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⚖️BMI Calculator

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What Is BMI and How Is It Calculated?

Body Mass Index (BMI) is a number calculated from your height and weight that estimates body fat. The formula is BMI = weight (kg) / height (m)². In imperial units: BMI = 703 × weight (lbs) / height (inches)².

The World Health Organization defines four categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). A BMI between 18.5 and 24.9 is the healthy range for most adults.

BMI is a quick screening tool but has real limitations. It does not account for muscle mass, bone density, or where fat is stored. Muscular athletes can have a high BMI without excess fat. For a full picture, combine BMI with waist circumference and consult a healthcare provider.

BMI Categories (WHO)

Underweight: Below 18.5 — may indicate malnutrition or other health issues
Normal weight: 18.5–24.9 — associated with lowest health risk for most people
Overweight: 25–29.9 — moderate increased risk of cardiovascular disease and diabetes
Obese Class I: 30–34.9 · Class II: 35–39.9 · Class III: 40+ (severe)

Frequently Asked Questions

What is a healthy BMI for adults?+

A BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight; 25–29.9 is overweight; 30 or above is obese. Asian populations may use lower thresholds (23 for overweight, 27.5 for obese).

Is BMI accurate for athletes?+

No. BMI cannot distinguish between fat and muscle. A muscular athlete may have a "overweight" BMI with very low body fat. In these cases, body fat percentage measured by a DEXA scan or skinfold test is more meaningful.

How do I calculate BMI in imperial?+

BMI = (weight in lbs × 703) ÷ (height in inches)². Example: 160 lbs, 5 ft 9 in (69 inches): (160 × 703) ÷ (69²) = 112,480 ÷ 4,761 = 23.6 — Normal weight.

What BMI is considered obese?+

A BMI of 30 or higher is classified as obese. Obesity is associated with increased risk of type 2 diabetes, heart disease, sleep apnea, and certain cancers. Even a 5–10% reduction in weight can significantly improve health outcomes.

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Understanding Body Mass Index

BMI estimates body fat from height and weight using the formula weight (kg) divided by height (m) squared. The World Health Organization classifies results as underweight (below 18.5), healthy (18.5–24.9), overweight (25–29.9) and obese (30 and above).

Worked example

A person 1.75 m tall weighing 70 kg has a BMI of 70 / (1.75 × 1.75) = 22.9, inside the healthy range. In imperial units, multiply weight in pounds by 703 and divide by height in inches squared.

Limitations you should know

BMI does not distinguish muscle from fat, so athletes often score "overweight" while carrying little fat, and it can underestimate risk in older adults who have lost muscle. Waist circumference and body-fat percentage give a fuller picture. BMI is a screening tool, not a diagnosis — discuss results with a healthcare professional.

Is BMI valid for children? Children use age- and sex-specific percentile charts rather than the adult cutoffs.