BMI Calculator (Body Mass Index)

What Is BMI (Body Mass Index)?

Body Mass Index (BMI) is a numerical measurement calculated from a person's weight and height. It is commonly used by healthcare professionals and public health organizations as a screening tool for classifying weight status.

BMI is calculated by dividing body weight in kilograms by the square of height in meters. The result is expressed in kilograms per square meter (kg/m²).

For most adults aged 20 and older, BMI is classified into four main categories: underweight, healthy weight, overweight, and obesity.

Although BMI is useful for population research and initial health screening, it cannot distinguish between body fat, muscle, bone, and other body tissues.

Why Is BMI Important?

BMI provides a quick, accessible measurement that can support discussions about weight-related health risks.

• Estimate body weight relative to height.
• Identify standard adult BMI categories.
• Support initial screening for possible weight-related health concerns.
• Compare population-level weight trends.
• Provide context alongside other health measurements.

BMI should not be treated as a complete assessment of health. Blood pressure, blood glucose, cholesterol, waist circumference, body composition, and medical history may provide important additional information.

How to Use the BMI Calculator

Our Body Mass Index Calculator can be used with metric or imperial measurements.

1. Select your preferred units: Choose metric or imperial measurements.
2. Enter your height: Provide your height in centimeters or feet and inches.
3. Enter your weight: Provide your current body weight in kilograms or pounds.
4. Calculate your BMI: Use the calculator to obtain your Body Mass Index.
5. Review your category: For adults aged 20 and older, compare the result with standard BMI categories.
6. Interpret your result: Consider the limitations of BMI and other relevant health information.

The BMI formula itself does not require age or sex. However, age is important when choosing the appropriate interpretation system. Children and teenagers need age- and sex-specific BMI-for-age assessments.

BMI Formula: How Is Body Mass Index Calculated?

BMI is calculated using a person's weight and height. The mathematical method is straightforward and works with either metric or imperial measurements.

1. Metric BMI Formula (Kilograms and Meters)

BMI = Weight (kg) ÷ [Height (m)]²

Where:

• Weight is measured in kilograms (kg).
• Height is measured in meters (m).
• Height is squared by multiplying the height by itself.

Example: BMI for 70 kg and 175 cm

Consider an adult weighing 70 kilograms with a height of 175 centimeters.

Step 1: Convert height into meters.

175 cm ÷ 100 = 1.75 m

Step 2: Square the height.

1.75 × 1.75 = 3.0625 m²

Step 3: Divide weight by height squared.

BMI = 70 ÷ 3.0625

BMI = 22.86 kg/m²

This result falls within the standard healthy weight BMI category for adults.

2. Imperial BMI Formula (Pounds and Inches)

If you use pounds and inches, the BMI formula includes a unit conversion factor.

BMI = [Weight (lb) × 703] ÷ [Height (in)]²

The factor 703 is a rounded conversion factor used when calculating BMI from pounds and inches.

Example: BMI for 160 lb and 5 ft 10 in

Step 1: Convert height into total inches.

(5 × 12) + 10 = 70 inches

Step 2: Square the height.

70 × 70 = 4,900 in²

Step 3: Multiply body weight by 703.

160 × 703 = 112,480

Step 4: Divide by height squared.

BMI = 112,480 ÷ 4,900

BMI = 22.96 kg/m²

The result is approximately 23.0, which is within the standard adult healthy weight BMI range.

3. BMI Formula Using Centimeters

You can also calculate BMI directly from weight in kilograms and height in centimeters:

BMI = [Weight (kg) × 10,000] ÷ [Height (cm)]²

Example:

BMI = (70 × 10,000) ÷ (175 × 175)

BMI = 700,000 ÷ 30,625

BMI = 22.86 kg/m²

All three formulas calculate the same index, with small differences possible from rounding and unit conversion.

BMI Chart for Adults: Underweight, Normal, Overweight and Obesity

The U.S. Centers for Disease Control and Prevention (CDC) uses the following BMI categories for adults aged 20 years and older.

Note: These categories are general screening classifications, not diagnoses. They do not account for individual differences in muscle mass, body fat distribution, or medical history.

What Does an Underweight BMI Mean?

A BMI below 18.5 falls within the underweight category for adults. Possible explanations include naturally low body weight, inadequate nutrition, medical conditions, or other individual factors.

A low BMI does not automatically indicate poor health, but unexplained weight loss or nutritional concerns should be discussed with a qualified healthcare professional.

What Is a Healthy BMI?

For adults, a BMI from 18.5 to less than 25.0 is classified as healthy weight under standard CDC categories.

