BMI CalculatorFree online fitness & health calculator — instant results, no sign-up
Body mass index with category, healthy weight range, BMI Prime and Ponderal Index.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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How the result changes with age
Each row below was calculated with this tool, keeping every other input at its default and changing only age from half to one and a half times the default value. Use it to see how sensitive the answer is before you enter your own numbers.
| Age (years) | BMI | Category | Healthy weight |
|---|---|---|---|
| 15 | 24.5 kg/m² | Normal | 56.7 – 76.6 kg |
| 23 | 24.5 kg/m² | Normal | 56.7 – 76.6 kg |
| 30 (default) | 24.5 kg/m² | Normal | 56.7 – 76.6 kg |
| 38 | 24.5 kg/m² | Normal | 56.7 – 76.6 kg |
| 45 | 24.5 kg/m² | Normal | 56.7 – 76.6 kg |
About the BMI Calculator
This calculator determines body mass index (BMI), weight category, healthy weight boundaries, BMI Prime, and the Ponderal Index based on height and weight. Adults use these metrics to assess body proportionality relative to standard weight thresholds.
- Age
- Age of the individual in years; standard BMI categories apply primarily to adults aged 20 and older.
- Sex
- Biological sex (Male or Female), used contextually alongside anthropometric tracking.
- Height
- Stature measured in centimeters, typically ranging from 100 to 250 cm.
- Weight
- Total body weight measured in kilograms, typically between 30 and 300 kg.
Formula and method
Height in meters: m = Height (cm) / 100 BMI = Weight (kg) / m² Healthy Weight Lower Bound = 18.5 × m² Healthy Weight Upper Bound = 25.0 × m² BMI Prime = BMI / 25 Ponderal Index = Weight (kg) / m³
- mHeight converted to meters
- BMIBody Mass Index in kg/m²
- BMI PrimeRatio of actual BMI to the standard upper-normal threshold of 25 kg/m²
- Ponderal IndexCorpulence index measuring weight scaled by height cubed in kg/m³
Height in centimeters is divided by 100 to obtain meters. BMI is calculated by dividing weight by height squared, and BMI Prime scales this value against a reference cutoff of 25 kg/m². Results are rounded to one decimal place, except BMI Prime which is rounded to two decimal places.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Age30 years
- SexMale
- Height175 cm
- Weight75 kg
Results
- BMI24.5 kg/m²
- CategoryNormal
- Healthy weight56.7 – 76.6 kg
- BMI Prime0.98
- Ponderal Index14 kg/m³
How to use the BMI Calculator
- 1Convert height in centimeters to meters by dividing by 100.
- 2Divide weight in kilograms by the square of height in meters to calculate BMI.
- 3Multiply the square of height in meters by 18.5 and 25 to establish the healthy weight range.
- 4Divide BMI by 25 to obtain BMI Prime, and divide weight by height cubed to obtain the Ponderal Index.
Reading your result
A BMI between 18.5 and 24.9 kg/m² falls in the Normal category, corresponding to a BMI Prime below 1.0. A BMI Prime above 1.0 indicates a weight above the standard normal ceiling for that height. The Ponderal Index offers an alternative proportionality check that scales body volume across three dimensions.
Limitations
The standard BMI calculation relies entirely on total body mass and height without distinguishing between muscle mass, bone density, and adipose tissue. It does not account for differences in fat distribution, such as visceral versus subcutaneous fat.
Common questions
What is BMI Prime?+
BMI Prime is the ratio of your calculated BMI to 25.0 kg/m² (the standard upper limit of the normal weight range). A value below 0.74 indicates underweight, 0.74 to 1.0 indicates normal weight, and values greater than 1.0 indicate overweight.
How is the Ponderal Index different from BMI?+
While BMI divides body weight by height squared, the Ponderal Index divides weight by height cubed. This cubic relationship often provides a more consistent measure of proportionality across exceptionally tall or short individuals.
How is the healthy weight range calculated?+
The healthy weight range reflects the weights that yield a BMI between 18.5 and 25.0 kg/m² for your specific height.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.