Body Mass Index (BMI) Calculator
The Body Mass Index (BMI) Calculator is a valuable tool for assessing an individual’s BMI value and corresponding weight status, while also taking age into account. To utilize the calculator, you can select either the “Metric Units” tab, which employs the International System of Units, or the “Other Units” tab, which allows you to convert units into either US or metric measurements. Furthermore, this calculator provides the Ponderal Index, which is explained in detail below.
Introduction to BMI
BMI is a measure of an individual’s leanness or corpulence based on their height and weight. Its purpose is to quantify tissue mass and serves as a general indicator of whether a person maintains a healthy body weight relative to their height. The calculated BMI value categorizes individuals as underweight, normal weight, overweight, or obese, based on predefined ranges. These ranges may vary by region and age, and may even have subcategories like severely underweight or very severely obese. While BMI is not a perfect measure of healthy body weight, it remains a useful indicator for identifying the need for further assessment or intervention. Below, you’ll find BMI categories used by the calculator:
BMI Table for Adults
The following table presents the World Health Organization’s (WHO) recommended body weight classifications for adults, applicable to both men and women aged 20 or older:
| Classification | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
BMI Chart for Adults
This graph represents BMI categories based on data from the World Health Organization. The dashed lines indicate subdivisions within the major categorizations.
BMI Table for Children and Teens (Ages 2-20)
For children and teenagers between the ages of 2 and 20, the Centers for Disease Control and Prevention (CDC) recommends BMI categorizations based on percentile ranges:
| Category | Percentile Range |
|---|---|
| Underweight | < 5% |
| Healthy weight | 5% – 85% |
| At risk of overweight | 85% – 95% |
| Overweight | > 95% |
BMI Chart for Children and Teens (Ages 2-20)
The CDC provides BMI-for-age percentile growth charts for both boys and girls.
Risks Associated with Being Overweight
Being overweight increases the risk of various serious diseases and health conditions, as outlined by the Centers for Disease Control and Prevention (CDC). These risks include:
- High blood pressure
- Imbalanced cholesterol levels (high LDL, low HDL, high triglycerides)
- Type II diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Reduced quality of life
- Mental health issues (clinical depression, anxiety, etc.)
- Physical discomfort and limitations
- Increased mortality risk compared to those with a healthy BMI
Generally, it’s advisable for individuals to aim for a BMI below 25 kg/m² and consult their healthcare provider for guidance on improving their health.
Risks Associated with Being Underweight
Being underweight also carries risks, including:
- Malnutrition, vitamin deficiencies, anemia
- Osteoporosis, which weakens bones and increases fracture risk
- Weakened immune function
- Growth and developmental issues, especially in children and teenagers
- Reproductive issues for women due to hormonal imbalances
- Increased risk of miscarriage in the first trimester for underweight women
- Complications following surgery
- Elevated mortality risk compared to those with a healthy BMI
In some cases, being underweight can indicate underlying conditions or diseases like anorexia nervosa, which have additional risks. It’s crucial to consult a healthcare professional if you suspect you or someone you know is underweight, especially if the cause isn’t apparent.
Limitations of BMI
Although BMI is widely used as an indicator of healthy body weight, it has its limitations. BMI doesn’t consider body composition and can be influenced by various factors, including age, sex, ethnicity, muscle mass, body fat, and activity level. In some cases, individuals may have a high BMI due to muscle mass, making it an imperfect measure of body fat.
