare provider for a complete health assessment.

BMI Calculator

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Healthy Weight Range

Weight to Reach Normal BMI

Category	BMI Range
Underweight	Below 18.5
Normal weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obesity Class I	30.0 – 34.9
Obesity Class II	35.0 – 39.9
Obesity Class III	40.0 and above

Understanding BMI

Body Mass Index (BMI) is a widely used screening tool that estimates body fat based on height and weight. While it doesn't measure body fat directly, BMI correlates with more direct measures of body fat and is a useful starting point for assessing weight-related health risks.

The BMI Formula

BMI can be calculated using either metric or imperial measurements:

Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) × 703) / height (in)²

BMI Categories

The World Health Organization defines these BMI categories for adults: Underweight (below 18.5), Normal (18.5-24.9), Overweight (25-29.9), and Obese (30+). These thresholds are based on statistical associations with health outcomes across large populations.

Limitations of BMI

BMI is a useful screening tool but has important limitations. It does not distinguish between muscle mass and fat mass — a muscular athlete may have a "high" BMI while being very fit. It also doesn't account for age, sex, ethnicity, bone density, or where body fat is distributed. Waist circumference and body fat percentage are complementary measurements worth considering.

When to Talk to a Doctor

If your BMI falls outside the normal range, or if you have concerns about your weight, consult a healthcare professional. They can perform a more comprehensive assessment including body composition, blood tests, family history, and other health markers that BMI alone cannot capture.

Frequently Asked Questions

BMI (Body Mass Index) is a number calculated from your height and weight. It's used as a screening tool to categorize weight status: underweight, normal, overweight, or obese. It's not a diagnostic tool — consult a doctor for health assessments.

BMI = weight (kg) / height (m)². In imperial units: BMI = (weight in lbs × 703) / (height in inches)². For example, a 170 lb person at 5'9" (69 inches): (170 × 703) / (69)² = 25.1.

BMI has limitations. It doesn't differentiate between muscle and fat, so muscular people may show a high BMI. It also doesn't account for age, sex, ethnicity, or fat distribution. It's a useful starting point but not a complete health picture.

A BMI between 18.5 and 24.9 is considered "normal weight" by the WHO. However, individual health depends on many factors beyond BMI. Some people are healthy at higher or lower BMI values depending on their body composition and overall health.