What Is BMI? How to Calculate and Understand Body Mass Index

Body Mass Index (BMI) is a simple number, calculated from your height and weight, that health organizations use as a quick screening indicator. This guide explains what BMI is, how to work it out, what the standard categories mean, and — just as importantly — what BMI cannot tell you.

What BMI Is

BMI is a single value derived from a person’s height and weight. It is meant to give a rough, at-a-glance idea of whether someone may be under-weight or over-weight relative to their height.

Health organizations have used BMI widely for decades, mainly because it is cheap, fast, and easy to calculate. It needs no special equipment beyond a scale and a way to measure height, which makes it practical for screening large groups of people.

It is important to frame BMI correctly from the start: it is a screening tool, not a measure of health or a diagnosis. A BMI number can suggest that a closer look might be worthwhile, but it does not, on its own, describe how healthy a person is.

The BMI Formula

BMI uses the same idea in both measurement systems: weight divided by height squared. The only difference is the units and a conversion factor for imperial measurements.

Metric formula:

BMI = weight (kg) ÷ height (m)²

So you divide your weight in kilograms by your height in metres multiplied by itself.

Imperial formula:

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

Here you multiply your weight in pounds by 703, then divide by your height in inches multiplied by itself. The 703 factor converts the result so it matches the metric scale.

Both formulas produce the same number for the same person — they are just two routes to the same value. If you would rather skip the arithmetic, our free BMI calculator does both versions for you and returns the category instantly.

A Worked Example

Suppose someone weighs 70 kg and is 1.75 m tall. Using the metric formula:

1. Square the height: 1.75 × 1.75 = 3.0625
2. Divide the weight by that result: 70 ÷ 3.0625 ≈ 22.9

So their BMI is approximately 22.9, which falls within the normal/healthy weight range in the standard adult chart below.

The Standard Adult BMI Categories

For adults, the World Health Organization publishes the following widely used ranges. The same chart applies to adult men and women.

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal / healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

The obesity range is sometimes divided further into classes — class I (30.0–34.9), class II (35.0–39.9), and class III (40.0 and above) — though this extra detail is not always needed for a basic screening.

These cut-off points are deliberately round numbers chosen for convenience. A BMI just above or just below a boundary is not meaningfully different from one just on the other side, so the categories are best read as broad bands rather than sharp lines.

Important Limitations

BMI is useful precisely because it is simple, but that simplicity is also its biggest weakness. It is worth understanding these limitations before reading much into any single number.

It does not measure body fat directly. BMI only uses height and weight, so it cannot tell muscle from fat. A very muscular person, such as an athlete, may have a high BMI and be classed as “overweight” despite carrying little excess fat. The reverse can also happen.

It ignores fat distribution. Two people with the same BMI can carry weight very differently, and where fat is stored can matter. BMI cannot capture this.

It does not account for age, sex, ethnicity, or body frame. These factors all affect how a given BMI relates to an individual, and a single universal scale cannot reflect them.

It is not designed for everyone. Standard adult BMI categories are not appropriate for children and teenagers, who are assessed using age- and sex-specific percentile charts instead. BMI also has limited usefulness for pregnant people, some athletes, and some older adults, whose body composition may not fit the assumptions behind the scale.

In short, BMI is a population-level screening tool. It works reasonably well for spotting general trends across large groups, but it is far less precise for describing any one individual.

A Closing Note

BMI is one rough data point — no more and no less. It can be a helpful starting point for a conversation, but it cannot diagnose health, and a number alone should never be a cause for alarm.

For any decisions about your health, the best step is to speak with a qualified healthcare professional. They can consider the full picture, including measurements and context that BMI leaves out. If you simply want to find your number and see which category it falls into, our free BMI calculator gives you both instantly.