How to Calculate BMI From Weight and Height

What the BMI Calculator does

The BMI Calculator turns a weight measurement and a height measurement into a Body Mass Index value, then assigns a category. It accepts metric and imperial unit choices, making the selected mode important when your measurements use different systems.

Use the calculator when you want a result from the measurements you enter rather than a category based on height or weight alone. The output gives you a rounded BMI figure alongside its category, so you can review both parts of the result together.

Enter measurements in the matching mode

1. Prepare the weight and height values you want to evaluate. Use measurements that belong to the same unit system, because the unit choice determines how the calculation handles them.

2. Enter the weight in the weight field and the height in the height field. In metric mode, type height on a centimeter scale: a height of 175 centimeters is entered as 175, not as 1.75.

3. Select metric or imperial mode so it corresponds to the measurements you entered. Metric mode interprets height in centimeters before calculating BMI, while imperial mode follows the calculator’s separate imperial calculation path.

4. Check the fields before submitting. The declared inputs are optional numeric weight and height values with defaults of 70 and 175, and the unit choice defaults to metric. If those starting values appear, replace them when they do not represent the measurements you intend to use.

5. Submit the calculation and inspect the returned BMI value and category. A weight or height of zero or below does not produce a BMI result; instead, the operation returns an error-bearing response, so correct that measurement with a positive value and try again.

Interpret the number and category

The BMI number is rounded to one decimal place, which makes the displayed result easier to scan while preserving a single decimal position. The category bands are Underweight below 18.5, Normal from 18.5 up to but not including 25, Overweight from 25 up to but not including 30, and Obese at 30 or higher.

Treat the threshold wording as part of the result-reading process. A value of 18.5 starts the Normal band, 25 starts Overweight, and 30 starts Obese; values just below those points remain in the preceding band. Compare the returned number with those boundaries and note the category supplied with it.

Your result reflects the weight, height, and unit mode submitted for that calculation. Changing a measurement or switching modes changes the basis of the calculation, so confirm those choices before using the result for comparison.

Worked example

A person checks a 70-kilogram weight against a 175-centimeter height while metric mode is selected.

Enter 70 kg as the weight, enter 175 cm as the height, keep metric mode selected, and submit the calculation.

The result contains a BMI value rounded to one decimal place and a BMI category.

Limitations

• The result is limited to the weight and height supplied through the selected unit mode; metric height is read in centimeters, while imperial mode uses its separate calculation path.

Common errors

• A height entered in inches while metric mode is selected is read on the centimeter scale, and a zero or negative measurement returns an error-bearing response. Choose the mode that matches your values, enter metric height in centimeters, and use positive measurements before submitting.

FAQ

How are BMI categories assigned?

The calculator assigns Underweight below 18.5, Normal from 18.5 to below 25, Overweight from 25 to below 30, and Obese at 30 or above. The BMI figure shown with the result has one decimal place.

Should I enter height in inches or centimeters?

Metric mode reads the height entry as centimeters, so enter 175 for 175 cm. When your measurements use the imperial system, select imperial mode so the separate imperial calculation path is used.

What happens if I enter zero or a negative number?

A zero or negative weight or height produces an error-bearing response rather than a BMI result. Change the affected entry to a positive measurement before submitting again.