BMI Formula
The equations for calculating BMI in both the International System of Units (SI) and the US customary system (USC) are provided below, using a 5’10”, 160-pound individual as an example:
USC Units: BMI = 703 × (mass (lbs) / height² (in)) = 703 × (160 / 70²) = 22.96 kg/m²
SI, Metric Units: BMI = (mass (kg) / height² (m)) = (72.57 / 1.782²) = 22.90 kg/m²
BMI Prime
BMI prime is a dimensionless value, which represents the ratio of an individual’s BMI to the upper limit of “normal” BMI, typically 25 kg/m² by WHO and CDC standards. BMI prime allows for a quick assessment of how one’s BMI compares to this upper limit.
| Classification | BMI | BMI Prime |
|---|---|---|
| Severe Thinness | < 16 | < 0.64 |
| Moderate Thinness | 16 – 17 | 0.64 – 0.68 |
| Mild Thinness | 17 – 18.5 | 0.68 – 0.74 |
| Normal | 18.5 – 25 | 0.74 – 1 |
| Overweight | 25 – 30 | 1 – 1.2 |
| Obese Class I | 30 – 35 | 1.2 – 1.4 |
| Obese Class II | 35 – 40 | 1.4 – 1.6 |
| Obese Class III | > 40 | > 1.6 |
Ponderal Index
The Ponderal Index (PI) is a measure similar to BMI, with the key difference being the cubing of height instead of squaring. This formula is more suitable for very tall or short individuals, as it provides a more accurate assessment than BMI in such cases. The PI formula is as follows:
USC Units: PI = (height (in) / ∛mass (lbs)) = (70 / ∛160) = 12.89 in/∛lbs
SI, Metric Units: PI = (mass (kg) / height³ (m)) = (72.57 / 1.783³) = 12.87 kg/m³
Why BMI is Used to Measure Overweight and Obesity
BMI is favored for its simplicity and cost-effectiveness. It relies solely on height and weight measurements, making it a readily accessible tool for health assessments. The calculation can be based on either kilograms and meters or pounds and inches, as shown in “How is BMI Calculated?”
Other Ways to Assess Excess Body Fatness Besides BMI
Alternative methods to assess body fatness include skinfold thickness measurements using calipers, underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and isotope dilution. These approaches are not always widely available, can be expensive, and often require the expertise of highly trained professionals. Additionally, standardizing these methods for consistent comparisons across studies and timeframes can be challenging.
How BMI is Calculated
The calculation of BMI is consistent for both adults and children and relies on the following formulas:
Measurement Units:
- Kilograms and meters (or centimetres): Formula: weight (kg) / [height (m)]²
- Pounds and inches: Formula: weight (lb) / [height (in)]² x 703
For further clarity, consider the examples provided for each formula.
How BMI is Interpreted for Adults
Adults aged 20 and above interpret BMI using standard weight status categories. These categories apply universally to men and women of all body types and age groups. The standard weight status categories associated with BMI ranges are detailed in the table provided.
Is BMI Interpreted the Same Way for Children and Teens as It Is for Adults?
BMI interpretation differs for children and teens, although the same formula is used as for adult BMI. Children and teens’ BMI must be age and sex-specific due to age-related changes in body fat and gender-based variations. CDC BMI-for-age growth charts factor in these distinctions and visually represent BMI as percentile rankings. These percentiles were derived from data collected from the U.S. population of 2- to 19-year-olds through various surveys spanning from 1963-65 to 1988-94.
Obesity among 2- to 19-year-olds is defined as a BMI at or above the 95th percentile compared to children of the same age and sex in the reference population from 1963 to 1994. Specific examples are provided for better understanding.
How Good is BMI as an Indicator of Body Fatness?
While the correlation between BMI and body fatness is relatively strong, individuals with the same BMI may have varying levels of body fat. Factors influencing these differences include gender, racial/ethnic group, age, and physical activity. The accuracy of BMI as an indicator of body fatness tends to improve with higher BMI and body fat levels. Nevertheless, health assessments conducted by trained healthcare providers are essential to evaluate an individual’s health status and risks.
Is an Athlete or Muscle-Bound Person with a BMI Over 25 Considered Overweight?
According to BMI weight status categories, individuals with a BMI between 25 and 29.9 are classified as overweight, while those with a BMI over 30 are considered obese. However, athletes may possess a high BMI due to increased muscle mass rather than excess body fat. Healthcare providers should perform comprehensive health assessments to accurately evaluate an individual’s health status and potential risks.
For further details on health problems associated with obesity, please refer to “Health Effects.“