Body mass index, or BMI, relates body mass to squared height. It provides a common way to describe weight relative to height without directly measuring body composition. This calculator accepts metric or imperial measurements, converts them to kilograms and metres and returns BMI along with a BMI Prime comparison.
The category bands on this page are adult bands. There is no age input, so it does not calculate a child’s BMI-for-age percentile. BMI is a screening measure rather than a diagnosis or a direct body-fat percentage. A result can help explain a height-and-weight relationship, but it cannot distinguish muscle from fat or describe health on its own.
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How to Use This Calculator
Enter standing height and choose centimetres, metres, inches or feet. The form takes a single number, so use total inches or decimal feet rather than writing separate feet and inches.
Enter weight and select kilograms or pounds. Check that the selected unit matches the scale reading.
Select the gender option required by the form. The current controller uses the same BMI equation and adult category thresholds for either option.
Calculate and read BMI and BMI Prime. The program rounds BMI to two decimal places before calculating Prime and assigning its category.
Use the result as one screening reference, keeping the original measurements and relevant age/context. For children or a clinical decision, use the appropriate assessment rather than applying these adult labels automatically.
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Formula Used
BMI = weight in kilograms / (height in metres)². BMI Prime = BMI/25. Pounds are converted using approximately 0.453592 kg per pound.
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Variables Explained
Input
Meaning in this calculator
Height
Standing height, converted to metres before squaring. A unit error is magnified because height appears squared.
Weight
Measured mass converted to kilograms. It is not an estimated ideal weight or a weight-loss target.
Gender
Required by the page, but it does not change the arithmetic in this implementation.
BMI
Ratio reported conventionally in kg/m² and displayed to two decimals.
BMI Prime
BMI divided by 25, making 25 correspond to 1.00. It is a scaled comparison, not a probability or percentage of body fat.
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Worked Example
For weight 70 kg and height 1.75 m, squared height is 1.75×1.75 =3.0625 m². BMI =70/3.0625 =22.8571, displayed as 22.86. BMI Prime =22.86/25 =0.9144, displayed as 0.91. This falls within the 18.5-to-below 25 adult band used by the page.
If weight changes to 80 kg at the same height, BMI becomes 26.12 and Prime 1.04 after rounding. If instead both height and weight are accidentally entered without matching units, the program can return an extreme answer rather than correcting the interpretation for you. For example,175 selected as metres does not mean 175 centimetres. Repeating the 70 kg example with 175 cm should give the same 22.86 and is a useful unit check.
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Why Use This Calculator?
The current adult category boundaries are below 18.5,18.5 to below 25,25 to below 30 and 30 or more. A value very close to a boundary may change category when rounded to two decimals, because the controller assigns categories after that rounding. The boundary is a classification convention, not a sudden change in an individual’s condition.
CDC describes BMI as one screening measure and distinguishes adult interpretation from child and teen assessment. It does not directly measure fat, muscle or the location of body fat. This page also has no inputs for pregnancy or other circumstances that can alter the usefulness of a simple weight-height comparison. Read its labels alongside appropriate clinical context rather than using a category alone to prescribe a diet, exercise programme or target weight.
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Common Mistakes
Do not enter five feet seven inches as 5.7 feet. Use 67 inches or the correctly converted decimal-foot value.
Do not enter pounds while leaving kilograms selected, or centimetres while leaving metres selected.
Do not interpret BMI Prime 0.91 as 91% body fat. It means BMI is about 0.91 times 25.
Do not use these adult category labels as a child growth assessment or as a diagnosis.
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Frequently Asked Questions
Does the gender selection change BMI?
Not in this implementation. The controller reads the field but applies the same weight/height² expression and the same adult category bands. Other assessments may use age or sex-specific references, but this page does not introduce those through its gender choice.
What is BMI Prime useful for?
It expresses BMI relative to 25. A Prime of 1.20 corresponds to BMI 30, while 0.80 corresponds to BMI 20. It is just a rescaled BMI value and does not add a separate measurement of health, fitness or tissue composition.
Why can a muscular person have a high BMI?
The equation only sees total mass and height. It does not separate muscle from fat or identify why weight is high relative to height. This is one reason the screening number should be considered alongside other information rather than treated as a body-fat measurement.
Can I calculate BMI for a teenager?
The arithmetic can be performed, but the displayed adult category is not a BMI-for-age percentile. This form has no age field or growth-chart reference. Use an age-appropriate child and teen assessment if that is the question being asked.
Why does changing height slightly affect the answer?
Height is squared in the denominator, so a small height error changes BMI more than the same proportional error in weight. Measure carefully, keep units correct and avoid treating small differences between rounded results as clinically meaningful by themselves.
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