BMI Calculator

What does this BMI Calculator do?

Body Mass Index (BMI) is a simple screening measure that relates your weight to your height, used worldwide as a quick indicator of whether someone falls into an underweight, normal, overweight, or obese range. It was developed in the 1830s and remains the most widely used weight-screening tool because it only needs two easy-to-measure inputs.

The formula

BMI = weight (kg) ÷ height (m)²

Your height is squared in meters, and your weight in kilograms is divided by that squared value. The result is a single number, independent of your actual height, that's compared against standard WHO categories.

Step-by-step example

For a person who is 170 cm (1.70 m) tall and weighs 68 kg: BMI = 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 ≈ 23.5, which falls in the "Normal" range (18.5–24.9).

Tips & common mistakes

• BMI doesn't distinguish between muscle and fat — a muscular athlete can register as "overweight" on BMI alone despite having low body fat.
• BMI categories are generally calibrated for adults; children, teenagers, pregnant women, and the elderly need different reference standards.
• Use BMI as one data point, not a diagnosis — waist circumference, body fat percentage, and overall health markers give a fuller picture.
• Make sure height is converted to meters (not left in centimeters) before squaring — a common calculation mistake when doing this by hand.

Where the BMI categories come from

The standard WHO/CDC BMI categories — underweight (below 18.5), normal (18.5–24.9), overweight (25–29.9), and obese (30 and above) — were established through large population studies correlating BMI with health outcomes and disease risk. They're a population-level statistical tool, originally developed for epidemiological research rather than individual clinical diagnosis, which is part of why individual exceptions (very muscular people, certain ethnic groups with different risk profiles at the same BMI, older adults) are common and expected.

BMI as a starting point, not an endpoint

Because BMI only needs height and weight, it's extremely quick and cheap to calculate at scale, which is exactly why it's used so widely in medical intake forms and public health screening. But it says nothing about where fat is distributed on the body (which matters for cardiovascular risk), how much of your weight is muscle versus fat, or your overall fitness and metabolic health. Waist circumference, body fat percentage (like the Navy method calculator on this site), and blood markers all add context BMI alone can't provide.

More tips

• BMI categories for children and teenagers use age- and sex-specific percentile charts rather than the fixed adult thresholds — never apply the adult BMI categories to a child's result.
• Tracking your own BMI over months or years to spot a trend is often more useful than fixating on a single reading, since day-to-day weight naturally fluctuates with hydration, food intake, and other short-term factors.

Frequently asked questions