However, having a BMI in this range does not guarantee good metabolic or cardiovascular health. Physical activity, diet quality, body fat distribution, and other health factors remain important.

What Does an Overweight BMI Mean?

A BMI from 25.0 to less than 30.0 is classified as overweight in adults. This category indicates body weight above the standard healthy BMI range for a given height.

It does not necessarily mean that an individual has excess body fat. People with substantial muscle mass may have higher BMI values without corresponding levels of excess body fat.

What Does an Obesity BMI Mean?

An adult BMI of 30.0 or greater falls within the obesity category. Higher BMI values are associated with increased risks of several chronic conditions at the population level.

However, BMI alone cannot establish the presence, severity, or cause of a medical condition. A clinical assessment should consider other health measurements and individual circumstances.

How to Calculate a Healthy Weight Range Using BMI

BMI can be used to estimate the body weight range corresponding to standard adult healthy BMI values at a given height.

The standard adult healthy BMI interval is 18.5 to less than 25.0 kg/m².

Healthy Weight Formula

Weight (kg) = Target BMI × [Height (m)]²

To calculate the lower and upper boundaries:

Lower Weight Boundary = 18.5 × Height (m)²

Upper Weight Boundary = 25 × Height (m)²

The upper boundary corresponds to BMI 25, which is excluded from the standard healthy category.

Example: Healthy BMI Weight Range for 170 cm

Height = 170 cm = 1.70 meters

Height squared = 1.70 × 1.70 = 2.89

Lower boundary: 18.5 × 2.89 = 53.47 kg

Upper boundary: 25 × 2.89 = 72.25 kg

For a height of 170 cm, the standard healthy BMI interval corresponds to a weight of approximately 53.47 kg to less than 72.25 kg.

This calculation represents a BMI-based screening interval, not an individualized ideal weight recommendation.

Healthy BMI Weight Chart by Height

Values are rounded to two decimal places. The upper boundary is excluded because BMI 25.0 begins the overweight category. These values are intended for adult BMI screening and are not individualized medical targets.

What Is BMI Prime?

BMI Prime is a dimensionless ratio comparing a person's BMI with a selected BMI reference threshold.

For standard adult BMI categories, the commonly used reference threshold is 25.

BMI Prime Formula

BMI Prime = BMI ÷ 25

BMI Prime Calculation Example

If your BMI is 22.5:

BMI Prime = 22.5 ÷ 25

BMI Prime = 0.90

This indicates that your BMI is 90% of the reference threshold of 25.

How to Interpret BMI Prime

• Below 0.74: Corresponds to BMI below 18.5.
• 0.74 to less than 1.00: Corresponds to the standard adult healthy BMI range.
• 1.00 to less than 1.20: Corresponds to BMI 25 to less than 30.
• 1.20 or higher: Corresponds to BMI 30 or greater.

BMI Prime is another way to express the same weight-to-height relationship. It does not provide a separate measurement of body fat or metabolic health.

What Is the Ponderal Index?

The Ponderal Index (PI), also called the corpulence index in some contexts, is another measurement relating body mass to height.

Unlike BMI, which divides weight by height squared, the mass-over-height-cubed version of the Ponderal Index divides weight by height cubed.

Ponderal Index Formula

Ponderal Index = Weight (kg) ÷ [Height (m)]³

Ponderal Index Example

For an adult weighing 70 kg with a height of 1.75 meters:

PI = 70 ÷ (1.75 × 1.75 × 1.75)

PI = 70 ÷ 5.359375

Ponderal Index = 13.06 kg/m³

The Ponderal Index may be useful in certain research contexts, but it has different applications and interpretation standards from adult BMI.

Important: The term Ponderal Index is also used for a reciprocal-style equation in some references. The formula and units must be specified when comparing results.

BMI Calculator for Men and Women

The standard adult BMI formula is the same for men and women:

BMI = Weight (kg) ÷ Height (m)²

The CDC's standard adult BMI categories also apply to both sexes. However, the relationship between BMI and body composition can differ.

BMI for Men

Men tend to have a greater average proportion of fat-free mass than women. Consequently, BMI alone may not accurately represent body fat percentage, especially among highly muscular individuals.

BMI for Women

Women generally have a higher average body fat percentage than men at the same BMI. Age, hormonal changes, and body fat distribution can also influence the relationship between BMI and health.

BMI categories should not be interpreted as a complete assessment of health for either sex. Waist measurements, body composition, and other relevant health indicators may provide additional context.

BMI for Children and Teenagers

BMI assessment for children and teenagers differs from BMI assessment for adults.

