BMI — Body Mass Index — is the most widely used screening tool for weight classification. Despite its limitations, it remains the standard starting point used by doctors and health organizations worldwide because it is fast, free, and requires only two measurements: height and weight. This guide explains how to calculate BMI using both metric and imperial units, what the categories mean, and how to interpret your result in context.
The BMI Formula (Metric Units)
BMI = Weight (kg) ÷ Height (m)²
Convert your height from centimeters to meters first by dividing by 100. Then square that number and divide your weight by it. Example: Height = 172 cm (1.72 m), Weight = 74 kg. BMI = 74 ÷ (1.72²) = 74 ÷ 2.9584 = 25.0 — borderline overweight.
The BMI Formula (Imperial Units)
BMI = (Weight in lbs × 703) ÷ Height in inches²
Example: You are 5 ft 8 in (68 inches) and weigh 165 lbs. BMI = (165 × 703) ÷ (68²) = 116,000 ÷ 4,624 = 25.1 — slightly overweight by WHO standards.
BMI Categories (WHO Standard)
| BMI Range | Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | Risk of nutritional deficiency, bone loss |
| 18.5 – 24.9 | Normal weight | Lowest risk for most weight-related conditions |
| 25.0 – 29.9 | Overweight | Moderately elevated metabolic risk |
| 30.0 – 34.9 | Obese (Class I) | High risk — diabetes, heart disease |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40+ | Obese (Class III) | Extremely high risk |
BMI Calculation Examples
Example 1: Female, 163 cm (1.63 m), 57 kg. BMI = 57 ÷ (1.63²) = 57 ÷ 2.6569 = 21.5 — Normal weight.
Example 2: Male, 183 cm (1.83 m), 100 kg. BMI = 100 ÷ (1.83²) = 100 ÷ 3.3489 = 29.9 — Overweight (borderline).
Example 3 (imperial): 5 ft 4 in (64 inches), 130 lbs. BMI = (130 × 703) ÷ (64²) = 91,390 ÷ 4,096 = 22.3 — Normal weight.
Important Limitations of BMI
BMI measures weight relative to height but does not distinguish between muscle, fat, and bone. A bodybuilder with very low body fat may have a BMI of 28 (overweight), while a sedentary person with high body fat may have a normal BMI due to low muscle mass. For individuals, BMI is best used as one data point among many, not as a definitive health verdict.
Waist circumference, body fat percentage, and metabolic blood markers (blood glucose, cholesterol, blood pressure) provide a more complete picture of health than BMI alone. Many healthcare providers now use waist-to-height ratio — a waist circumference less than half your height is a simple and effective target — in addition to BMI for a more meaningful assessment.
Use Our Free BMI Calculator
Skip the manual calculation and use our free BMI Calculator — enter your height and weight and get your BMI score and category instantly, no sign-up required.
Frequently Asked Questions
Is BMI the same for men and women?
The WHO uses the same BMI thresholds for all adults regardless of sex. However, women naturally carry a higher percentage of body fat than men at the same BMI due to physiological differences. This means a woman and a man with identical BMIs may have different body compositions. Some researchers advocate for sex-specific BMI thresholds, though WHO guidelines have not yet formally adopted them.
Does BMI apply to children?
No — for children and teenagers (under 18), BMI is assessed using age- and sex-specific growth percentile charts, not the fixed adult thresholds. A child at the 85th–94th percentile for BMI-for-age is classified as overweight; at the 95th percentile or above, as obese. Always use a pediatric assessment for anyone under 18.
Can you have a high BMI and still be healthy?
Yes. BMI is a population screening tool, not a personal health diagnosis. People with higher muscle mass (athletes, strength trainers) often have elevated BMIs that do not reflect elevated health risk. Conversely, people in the “normal” BMI range can have poor metabolic health. Blood pressure, blood glucose, cholesterol, and fitness level are all more directly linked to health outcomes than BMI alone.
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