BMI calculator.
Check your body mass index using metric or imperial units, then review categories, formulas, examples, and key limitations.
Use the BMI Calculator.
Results update automatically as you type. No account is required.
Enter your values
Your BMI on the scale
Where your result falls across the standard adult categories.
About the BMI calculator
Body Mass Index, usually shortened to BMI, is a quick way to compare body weight with height. It turns height and weight into one number that can help indicate whether an adult may be underweight, within a commonly used healthy range, overweight, or in an obesity category.
This BMI calculator works with both metric and imperial units. Enter your height and weight, choose the unit system, and PRECISR instantly calculates your BMI, shows the matching adult category, and places your result on the visual BMI scale above.
What BMI measures
BMI is not a direct measurement of body fat. It is a screening number based on height and weight. Because it is simple and consistent, BMI is widely used in health education, population research, and first-pass weight status checks. It can be useful for spotting patterns, but it should not be treated as a complete medical diagnosis.
A person with a high BMI may have excess body fat, but may also have more muscle mass. A person with a normal BMI can still have health risks depending on activity level, waist size, blood pressure, blood sugar, cholesterol, family history, and other factors.
BMI formulas
| Unit system | Formula | Example |
|---|---|---|
| Metric | BMI = weight kg ÷ height m² | 72 kg ÷ 1.78² = 22.7 |
| Imperial | BMI = 703 × weight lb ÷ height in² | 703 × 160 ÷ 70² = 23.0 |
BMI categories for adults
The table below uses common adult BMI ranges. These ranges are intended for adults and are not the same as children’s BMI percentiles.
| BMI range | Category | What it usually suggests |
|---|---|---|
| Below 16 | Severe thinness | Very low weight for height; medical review may be important. |
| 16 – 16.9 | Moderate thinness | Below the usual adult weight range. |
| 17 – 18.4 | Mild thinness | Slightly below the usual adult weight range. |
| 18.5 – 24.9 | Healthy weight | Commonly considered the normal adult BMI range. |
| 25.0 – 29.9 | Overweight | Above the usual range; health risk may rise for some people. |
| 30.0 – 34.9 | Obesity class I | Higher weight status that may increase health risk. |
| 35.0 – 39.9 | Obesity class II | Higher obesity category; professional guidance may be useful. |
| 40.0 and above | Obesity class III | Highest adult BMI category; often linked with greater health risk. |
Worked BMI examples
| Example | Calculation | Result |
|---|---|---|
| Metric | Weight 72 kg, height 178 cm → 72 ÷ 1.78² | BMI ≈ 22.7, healthy weight range |
| Imperial | Weight 160 lb, height 70 in → 703 × 160 ÷ 70² | BMI ≈ 23.0, healthy weight range |
| Higher BMI example | Weight 210 lb, height 70 in → 703 × 210 ÷ 70² | BMI ≈ 30.1, obesity class I |
Healthy BMI range and healthy weight
For many adults, the healthy BMI range is commonly listed as 18.5 to 24.9. You can also use BMI to estimate a rough healthy weight range for a given height. For example, the lower end can be estimated by 18.5 × height² and the upper end by 24.9 × height², using height in metres.
This range is only a guide. It does not replace personal medical advice, especially for people who are pregnant, older adults, athletes, people with high muscle mass, or anyone managing a health condition.
BMI Prime
BMI Prime compares your BMI with the upper end of the commonly used healthy adult range. It is calculated as:
BMI Prime = BMI ÷ 25
A BMI Prime below 1 means the BMI is below 25. A BMI Prime above 1 means the BMI is above 25. This can be helpful when comparing how close a result is to the upper healthy-range boundary.
Ponderal Index
The Ponderal Index is another height-and-weight ratio. Instead of dividing by height squared like BMI, it divides by height cubed. This can sometimes be useful for very short or very tall people, where BMI may exaggerate or understate body size.
| Measure | Formula | Main idea |
|---|---|---|
| BMI | weight ÷ height² | Common adult screening number. |
| Ponderal Index | weight ÷ height³ | Alternative ratio that may behave differently at height extremes. |
Limitations of BMI
BMI can be helpful, but it has important limitations. It does not separate fat, muscle, bone, and water weight. It also does not show where body fat is stored. Waist circumference, body fat percentage, blood pressure, blood tests, physical activity, and medical history may give a fuller picture.
BMI may be less accurate for athletes and bodybuilders, older adults, pregnant people, children and teenagers, and people whose body composition differs from the population averages used to create broad BMI categories.
BMI for children and teenagers
Children and teenagers should not usually be evaluated with the same fixed adult BMI ranges. Pediatric BMI is typically compared with BMI-for-age percentiles because body fat changes with age and differs between boys and girls during growth. This calculator is designed for adult BMI estimates.
Health context
A BMI below or above the commonly used range can be a reason to look more closely at health, nutrition, activity, and body composition. Being underweight may be linked with nutritional deficiencies or other health concerns. A high BMI may be associated with higher risk of conditions such as high blood pressure, type 2 diabetes, sleep apnea, and heart disease. The actual risk for an individual depends on many factors.
⚠ Disclaimer: BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, fat distribution, age, pregnancy, or ethnicity. Athletes may have a high BMI due to muscle, not fat. Children and teenagers use BMI-for-age percentiles rather than fixed adult ranges. Always consult a qualified healthcare professional for medical assessment.
FAQ
How does the BMI calculator work?
Enter your height and weight, then choose metric or imperial units. The calculator applies the standard BMI formula and shows your BMI category instantly.
What is the BMI formula?
For metric units, BMI = weight in kilograms ÷ height in metres squared. For imperial units, BMI = 703 × weight in pounds ÷ height in inches squared.
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is commonly considered a healthy weight range. BMI below 18.5 is usually underweight, 25 to 29.9 is overweight, and 30 or higher is in an obesity category.
Is BMI accurate?
BMI is useful as a quick screening tool, but it is not perfect. It does not directly measure body fat or account for muscle mass, body frame, pregnancy, age, sex, ethnicity, or fat distribution.
Why can athletes have a high BMI?
BMI uses total weight, not body composition. Muscle is dense, so muscular athletes can have a high BMI even when their body fat level is not high.
Can BMI be used for children?
Children and teenagers usually use BMI-for-age percentiles instead of fixed adult BMI ranges. This calculator is intended for adult BMI estimates.
What is BMI Prime?
BMI Prime is BMI divided by 25, the upper end of the common healthy adult BMI range. A BMI Prime above 1 means the BMI is above 25.
What should I do if my BMI is high or low?
Treat the result as a starting point. A healthcare professional can interpret it alongside waist size, activity level, body composition, blood pressure, lab results, medical history, and personal goals.