BMI Formula: How to Calculate Body Mass Index

If you already know the formula, the value of this page is understanding why it works and how to use it correctly. BMI is not a mysterious number. It is just a height and weight ratio with a squared-height adjustment. That makes it simple enough to calculate by hand, but it also means the result has clear limits.

This page shows the metric formula, the U.S. customary formula, step-by-step examples, the reason height is squared, and what the formula actually represents. If you want the bigger context around BMI as a screening tool, read What Is BMI? after this page.

If you want the number now, use the BMI calculator. If you want to understand the math first, keep reading.

BMI formula

The standard BMI formula is straightforward:

In this formula, weight is measured in kilograms and height is measured in meters. Height is squared, which means you multiply the height by itself before dividing. That squared term is what makes BMI scale more fairly across different heights.

Diabetes Canada uses the same shorthand, BMI = kg/m², and notes that BMI is a simple classification based on height and weight. That is why the formula is so easy to apply in clinics, research, and calculators.

A clean way to think about it is this: BMI does not ask what your weight is made of. It only asks how much weight you have for your height. That is enough for screening, but not enough for a full body-composition assessment.

BMI formula (US units)

If you use pounds and inches, the formula is adjusted with a conversion factor:

The number 703 exists because pounds and inches do not line up with the metric units used in the standard BMI equation. It is a conversion factor that makes the U.S. version produce the same BMI result as the metric version.

The logic is the same in both systems. In both cases, BMI compares weight with height, then uses height squared so that taller people are not automatically treated as if they were proportionally heavier. If you want a source that shows both formulas side by side, the CDC calculates BMI with the same metric and U.S. equations.

How to calculate BMI step by step

Manual calculation is useful because it shows exactly where the number comes from. Once you see the steps once or twice, the BMI formula becomes much easier to remember.

Metric example

Suppose a person weighs 70 kg and is 1.75 m tall.

1. Square the height: 1.75 × 1.75 = 3.0625
2. Divide the weight by the squared height: 70 / 3.0625
3. The final BMI is about 22.9

That result falls in the usual healthy weight range for adults. The calculation itself is simple, but the interpretation still depends on context such as muscle mass, waist size, age, and health markers.

US example

Now suppose a person weighs 180 lb and is 70 in tall.

1. Multiply the weight by 703
2. Square the height: 70 × 70 = 4900
3. Divide the adjusted weight by the squared height
4. The result is about 25.8

Cleveland Clinic describes BMI as a calculation based on height and weight, and the CDC publishes the same metric and U.S. formulas used here. The manual method and the calculator method should give the same answer if the inputs are accurate.

Why the BMI formula uses height squared

Taller people naturally weigh more because bigger bodies require more mass. If height were not squared, the formula would not scale fairly across different body sizes. Squaring height helps normalize the relationship between size and weight so that BMI can compare people of different heights using one common scale.

That said, the scaling is not perfect. Some researchers and health writers note that height-squared is still a simplification, which means BMI can understate or overstate weight status in some very short or very tall people. Medical News Today has discussed this limitation in its BMI explainers, and the broader point is straightforward: BMI is useful, but it is still an approximation.

The important thing is not to expect the formula to do more than it was designed to do. BMI is a practical screening measure, not a three-dimensional measurement of the human body.

What the BMI formula represents

The BMI formula represents weight relative to height. That makes it a general estimate of body size, and in population studies it correlates with body fat and health risk. It is useful because it quickly turns height and weight into a standardized number that can be compared across many people.

It is also helpful because the number is easy to interpret. Lower values usually indicate a lighter body for the person's height, and higher values usually indicate a heavier body for the person's height. The categories give that number a practical meaning without requiring a complicated interpretation step.

The limitation is that this relationship is indirect. BMI can suggest that body fat might be higher or lower than expected, but it does not directly inspect body tissue. If you want the larger explanation of what the number means, the What Is BMI? page gives the broader context.

Limitations of the BMI formula

The BMI formula does not measure body fat directly. Diabetes Canada makes this point clearly, noting that body composition and fat distribution can vary widely among people with similar BMI values. That is one of the main reasons BMI should never be used as a standalone answer to health questions.

BMI also does not distinguish muscle from fat. That matters for athletes, strength-trained adults, and other people with more lean mass. Two people can have the same BMI and very different body composition, which means the same number can describe very different realities.

BMI also does not account for where fat is stored. Abdominal fat and body-fat distribution can matter for health, but the formula does not show that. If you want a deeper explanation of those gaps, read Is BMI Accurate? and BMI vs Body Fat.

When to use the BMI formula

The BMI formula is most useful when you need a quick estimate, a simple screening number, or a way to track general trends over time. That is why it remains so common in health settings: it is fast, repeatable, and easy to understand.

It is especially useful at the start of an evaluation. If BMI is clearly outside the healthy range, that can be a prompt to look more closely at waist measurement, activity, sleep, diet patterns, family history, and other health markers. The formula helps decide when the conversation should continue.

For a more practical next step, compare the number with the BMI calculator so you can see the result instantly after understanding the math.

Frequently asked questions