BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple screening metric that estimates body fat based on height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains the most widely used population-level measure of weight status. BMI is used by healthcare providers, insurers, and public health organizations worldwide.

BMI is calculated by dividing weight in kilograms by the square of height in meters: BMI = weight (kg) / height (m) squared. In imperial units, the formula is BMI = (weight in pounds / height in inches squared) multiplied by 703. The result is a dimensionless number that falls into one of four standard categories.

BMI categories

• Below 18.5 — Underweight: May indicate malnutrition, eating disorders, or other health conditions.
• 18.5 to 24.9 — Normal weight: Associated with the lowest risk of weight-related health problems.
• 25.0 to 29.9 — Overweight: Higher risk for cardiovascular disease, type 2 diabetes, and hypertension.
• 30.0 and above — Obese: Significantly elevated risk for multiple chronic diseases including heart disease, stroke, and certain cancers.

How to use this tool

Enter your height and weight in either metric (cm/kg) or imperial (feet-inches/pounds) units. The calculator instantly computes your BMI and displays which category it falls into. The visual scale shows where your BMI sits relative to the standard ranges.

Limitations of BMI

BMI is a screening tool, not a diagnostic measure. It does not distinguish between muscle mass and fat mass, so athletes and bodybuilders may have a high BMI with very low body fat. It also does not account for bone density, age, sex, ethnicity, or fat distribution. A person with a normal BMI can still carry excess visceral fat around their organs.

For a more complete assessment, healthcare professionals use additional measures like waist circumference, waist-to-hip ratio, body fat percentage (via DEXA scans or calipers), and blood markers. BMI is most useful as a quick population-level screening tool rather than an individual diagnostic.

Practical health tips

If your BMI falls outside the normal range, the most effective first steps are regular physical activity (150 minutes per week of moderate exercise) and balanced nutrition. Small, sustainable changes are more effective than crash diets. Consult a healthcare professional before making significant changes to your diet or exercise routine, especially if you have existing health conditions.

Worked example

For someone 1.75 m tall weighing 72 kg, BMI is 72 / (1.75 x 1.75) = 72 / 3.0625 = about 23.5, which falls in the 18.5-24.9 "normal" range. In imperial units the formula is weight in pounds x 703 / height in inches squared. The value only classifies the ratio of weight to height; it says nothing about where that weight sits or what it is made of.

Common mistakes

BMI's biggest limitation is that it cannot distinguish muscle from fat, so a lean, muscular athlete can register as "overweight" while someone with low muscle mass may sit in the normal range yet carry excess fat. It also was derived from adult populations, so it should not be applied directly to children, pregnant people, or the very elderly without adjusted charts. Treat it as a rough screening tool, not a diagnosis.

Frequently asked questions

Is BMI accurate for children?

Children and teenagers use age- and sex-specific BMI percentile charts rather than the adult categories. A child in the 85th percentile is considered overweight, and the 95th percentile is considered obese. Growth patterns vary significantly during childhood, so percentile-based assessment is more appropriate than fixed thresholds.

What is a healthy BMI for older adults?

Research suggests that slightly higher BMI values (25–27) may be protective for adults over 65, as moderate body reserves help recovery from illness. The standard BMI categories were developed primarily for younger adults and may not apply directly to elderly populations.

Is BMI accurate for athletes?

Often not. Because muscle is denser than fat, athletes with high muscle mass frequently score in the overweight band despite low body fat. For them, measurements like waist circumference, body-fat percentage, or a DEXA scan give a far more meaningful picture than BMI alone.