BMI Equation

A doctor mentions your BMI is 27 – but how was that number actually calculated? The BMI equation is a straightforward mathematical formula that relates your weight to your height, producing a single number used worldwide to screen for weight-related health risks. Below you’ll find both versions of the formula, a worked example, and a reference chart.

What Is the BMI Equation?

The Body Mass Index (BMI) equation divides a person’s weight by the square of their height. Belgian mathematician Adolphe Quetelet developed this ratio in the 1830s while studying social physics, which is why it is sometimes called the Quetelet index.

The result is a dimensionless number that places a person into one of several weight-status categories – underweight, normal weight, overweight, or obese – established by the World Health Organization WHO.

The BMI Formula: Metric and Imperial

The equation exists in two forms, depending on the units you use.

Metric BMI Equation

BMI = weight (kg) ÷ height (m)²

1. Measure your weight in kilograms.
2. Measure your height in meters and square it (multiply it by itself).
3. Divide weight by the squared height.

Imperial BMI Equation

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

1. Measure your weight in pounds.
2. Measure your height in inches and square it.
3. Divide weight by the squared height.
4. Multiply the result by 703.

The constant 703 converts imperial units so both formulas produce the same BMI value for the same person.

Step-by-Step Calculation Example

Using the metric formula for a person who weighs 75 kg and is 1.78 m tall:

BMI = 23.7 – which falls in the normal weight category.

The same person in imperial units: 165.3 lbs, 70.1 in.

Rounding differences account for the 0.1 variation; both methods yield the same BMI.

BMI Categories for Adults

The WHO classifies BMI results for adults aged 20 and older as follows:

For children and teens aged 2–19, the CDC recommends using BMI-for-age percentile charts because body composition varies significantly during growth.

How Accurate Is the BMI Equation?

The BMI equation is a screening tool, not a diagnostic instrument. It estimates weight-related risk but has well-documented limitations:

• No distinction between fat and lean mass. A muscular person can register as overweight despite low body fat.
• No measure of fat distribution. Abdominal (visceral) fat carries higher health risk than fat stored in the hips or thighs, but BMI cannot tell the difference.
• Age and sex blind. Older adults tend to carry more fat than younger adults at the same BMI; women naturally carry more fat than men at the same BMI.
• Ethnic variability. Research shows that health risks begin at lower BMI values for some Asian populations – the WHO suggests a revised overweight threshold of 23.0 for these groups.

For a fuller picture of metabolic health, clinicians pair BMI with waist circumference, body fat percentage, blood pressure, and blood markers.

When to Use a Different Metric

Consider alternatives when the BMI equation may misclassify:

• Athletes and bodybuilders – body fat percentage via skinfold calipers or DEXA scans is more reliable.
• Elderly adults – the waist-to-height ratio may better predict cardiovascular risk.
• Pregnant women – BMI is not applicable during pregnancy; weight gain guidelines from ACOG should be followed instead.

This article provides general information and is not a substitute for professional medical advice. Consult a healthcare provider for personalized assessment.

Quick Reference: BMI Equation at a Glance

• Metric: BMI = kg ÷ m²
• Imperial: BMI = 703 × lbs ÷ in²
• Normal range: 18.5 – 24.9
• Best for: Population-level screening and trend tracking
• Limitation: Does not measure body composition directly

Use the calculator above to find your BMI instantly – no manual arithmetic required.