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BMI Calculator

Calculate your Body Mass Index (BMI) — a widely used screening tool for body weight relative to height. Works in metric (cm / kg) and imperial (ft·in / lbs).

Units

We do our best to keep EasyCalc tools accurate and reliable, but they should not be considered medical advice or a substitute for professional healthcare.

What Is BMI?

Body Mass Index is a number calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and is now the most widely used population-level screening indicator for weight status.

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lbs) ÷ height (in)²

For example, a person who is 170 cm tall and weighs 70 kg has a BMI of 70 ÷ 1.70² = 24.2 — in the normal range.

BMI Categories

The World Health Organization and the National Heart, Lung, and Blood Institute classify adult BMI as follows:

CategoryBMI
UnderweightBelow 18.5
Normal weight18.5 – 24.9
Overweight25.0 – 29.9
Obese (Class I)30.0 – 34.9
Severely Obese (Class II+)35.0 and above

These thresholds apply to adults aged 18 and older and are the same for men and women.

Limitations of BMI

BMI is useful at the population level but has well-known limitations when applied to individuals:

  • Muscle vs fat — BMI measures total mass relative to height, not body composition. A muscular athlete may register as overweight despite having low body fat.
  • Fat distribution — BMI says nothing about where fat is stored. Abdominal fat carries greater health risk than fat in other areas; measures like waist-to-height ratio capture this directly.
  • Age and sex — Older adults tend to carry more body fat at the same BMI. Women typically have more body fat than men at the same BMI value.
  • Ethnicity — Research shows elevated metabolic risk at lower BMI values in people of Asian descent. Adjusted thresholds (overweight ≥ 23.0, obese ≥ 27.5) are recommended for these populations.

When to Use BMI

BMI is a quick, free screening tool for potential weight-related health issues and requires no special equipment. It correlates with cardiometabolic risk across large populations and is routinely used as one input in clinical health assessments.

For a more complete picture, BMI should be read alongside other measures such as waist circumference, waist-to-height ratio, blood pressure, and blood lipid levels. No single number replaces a full clinical assessment.

Sources

Frequently Asked Questions

What is a healthy BMI?

A BMI between 18.5 and 24.9 is considered the normal (healthy weight) range for most adults. Values below 18.5 indicate underweight; 25–29.9 indicates overweight; 30 or above indicates obesity.

How is BMI calculated?

BMI equals weight in kilograms divided by height in meters squared: BMI = kg / m². In imperial units the formula is BMI = 703 × lbs / in². For example, someone who is 5 ft 10 in tall and weighs 160 lbs has a BMI of 703 × 160 / 70² ≈ 23.0.

Is BMI the same for men and women?

Yes — the standard adult BMI formula and the 18.5 / 25 / 30 cutoffs are the same for men and women. However, women typically carry more body fat than men at the same BMI, which is one of the index's known limitations.

Does BMI apply to children?

No. For people under 18, BMI is interpreted as a percentile relative to other children of the same age and sex. The adult categories shown here do not apply to children or teenagers.

What are BMI's main limitations?

BMI does not distinguish fat from muscle or reveal where fat is stored. Muscular athletes can be classified as overweight despite low body fat. It also does not account for age, sex, or ethnicity. For Asian populations, research supports lower cutoffs: overweight at ≥ 23.0 and obese at ≥ 27.5.