How to Calculate BMI (Body Mass Index), With the Formula and Examples
BMI = weight in kilograms ÷ height in metres squared. If you measure in pounds and inches, BMI = 703 × weight (lb) ÷ height (in) squared. This guide shows both formulas worked end to end, the standard WHO category table, and what BMI can and cannot tell you.
BMI relates your weight to your height as a single screening number. In metric units it is
BMI = weight (kg) ÷ height (m)². If you measure in pounds and inches, it isBMI = 703 × weight (lb) ÷ height (in)². The result is unitless and conventionally rounded to one decimal place.
BMI is used as a quick screening figure, not a measurement of body fat. You can compute it with the BMI calculator or by hand in a few steps.
The two formulas
There are two versions of the same calculation, so pick the one that matches your units. The conversion factor in the imperial version rescales pounds and inches into the same metric result, so both give the same BMI for the same person. BMI is reported as a plain number with no units, technically kilograms per square metre.
Metric worked example
Work in metres, not centimetres. Take someone who weighs seventy kilograms and stands one and three-quarter metres tall. Square the height, divide weight by that figure, then round to one decimal place. A height given in centimetres must be divided by one hundred to reach metres before you square it.
That result falls in the normal range on the WHO table below.
Imperial worked example
Convert the height to whole inches first. For someone measured in pounds and inches, turn the feet-and-inches height into a single inch count before anything else, since one foot equals twelve inches. Then square that inch count, multiply the weight by the conversion factor, divide, and round. Forgetting to convert feet to inches is the most common mistake here.
That result sits just inside the overweight range.
What the BMI categories mean
For adults aged twenty and over, the World Health Organization uses a standard set of categories that are the same regardless of sex. The cut-offs are population screening thresholds, not personal targets, so a value a tenth or two either side of a boundary does not signal a meaningful change in health.
By this table, the metric example lands in normal weight and the imperial example in overweight.
What BMI does not tell you
BMI is a screening tool, not a diagnosis. It estimates whether weight may be a concern at a population level, but it cannot measure health on its own. It does not separate muscle from fat, so a muscular person can score as overweight while carrying little body fat. It ignores fat distribution, even though where fat sits on the body matters for health. It is calibrated for adults, so children and teenagers use age- and sex-specific percentile charts instead of the fixed adult cut-offs. The standard thresholds also do not adjust for age, sex, or ethnicity, and some health bodies use lower thresholds for certain populations.
For these reasons BMI is best read as one input among several, and any concern about weight is a matter for a qualified clinician rather than a single number.
Quick reference
Convert centimetres to metres and feet to inches before squaring, round to one decimal place, and report the result as a unitless number. BMI is a screening figure, not a diagnosis. To skip the arithmetic, enter your weight and height into the BMI calculator and it returns your BMI and category in both unit systems.
Common questions
For most adults the WHO classes the normal range as roughly the middle band of the category table. That range is a screening band, not a guarantee of health, and it does not replace a clinical assessment.
Not reliably. Because muscle is denser and heavier than fat, athletes and other very muscular people often record a BMI in the overweight or obese range despite low body fat. For these individuals, measures such as body-fat percentage are more informative than BMI alone.
BMI falls when weight goes down relative to height, since height is fixed for adults. Reducing weight lowers the numerator in the formula and therefore the BMI. The amount and means of any weight change are individual matters best discussed with a healthcare professional.