Because children and adolescents are still growing, their BMI must be interpreted relative to age- and sex-specific growth references.

The CDC uses BMI-for-age percentiles for children and teenagers aged 2 through 19 years.

Severe obesity in children and teens uses additional criteria, including percentages of the age- and sex-specific 95th BMI percentile.

Important: A calculator that only applies adult BMI categories should not classify children or teenagers using adult thresholds.

For children and adolescents, use an appropriate BMI-for-age calculator or consult a qualified healthcare professional.

BMI and Health Risks: What Does Your BMI Indicate?

BMI categories are associated with differences in health risks across populations, but an individual's BMI cannot diagnose a medical condition.

Potential Concerns Associated With Low BMI

Depending on the cause and individual circumstances, low body weight may be associated with:

• Inadequate nutritional intake.
• Nutrient deficiencies.
• Reduced muscle mass.
• Lower bone density in some individuals.
• Underlying medical conditions.

Potential Concerns Associated With Higher BMI

At the population level, higher BMI values, especially in obesity categories, are associated with increased risk of conditions such as:

• Type 2 diabetes.
• High blood pressure.
• Cardiovascular disease.
• Obstructive sleep apnea.
• Osteoarthritis.
• Certain types of cancer.

Risk depends on multiple factors, including abdominal fat distribution, physical fitness, genetics, metabolic health, and medical history.

Neither a low nor a high BMI should be used alone to establish an individual's health status.

Limitations of BMI: Why BMI Is Not Always Accurate

Although BMI is widely used, it has important limitations when evaluating individual health and body composition.

1. BMI Does Not Distinguish Muscle From Fat

Muscle and fat both contribute to body weight. A person with substantial muscle mass may have a high BMI even without excessive body fat.

2. BMI Does Not Measure Body Fat Percentage

BMI is calculated solely from height and weight. It does not determine the percentage of body weight made up of fat.

3. BMI Does Not Show Fat Distribution

Where body fat is stored can influence health risks. BMI does not distinguish abdominal fat from fat stored elsewhere.

4. BMI May Differ in Meaning Across Populations

People of different ages, sexes, and ancestral backgrounds may have different body composition or metabolic risk at the same BMI.

5. BMI Does Not Evaluate Overall Fitness

Two individuals with identical BMI values may have very different levels of cardiovascular fitness, strength, metabolic health, and physical activity.

6. BMI Has Special Limitations During Pregnancy

Pregnancy-related weight changes affect BMI. Standard adult BMI interpretation is not suitable as a standalone assessment of weight status during pregnancy.

For a more complete assessment, BMI should be considered alongside relevant clinical measurements and professional evaluation when appropriate.

BMI vs. Body Fat Percentage: What Is the Difference?

BMI and body fat percentage measure different aspects of body composition.

For additional context about body composition, use our Body Fat Calculator.

BMI vs. FFMI: Which Measurement Is More Useful?

Fat-Free Mass Index (FFMI) measures fat-free mass relative to height squared, while BMI uses total body weight.

BMI Formula

BMI = Total Body Weight (kg) ÷ Height (m)²

FFMI Formula

FFMI = Fat-Free Mass (kg) ÷ Height (m)²

FFMI can help provide additional information about fat-free mass among people interested in body composition or resistance training.

However, neither BMI nor FFMI directly measures overall health or physical performance.

Learn more using our FFMI Calculator.

How to Support a Healthy Weight and Overall Health

A balanced approach to weight management focuses on nutrition, physical activity, sleep, and sustainable habits rather than relying exclusively on a BMI target.

1. Eat a Balanced Diet

Include a variety of vegetables, fruits, whole grains, protein sources, and other nutrient-rich foods appropriate to your individual needs.

2. Stay Physically Active

Regular movement supports cardiovascular health, physical function, and weight management. Choose activities appropriate to your fitness level and medical circumstances.

3. Include Strength Training

Resistance training can help maintain or develop muscle mass, which is important for physical function and healthy aging.

4. Get Adequate Sleep

Consistent, sufficient sleep supports many aspects of health and can influence appetite regulation and everyday activity.

5. Avoid Extreme Dieting

Very restrictive diets may be difficult to sustain and can lead to inadequate nutrient intake. Individualized support may be appropriate when significant weight changes are being considered.

6. Consider Other Health Indicators

Waist circumference, blood pressure, blood glucose, cholesterol, physical fitness, and medical history may provide a more complete picture of health than BMI alone.

If you want to estimate your daily energy needs, try our TDEE Calculator.

Frequently Asked Questions About BMI

What does BMI stand for?

