You can calculate your body mass index (BMI) by dividing your weight in kilograms by your height in meters squared. For example, if you weigh 70 kg and are 1.75 meters tall, your BMI is 22.9. If you use pounds and inches, divide your weight in pounds by your height in inches squared, then multiply by 703. BMI helps assess weight relative to height.
What Is BMI?
Body mass index (BMI) is a numerical measure that compares a person's weight with their height. Healthcare professionals use it as a screening tool to help assess weight status and identify people who may benefit from further evaluation. According to the Centers for Disease Control and Prevention (CDC), BMI is a quick, low-cost measure that can provide useful information about potential health risks when considered alongside other health indicators.
BMI does not directly measure body fat or show where fat is stored in the body. Instead, it provides a simple starting point for understanding weight in relation to height. Its meaning depends on factors such as age and, in some circumstances, ethnicity, muscle mass, and overall health.
How Do You Calculate BMI Using the Formula?
You can calculate BMI yourself using a calculator and two measurements: your weight and your height. The formula depends on which measurement units you use.
Metric formula: kilograms and meters
If you measure your weight in kilograms and your height in meters, use this formula:
BMI = Weight (kg) ÷ [Height (m) × Height (m)]
In other words, multiply your height by itself, then divide your weight by that number.
Imperial formula: pounds and inches
If you use pounds for weight and inches for height, use the following formula:
BMI = [Weight (lb) ÷ Height (in)²] × 703
The number 703 converts the result to the same BMI scale used with metric measurements. The CDC's BMI calculation guidance provides both formulas and explains how to apply them.
How to Calculate BMI Step by Step
Follow these steps to work out your BMI accurately:
- Measure your weight. Use a reliable weighing scale and record your weight in kilograms or pounds.
- Measure your height. Stand upright without shoes and record your height in meters or inches.
- Square your height. Multiply your height by itself. For example, 1.75 × 1.75 = 3.0625.
- Divide your weight by the squared height. If using kilograms and meters, divide your weight in kilograms by the squared height in meters.
- Round the result. You can usually record BMI to one decimal place for easier reading.
Make sure your measurements use the correct units. Entering centimeters instead of meters into the metric formula without adjusting the calculation will produce an incorrect result.
How Do You Calculate BMI With an Example?
Consider an adult who weighs 70 kg and has a height of 175 cm. To calculate BMI, first convert the height into meters: 175 cm equals 1.75 m.
Step 1: Square the height.
1.75 × 1.75 = 3.0625
Step 2: Divide the weight by the squared height.
70 ÷ 3.0625 = 22.857...
Step 3: Round to one decimal place.
BMI = 22.9
For an adult, a BMI of 22.9 falls within the standard healthy-weight category. This classification describes weight relative to height; it does not establish whether the person is healthy in every other respect.
What Do BMI Numbers Mean for Adults?
For most adults aged 20 and older, BMI is interpreted using standard categories. These ranges help healthcare professionals identify weight patterns that may warrant closer assessment.
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 or higher | Obesity |
According to the CDC's BMI frequently asked questions, BMI is a screening measure rather than a complete assessment of an individual's health. The categories provide a useful reference, but a result outside the healthy-weight range does not automatically mean that someone has a medical condition.
Some clinical guidelines also use lower BMI thresholds when assessing health risks in people from certain ethnic backgrounds, including South Asian populations. For example, the UK National Health Service (NHS) guidance on overweight and obesity describes lower thresholds for several ethnic groups. A healthcare professional can help interpret the appropriate range for an individual's circumstances.
Can You Calculate BMI Without a Calculator?
Yes. You can calculate BMI manually using the formula and basic arithmetic. However, a calculator can make the process faster and reduce calculation errors, particularly when working with decimal measurements.
For metric measurements, divide weight in kilograms by height in meters multiplied by itself. For example, a person weighing 60 kg with a height of 1.60 m would calculate BMI as follows:
60 ÷ (1.60 × 1.60) = 60 ÷ 2.56 = 23.4
For imperial measurements, multiply the result of dividing pounds by squared inches by 703. Alternatively, you can use an online BMI calculator and enter your height and weight in the units requested.
How Is BMI Calculated for Children and Teenagers?
BMI is interpreted differently in children and teenagers because their bodies are still growing. Rather than applying the standard adult categories, healthcare professionals compare a child's BMI with reference values for people of the same age and sex. This comparison produces a BMI-for-age percentile.
For children and teenagers aged 2 through 19, the CDC Child and Teen BMI Calculator reports BMI, BMI percentile, and the corresponding category using CDC growth charts. A child's percentile helps show how their BMI compares with that of peers in the reference population.
Children under 2 years old require different growth assessment methods. If you are checking a child's weight or growth, a pediatric healthcare professional can help interpret the measurements in the context of the child's development.
What Are the Limitations of BMI?
Although BMI is easy to calculate, it does not provide a complete picture of body composition or health. It cannot distinguish muscle from fat, measure body fat directly, or show how fat is distributed around the body.
- Muscular individuals: People with substantial muscle mass may have a higher BMI even when they do not have excess body fat.
- Older adults: Changes in muscle mass and body composition can affect how accurately BMI reflects an individual's health.
- Different ethnic backgrounds: Health risks associated with a particular BMI can vary across populations, so standard adult thresholds may not be appropriate for every individual.
- Pregnancy: Weight changes during pregnancy affect BMI interpretation, so the usual adult categories should not be used on their own to assess pregnancy-related weight changes.
- Children and teenagers: Adult BMI categories are not suitable for growing children, whose results must be interpreted using age- and sex-specific references.
The CDC's overview of BMI emphasizes that BMI should be considered alongside other information, including blood pressure, cholesterol levels, medical history, and physical examination findings. Depending on the situation, a healthcare professional may also assess waist circumference, weight trends, and other relevant measures.
When Should You Discuss Your BMI With a Healthcare Professional?
Consider discussing your BMI with a healthcare professional if your result is outside the standard adult range, your weight has changed unexpectedly, or you have concerns about nutrition, growth, or weight-related health risks. An appointment can help clarify what the number means for you and whether any additional assessment is appropriate.
Rather than relying on a single BMI reading, healthcare professionals may consider changes over time, medical history, lifestyle, physical findings, and other measurements. This broader assessment helps put BMI into context and supports more individualized health decisions.