BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a measure of body fat based on your height and weight. It's widely used by health organizations, including the World Health Organization (WHO), as a simple screening tool to identify whether a person is underweight, at a healthy weight, overweight, or obese.

BMI Formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

BMI Categories (WHO Standards)

• Underweight: BMI below 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obese: BMI 30 or higher

Limitations of BMI

BMI is a useful screening tool but doesn't account for muscle mass, bone density, overall body composition, or differences based on age, sex, or ethnicity. Athletes with high muscle mass may have a high BMI but low body fat. Similarly, older adults may have a normal BMI but higher body fat. For a more complete health assessment, consult a healthcare professional.

Worked Calculation Examples

Seeing the formula applied makes it easier to understand how BMI is derived. In metric units, a person who is 170 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.70)² = 70 ÷ 2.89 ≈ 24.2, which falls in the normal range. In imperial units, someone who is 5 feet 7 inches (67 inches) tall and weighs 154 lb has a BMI of 703 × 154 ÷ 67² = 108,262 ÷ 4,489 ≈ 24.1 — the same result expressed in a different unit system. The healthy weight range for a given height is calculated by multiplying the height in meters squared by 18.5 and 24.9. For a 170 cm person, that range is roughly 53.5 kg to 71.9 kg, which the calculator displays alongside the BMI value.

BMI for Different Populations

The standard BMI thresholds were developed primarily from data on white European populations, and they may not apply equally to everyone. Some Asian populations face health risks at lower BMI values, prompting the WHO to suggest a modified threshold where a BMI of 23 or above may indicate increased risk. For children and teens aged 2 to 19, BMI is calculated the same way but interpreted using age- and sex-specific percentile charts rather than the adult categories — a child at the 85th percentile for their age is considered overweight. Older adults may have a higher body fat percentage at the same BMI due to age-related muscle loss. The calculator provides the standard adult categories, but individual context always matters when interpreting the result.

Practical Use Cases

BMI is most useful as a quick screening tool and a way to track changes over time. Here are some common scenarios where it adds value:

• Routine check-ups: Doctors use BMI as one of several indicators during annual physical exams.
• Weight management: Track your BMI monthly to see whether your diet and exercise plan is moving you toward a healthy range.
• Insurance and employment: Some life insurance and occupational health assessments use BMI as a risk factor.
• Public health research: BMI data helps track population-level trends in obesity and undernutrition.
• Fitness goals: Use the healthy weight range to set a realistic target weight for your height.

Tips for Accurate Measurement

Measure your height and weight under consistent conditions to get a reliable BMI. Weigh yourself in the morning before eating, wearing minimal clothing, and use the same scale each time. Measure height without shoes, standing straight against a wall. Small measurement errors can shift your BMI by a full point, so precision matters — especially near category boundaries. Remember that BMI is a snapshot, not a diagnosis: a single reading is less informative than the trend over weeks and months. If your BMI falls outside the normal range, or if you have concerns about body composition, muscle mass, or overall health, talk to a healthcare professional who can assess factors BMI does not capture.