BMI stands for Body Mass Index. It is a measurement calculated from body weight and height, commonly used to classify weight status in adults.

How do I calculate BMI?

Divide your body weight in kilograms by your height in meters squared. For example, a person weighing 70 kg and measuring 1.75 m tall has a BMI of approximately 22.86.

What is a healthy BMI for adults?

Under standard CDC categories, a BMI of 18.5 to less than 25.0 is classified as healthy weight for adults aged 20 and older. This classification does not guarantee good health.

What BMI is considered overweight?

For adults, a BMI from 25.0 to less than 30.0 is classified as overweight under standard CDC categories.

What BMI is considered obesity?

An adult BMI of 30.0 or greater falls within the obesity category. This category is further divided into three classes based on BMI level.

Is BMI calculated differently for men and women?

No. The standard BMI formula is identical for adult men and women. However, body composition can differ at the same BMI, so interpretation should consider individual characteristics.

Is BMI accurate for athletes and bodybuilders?

BMI may be less informative for highly muscular individuals because it cannot distinguish muscle mass from fat mass. Body composition measurements may provide additional context.

Does BMI measure body fat percentage?

No. BMI measures body weight relative to height. It does not directly calculate body fat percentage or identify where fat is stored.

What is BMI Prime?

BMI Prime is calculated by dividing BMI by a reference BMI threshold, commonly 25 for standard adult classifications. It shows how a BMI compares proportionally with that threshold.

What is the difference between BMI and BMR?

BMI is an index of body weight relative to height. Basal Metabolic Rate (BMR) describes energy expenditure under basal resting conditions. They measure different things.

Can I calculate BMI using pounds and feet?

Yes. Convert your height into total inches, then use BMI = Weight (lb) × 703 ÷ Height (in)². You can also use the imperial measurement option in the calculator.

What is the ideal weight for my height?

BMI can be used to calculate a weight interval corresponding to the standard adult healthy BMI category. However, it cannot determine an individualized ideal weight for every person.

Is BMI accurate for children?

Children and teenagers require BMI-for-age percentile interpretation because their bodies are growing. Standard adult BMI categories should not be applied to children or teenagers.

Does age affect BMI?

Age is not included in the standard BMI formula. However, age can affect body composition and the interpretation of results. Children and teenagers require age-specific assessment.

Can someone have a healthy BMI but excess body fat?

Yes. A person can fall within the standard healthy BMI category while having a higher body fat percentage or unfavorable fat distribution. BMI alone cannot evaluate these factors.

Can someone have a high BMI but low body fat?

Yes. Some highly muscular individuals have elevated BMI values despite relatively low body fat percentages.

Is BMI the same worldwide?

The mathematical BMI formula is widely used internationally. However, some health organizations recommend additional screening thresholds for particular populations because weight-related metabolic risks may differ.

How often should I check my BMI?

There is no universal requirement to calculate BMI at a particular frequency. Rechecking it when your weight changes meaningfully or during a routine health assessment may be useful.

Can BMI diagnose obesity-related diseases?

No. BMI is a screening measure, not a diagnostic test. Healthcare professionals assess medical history, physical findings, and other relevant measurements to evaluate health risks or diseases.

Should I use BMI during pregnancy?

Standard adult BMI categories should not be used alone to assess weight status during pregnancy. Pregnancy-related weight recommendations generally consider pre-pregnancy BMI and individualized clinical factors.

Scientific References and Trusted Sources

The following resources explain BMI calculations, adult BMI categories, BMI-for-age interpretation, and clinical limitations.

1. Centers for Disease Control and Prevention (CDC). Adult BMI Calculator and Categories. Read CDC guidance.
2. Centers for Disease Control and Prevention (CDC). Child and Teen BMI Categories. Read CDC guidance.
3. World Health Organization (WHO). Obesity and Overweight. Read WHO fact sheet.
4. National Heart, Lung, and Blood Institute (NHLBI). Calculate Your Body Mass Index. Visit NHLBI resource.

Medical Disclaimer

This BMI Calculator is provided by FitnessCalculator.org for general educational and informational purposes only. Results are estimates based on the measurements entered and standard body mass index formulas.

BMI is a screening measure and cannot diagnose obesity-related diseases, determine body fat percentage, or replace a comprehensive medical assessment.

Adult BMI categories are not appropriate for interpreting growth in children and teenagers. Pregnancy and certain medical conditions also require individualized assessment.

If you have concerns about your weight, nutritional status, unexplained weight changes, or overall health, consult an appropriately qualified healthcare professional.

For additional information, please review our Medical Disclaimer.