BMI Calculator

What Is a BMI Calculator?

A BMI Calculator is a free online tool that calculates your Body Mass Index (BMI) using your height and weight. BMI is a screening measure that helps assess whether an adult's weight falls within a standard range for their height.

Our body mass index calculator supports metric (cm/kg) and imperial (ft/in/lb) units. It provides your BMI score, adult weight category, BMI Prime, Ponderal Index, and a BMI-based healthy weight range.

For children and teenagers aged 2–19, BMI must be interpreted using age- and sex-specific BMI percentile charts rather than adult BMI categories.

How to Use the BMI Calculator

Follow these simple steps to calculate BMI online:

1. Select Units: Choose Metric (cm/kg) or Imperial (ft/in/lb).

2. Enter Your Age: Provide your age so the calculator can display the appropriate BMI interpretation guidance.

3. Select Gender: Choose Male or Female.

4. Enter Height: Add your height in centimeters or feet and inches, depending on your selected units.

5. Enter Weight: Enter your weight in kilograms or pounds.

6. Click Calculate: View your BMI score and available results.

For adults aged 20 and older, the calculator displays a BMI category, healthy weight range, BMI Prime, Ponderal Index, and BMI spectrum.

You can also view BMI Progress Tracking to compare calculations made during your session.

How to Calculate BMI: Formula and Example

The BMI formula divides weight by height squared. The formula changes slightly depending on whether you use metric or imperial measurements.

Metric BMI Formula

$\mathrm{BMI}=\frac{\text{Weight (kg)}}{[\text{Height (m)}]^2}$

Imperial BMI Formula

$\mathrm{BMI}=\frac{703\times\text{Weight (lb)}}{[\text{Height (in)}]^2}$

BMI Calculation Example

Suppose a person has:

• Height: 180 cm (1.80 m)

• Weight: 65 kg

• Age: 30 years

Using the metric formula:

$\mathrm{BMI}=\frac{65}{1.80^2}$

$\mathrm{BMI}=\frac{65}{3.24}\approx20.06$

Result: A BMI of 20.06 falls within the standard healthy weight category for adults.

The BMI calculation uses the same height-to-weight formula for adult men and women. However, BMI does not directly measure body fat or account for differences in muscle mass and body composition.

Adult BMI Chart and Categories

The following BMI chart shows the standard adult BMI classifications used for people aged 20 and older.

These categories are used for screening, not diagnosing a health condition.

The calculator's visual BMI spectrum uses the broader categories of Underweight, Normal, Overweight, and Obesity.

For more information, see the CDC's adult BMI guidance.

Understanding Your BMI Calculator Results

After you calculate BMI, the tool displays several measurements to help you understand the result.

BMI Score and Category

Your BMI score represents the relationship between your weight and height.

For adults, the calculator identifies whether the score falls within the underweight, normal weight, overweight, or obesity range.

For example, a BMI of 20.06 is classified as normal weight for an adult.

Healthy Weight Range

The calculator estimates an adult healthy weight range using BMI thresholds of 18.5 and 24.9.

The formulas are:

$W_{\min}=18.5\times H^2$

$W_{\max}=24.9\times H^2$

Where (H) is height in meters and weight is measured in kilograms.

For a person who is 180 cm tall, the calculator displays an estimated healthy weight range of approximately 59.9–81.0 kg.

This is a BMI-based reference range, not a personalized ideal weight recommendation.

You can also compare other weight estimates using our Ideal Weight Calculator.

BMI Prime

BMI Prime compares your BMI with the standard adult overweight threshold of 25.

$\text{BMI Prime}=\frac{\mathrm{BMI}}{25}$

For a BMI of 20.06:

$\text{BMI Prime}=\frac{20.06}{25}\approx0.80$

A BMI Prime of 1.0 corresponds to a BMI of 25. Values below 1.0 include both underweight and healthy-weight BMI values.

Ponderal Index

The Ponderal Index is another height-to-weight measurement that uses height cubed instead of squared.

$PI=\frac{\text{Weight (kg)}}{[\text{Height (m)}]^3}$

For someone weighing 65 kg at 1.80 m:

$PI=\frac{65}{1.80^3}\approx11.1\ \text{kg/m}^3$

The Ponderal Index provides another way to describe body proportions, but it does not directly measure body fat or diagnose health conditions.

BMI Progress Tracking

The BMI Progress Tracking chart shows BMI values from calculations made during your session.

You can change your height or weight, calculate again, and compare the displayed BMI results.

BMI Calculator for Women and Men

A BMI calculator for women and a BMI calculator for men use the same standard adult BMI formula.

For example, an adult man and woman with the same height and weight will have the same calculated BMI.

However, the same BMI does not necessarily mean they have the same body fat percentage or health risk. Age, muscle mass, body composition, and other factors can affect how BMI should be interpreted.

Our calculator includes a gender selection, but the basic BMI calculation is based on height and weight.

For a more detailed estimate of body composition, you can explore our Lean Body Mass Calculator.

BMI for Children and Teenagers

BMI is calculated using the same height and weight formula for children, teenagers, and adults. However, the interpretation is different.

For children and teenagers aged 2–19, BMI should be evaluated using BMI-for-age percentiles that account for age and sex.

Adult BMI categories and adult healthy weight ranges should not be used to classify children and teenagers.

For example, a 15-year-old with a BMI of 20.06 cannot be classified using the adult healthy weight range alone. Their BMI needs to be compared with an appropriate age- and sex-specific growth chart.

Our calculator displays the BMI value and advises users aged 2–19 to use BMI-for-age percentile guidance.

For more information, see the CDC's child and teen BMI guidance.

Is BMI an Accurate Measure of Health?

BMI is a useful screening tool, but it has limitations.

It does not directly measure body fat, distinguish muscle from fat, or show where body fat is distributed.

For example, an athlete with substantial muscle mass may have a higher BMI without having excess body fat. Similarly, someone with a BMI in the standard healthy range may still have health risks that BMI cannot identify.

BMI is best interpreted alongside other information, such as waist measurements, medical history, and relevant health assessments.

The World Health Organization provides additional information about BMI and weight-related health risks.