BMI Calculator

What BMI Measures

Body Mass Index is a ratio of weight to height used as a quick population-level screening tool. For metric units, divide weight in kilograms by height in meters squared. For imperial units, multiply weight in pounds by 703, then divide by height in inches squared.

Enter measurements taken without shoes and with light clothing for the most consistent reading across repeated weigh-ins.

Standard Categories

For adults, the WHO classification places a BMI under 18.5 in the underweight range, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese, with further classes above 35 and 40. Children and teens require age-specific percentile charts instead, since body composition changes rapidly throughout growth and single adult cutoffs do not apply to them.

• Underweight: below 18.5
• Healthy range: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obesity: 30 and above

How to Use This Tool

Enter an accurate height and weight, choose your unit system, and read the resulting index and category. Recalculate after meaningful weight change, and record dates alongside values if you are tracking trends over months rather than reacting to daily fluctuations. Note context such as recent training or illness that can temporarily shift the reading.

Limitations You Should Know

BMI does not distinguish muscle from fat, so very muscular individuals may score overweight despite low body fat. It ignores fat distribution, age-related changes, pregnancy, and differences across ethnic groups with different metabolic risk patterns. The same BMI can represent very different health profiles.

When to Talk to a Professional

Treat BMI as one data point among many: waist circumference, blood pressure, blood sugars, and lifestyle matter as much or more. Discuss results with a doctor or registered dietitian who can interpret them in the context of your medical history and goals.