Quick Answer
BMI (Body Mass Index) is calculated as weight in kilograms divided by height in meters squared (BMI = kg / m²). A BMI between 18.5 and 24.9 is generally considered a healthy weight range for most adults.
What Is BMI?
Body Mass Index (BMI) is a simple screening measurement that uses your height and weight to estimate whether your body weight falls into a healthy range. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and is still the most widely used weight-screening tool by health organizations worldwide, including the World Health Organization (WHO) and the CDC.
BMI doesn't measure body fat directly, it's a ratio of mass to height. Because of this, it can be less accurate for certain groups: athletes with high muscle mass may show a 'high' BMI despite having low body fat, while older adults can have a 'normal' BMI while carrying excess body fat. Despite these limitations, BMI remains a useful first-pass indicator for population-level health screening and tracking weight trends over time for any individual.
Doctors typically use BMI alongside other measurements, waist circumference, body fat percentage, and overall health history, to get a fuller picture before making any health recommendations.
In Practice
A 38-year-old man is 5 feet 10 inches tall (177.8 cm) and weighs 95 kg. His BMI is 95 ÷ (1.778 × 1.778) = 30.1 — just into the obese category. Yet he runs three times a week, has normal blood pressure and feels healthy. His doctor explains that his BMI is elevated primarily because of above-average muscle mass in his legs and shoulders, not excess fat. A body fat percentage test confirms 19% body fat — well within the healthy range for his age. This is a classic example of why BMI is a useful starting point but never the whole story. The calculator correctly flags his weight; his doctor provides the context.
How to Use
- 1Choose your preferred unit system, metric (kg, cm) or imperial (lb, in).
- 2Enter your current weight.
- 3Enter your height.
- 4The calculator instantly returns your BMI value and which weight category it falls into.
Formula
BMI = weight (kg) ÷ [height (m)]²BMI = 703 × weight (lb) ÷ [height (in)]²Example: A person weighing 70 kg with a height of 1.75 m
- Height squared = 1.75 × 1.75 = 3.0625 m²
- BMI = 70 ÷ 3.0625
Common Mistakes to Avoid
✕ Entering 5 feet 6 inches as 5.6 feet
✓ 5 feet 6 inches is 5.5 feet (66 inches total), not 5.6. To avoid confusion, convert to centimetres directly: 5 ft 6 in equals 167.6 cm. Use the cm field in this calculator to eliminate unit errors.
✕ Treating BMI as a definitive health verdict
✓ BMI is a screening tool, not a diagnosis. Muscular individuals often show high BMI with low body fat, while some people at normal BMI carry excess abdominal fat. Use BMI alongside waist circumference and your doctor's assessment.
Tips & Things to Know
- BMI categories: below 18.5 = Underweight, 18.5–24.9 = Normal, 25–29.9 = Overweight, 30 and above = Obese.
- BMI is less reliable for bodybuilders, pregnant women, children, and people over 65, these groups should rely more on a doctor's assessment than the BMI number alone.
- Tracking your BMI trend over months is often more useful than a single reading, since it shows the direction your weight is moving.
- BMI says nothing about where fat is stored on the body. Waist circumference is often considered alongside BMI because abdominal fat carries different health risks than fat stored elsewhere.