BMI Calculator

Calculate your Body Mass Index (BMI) in metric or imperial units and learn which weight category you fall into based on World Health Organization ranges.

Understanding BMI

BMI is weight in kilograms divided by height in metres squared. It is a screening tool, not a diagnosis, and does not measure body composition.

What BMI Measures

Body Mass Index is a screening tool that estimates body fatness based on height and weight. It is calculated by dividing weight in kilograms by height in meters squared. BMI categories include underweight below 18.5, normal weight from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 and above. While BMI does not directly measure body fat, it correlates with more direct measures and serves as an accessible initial assessment of weight-related health risk.

Limitations of BMI

BMI does not distinguish between muscle and fat mass, meaning muscular individuals may be classified as overweight despite low body fat. It does not account for fat distribution, age-related changes in body composition, or ethnic differences in body fat patterns. Athletes, elderly individuals, and certain ethnic groups may be misclassified. For a more complete picture, combine BMI with waist circumference, body fat percentage from our Body Fat Calculator, and other health markers.

Using BMI for Health Planning

Despite its limitations, BMI remains useful as a population-level screening tool and starting point for health discussions. Track BMI trends over time rather than focusing on a single measurement. Combine with other metrics for a comprehensive health assessment. If your BMI indicates overweight or obesity, our Calorie Calculator and Fitness Guide provide evidence-based strategies for healthy weight management.

BMI Categories and Health Risk

Research consistently shows correlations between BMI categories and health outcomes at the population level. BMI values in the overweight and obese ranges are associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and joint problems. However, individual risk depends on many factors beyond BMI including fitness level, fat distribution, family history, and metabolic health markers. Some individuals with normal BMI have poor metabolic health while some with elevated BMI are metabolically healthy. Use BMI as one data point among many in assessing your health status.

Frequently Asked Questions

How is BMI calculated?

Body Mass Index is calculated by dividing your weight in kilograms by the square of your height in metres (BMI = kg ÷ m²). In imperial units, the formula is weight in pounds divided by height in inches squared, multiplied by 703. The result is a single number used to screen for underweight, healthy weight, overweight, and obesity ranges. Our calculator accepts both metric and imperial units and returns your BMI along with the World Health Organization category it falls into.

What are the BMI categories?

Using World Health Organization ranges for adults, a BMI below 18.5 is classed as underweight, 18.5 to 24.9 as healthy weight, 25 to 29.9 as overweight, and 30 or above as obese. These thresholds are screening guidelines, not diagnoses, and the same number can mean different things for different bodies. They are most useful for spotting broad trends across populations and for prompting a closer look at your health rather than for labelling any individual.

Is BMI accurate for everyone?

No. BMI is a simple ratio of weight to height and cannot tell the difference between muscle and fat. Very muscular people, such as athletes, may register as overweight despite low body fat, while some people within a healthy BMI may carry excess fat. It also does not account for age, sex differences in body composition, or where fat is stored. Treat BMI as one quick screening tool to be considered alongside measures like body fat percentage, waist circumference, and overall fitness.

What should I do if my BMI is outside the healthy range?

A BMI outside the healthy range is a prompt to look more closely, not a reason to panic. Consider it together with other indicators such as waist measurement, activity level, diet, and how you feel. If you are concerned, a doctor or registered dietitian can interpret your BMI in context and recommend safe, sustainable changes. Sudden or extreme measures are rarely helpful; gradual improvements to nutrition and activity tend to produce lasting results.

Does BMI differ for children and adults?

Yes. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult categories, because growing bodies change composition with age. The calculator's standard categories apply to adults. For children, a pediatric BMI-for-age percentile from a healthcare provider is the appropriate measure.