How to Calculate BMI and What Your Number Means

A plain-language guide to body mass index — the formula, the ranges, and where BMI has real limits.

What is BMI?

Body mass index, or BMI, is a number calculated from your height and weight. It was developed in the 1800s by Belgian mathematician Adolphe Quetelet as a way to study population-level weight patterns — not to diagnose individuals. Despite that origin, BMI became a widely used screening tool in medicine and public health because it is fast, free, and requires no equipment beyond a scale and a measuring tape.

BMI does not measure body fat directly. It estimates whether your weight is appropriate for your height by expressing weight relative to height squared. The result is a single number used to place people into broad categories.

The BMI formula

There are two versions of the formula depending on which units you use:

  • Metric: BMI = weight (kg) ÷ height (m)2
  • Imperial: BMI = (weight (lbs) ÷ height (in)2) × 703

For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial terms, a person weighing 154 lbs at 5 feet 9 inches (69 inches) has a BMI of (154 ÷ 692) × 703 = 22.7.

You can skip the math with the free BMI Calculator on this site, which handles both metric and imperial inputs instantly.

What the BMI ranges mean

The standard adult BMI categories used by the CDC and WHO are:

  • Below 18.5 — Underweight
  • 18.5 to 24.9 — Healthy weight
  • 25.0 to 29.9 — Overweight
  • 30.0 and above — Obesity (further divided into Class 1, 2, and 3)

These thresholds were set based on population studies linking weight categories to health risks such as cardiovascular disease, type 2 diabetes, and hypertension. At the extremes — very low or very high BMI — the associations with health problems are generally strong. In the middle ranges, the picture is more nuanced.

Where BMI falls short

BMI has well-documented limitations that are worth understanding before placing too much weight (so to speak) on your number.

It does not distinguish muscle from fat. A muscular athlete may have a BMI that technically places them in the "overweight" range despite having low body fat. Conversely, someone with low muscle mass may have a "healthy" BMI but carry a higher-than-ideal proportion of body fat.

It does not account for where fat is stored. Fat carried around the abdomen (visceral fat) is more closely linked to health risks than fat carried in the hips or thighs. BMI cannot detect this difference.

It may not apply equally across different ethnic groups. Research suggests that people of Asian descent may face increased health risks at lower BMI thresholds, while some studies have found the standard ranges less predictive in certain other populations.

It does not account for age or sex differences in body composition. Older adults tend to have more body fat at the same BMI than younger adults. Women generally have more body fat than men at the same BMI.

How BMI is actually used

Despite its limitations, BMI remains useful as a population-level screening tool and a starting point for conversations about weight and health. Clinicians typically use it alongside other measures — waist circumference, blood pressure, cholesterol, blood glucose, and lifestyle factors — rather than relying on it in isolation.

For personal use, BMI is a reasonable rough check. If your number is well within the healthy range and you feel good, it is unlikely to be cause for concern. If it falls outside the healthy range, it is worth discussing with a doctor alongside a fuller picture of your health.

Calculate your BMI now

Use the free BMI Calculator on this site to get your number instantly — no signup required. You can also use the Ideal Weight Calculator to see weight ranges for your height based on several common formulas.