This free BMI calculator estimates your Body Mass Index using your height and weight, in either metric or imperial units. Enter your measurements, and the tool instantly returns your BMI value and the standard weight category it falls into. Note: BMI is a general population screening tool, not a medical diagnosis. Please consult a healthcare professional for personal health guidance.
How to Use the BMI Calculator
- Choose metric (kilograms and centimetres) or imperial (pounds and inches).
- Enter your weight and height.
- Your BMI and the corresponding weight category appear instantly.
The BMI Formula
BMI is calculated differently depending on the unit system you use:
- Metric: BMI = weight (kg) ÷ height (m) ²
- Imperial: BMI = 703 × weight (lb) ÷ height (in) ²
The metric version is the primary definition; the imperial version multiplies by 703 to convert pounds and inches into compatible units. Both produce the same result for the same person.
Worked Example
A person weighs 70 kg and stands 1.75 m tall. Metric BMI: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. That falls in the Normal weight range. Using imperial: 154 lb and 69 inches. 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 ≈ 22.7 — essentially the same value (the small difference is rounding in the unit conversion).
BMI Categories
The World Health Organization defines four main adult BMI categories:
- Under 18.5 — Underweight
- 18.5 – 24.9 — Normal weight
- 25.0 – 29.9 — Overweight
- 30.0 and above — Obese
Some health organizations further subdivide the obese category (Class I: 30–34.9, Class II: 35–39.9, Class III: 40+) to stratify health risk more precisely, but the four main bands are the most widely used reference points.
Why Use a BMI Calculator?
BMI offers a quick, equipment-free way to see where your weight stands relative to the population norm for your height. It is used by healthcare systems worldwide as an initial screening metric because it correlates, on a population level, with health risks associated with being underweight, overweight, or obese. For individuals, it is a useful first signal — a BMI in the overweight or obese range is often a prompt to discuss lifestyle changes with a doctor, while a BMI in the underweight range can flag nutritional concerns.
BMI Limitations: What the Number Does Not Tell You
BMI is a population screening tool, not a precise measure of individual health or body composition. It does not distinguish between fat mass and muscle mass, so a highly trained athlete with dense muscle may have a high BMI despite very low body fat. Conversely, an older adult with low muscle mass but significant visceral fat might have a normal BMI while carrying real metabolic risk. BMI also does not account for age, sex, ethnicity, or where body fat is distributed — abdominal fat carries higher risk than fat stored elsewhere. For a complete picture, clinicians consider BMI alongside waist circumference, blood pressure, blood glucose, and other markers. Think of your BMI as one data point, not a verdict.
Frequently Asked Questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered the normal or healthy range. However, healthy body composition can vary by age, sex, and ethnicity, so the number should be interpreted alongside other health measures.
Is BMI accurate for athletes?
Not always. Athletes with high muscle mass may have a BMI in the overweight range despite low body fat. For muscular individuals, BMI overestimates body fat because it cannot distinguish between muscle and fat tissue.
Is this a medical tool?
No. This calculator provides a BMI estimate for general informational purposes. It is not a substitute for professional medical advice, diagnosis, or treatment. Consult a qualified healthcare provider for personal health assessments.