BMI Calculator — Free Body Mass Index Tool

What is BMI?

Body Mass Index (BMI) is a simple numerical value calculated from your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and is widely used today as a quick screening tool for body weight categories.

The formula is straightforward: BMI = weight (kg) ÷ height² (m²). For imperial units, the formula adjusts to: BMI = 703 × weight (lb) ÷ height² (in²).

BMI categories

BMI Range	Category	Health Risk
Below 18.5	Underweight	Increased risk of nutritional deficiency
18.5 – 24.9	Normal weight	Lowest health risk
25.0 – 29.9	Overweight	Moderate increased risk
30.0 and above	Obese	High to very high increased risk

Limitations of BMI

While BMI is useful as a general population screening tool, it has well-documented limitations. It does not distinguish between fat mass and muscle mass, meaning a very muscular athlete may have a "high" BMI despite having very low body fat. It also doesn't account for how fat is distributed in the body — abdominal fat carries higher health risks than fat stored in other areas.

For a more complete picture of your health, consider also tracking waist circumference, body fat percentage, and other metabolic markers in consultation with your doctor.

Frequently asked questions

How accurate is BMI?

BMI is a reasonable proxy for body fatness at the population level, but individual accuracy varies. Studies show it correctly classifies excess body fat in roughly 50–70% of individuals. It is less accurate for athletes, older adults, and people of certain ethnicities.

Is BMI different for men and women?

The standard BMI formula and categories are the same for adult men and women. However, women tend to have more body fat than men at the same BMI, so some researchers argue for sex-specific thresholds. The standard categories remain the most widely used clinical reference.

What BMI is considered healthy?

The World Health Organization defines a BMI between 18.5 and 24.9 as "normal weight" and the range associated with the lowest health risk in adults. For Asian populations, some health organisations use lower thresholds (e.g. 23.0 for overweight).

How do I calculate BMI manually?

In metric units: divide your weight in kilograms by your height in metres squared. For example, a person who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.