How to Calculate BMI: Formula, Units and a Worked Example

BMI (body mass index) is weight in kilograms divided by height in meters squared. In U.S. units, it is weight in pounds divided by height in inches squared, multiplied by 703. The result is a single number used to screen for weight categories, not a direct measure of body fat.

The formula

The CDC gives two ways to calculate BMI, one for metric units and one for U.S. customary units (CDC BMI FAQ).

Metric:  BMI = weight (kg) / height (m)^2
U.S.:    BMI = weight (lb) / height (in)^2 x 703

The 703 in the U.S. version is a conversion factor. It lets pounds and inches produce the same result that kilograms and meters would.

For adults aged 20 and older, the CDC groups the result into these categories (CDC adult BMI categories):

Obesity is divided further: class 1 is 30 to less than 35, class 2 is 35 to less than 40, and class 3 is 40 or higher. The CDC page presents BMI as one health indicator that is considered alongside other factors.

Using it in NurseTools

NurseTools is an iPhone app for nurses that is coming soon to the App Store. Nothing is downloadable yet. The BMI calculator will be one of two interactive tools available at launch.

Here is how it is planned to work:

• Choose metric or U.S. units. Switching converts the values you already entered, in place.
• Enter weight and height in fields with steppers.
• The result updates live as you type.
• A scale marker shows where the result sits. It is neutral, with no diagnostic colors.
• Copy result and Reset buttons are included.
• A “Formula & details” section shows the formula, the units, the source, the version and the review status.

The tool shows the math. It does not interpret the result or recommend care. NurseTools has no account, never asks for patient data and works offline.

Worked example

This example uses made-up numbers. It is not a patient.

Metric version: weight 70 kg, height 1.75 m.

1. Square the height: 1.75 x 1.75 = 3.0625.
2. Divide weight by that: 70 / 3.0625 = 22.857…
3. Round to one decimal place: 22.9.

U.S. version, with different made-up numbers: weight 154 lb, height 69 in.

1. Square the height: 69 x 69 = 4,761.
2. Multiply the weight by 703: 154 x 703 = 108,262.
3. Divide: 108,262 / 4,761 = 22.739…
4. Round to one decimal place: 22.7.

Reading the result against the CDC table above, both numbers fall in the 18.5 to less than 25 row. Rounding to one decimal place matters near a boundary, so it helps to know which rounding a tool uses. The “Formula & details” section in NurseTools is where that kind of detail will be visible.

Limits and common pitfalls

• BMI is not a direct measure of body fat. The CDC says it does not distinguish between fat, muscle and bone mass (CDC: About BMI). Two people with the same BMI can have very different body composition.
• Adult categories do not apply to children and teens. For ages 2 through 19, BMI is compared with children of the same age and sex using BMI-for-age percentiles (CDC child and teen BMI).
• Unit mix-ups are easy to make. Pounds with meters, or kilograms with inches, give a meaningless number. Check that weight and height use the same system.
• Forgetting to square the height is a common arithmetic slip. Dividing by height alone gives a much larger result.
• Forgetting the 703 in the U.S. formula gives a number far too small.
• Rounding differs between tools. One decimal place is common, but a value just under or over a category line can change depending on when rounding happens.
• BMI is one input among many. The CDC advises considering it with other factors, such as medical history, health behaviors, physical exam findings and lab results (CDC: About BMI).

A BMI number answers a narrow question: how weight compares with height. Anything beyond that comes from the rest of the clinical picture.