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Free BMI Calculator

Calculate your Body Mass Index instantly using your height and weight. Evaluate where you stand on standard adult health categories.

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Calculate your BMI in seconds using US or metric units.

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Educational Resources

Transparent mathematical formulas, adult reference charts, and context explaining the clinical limitations of BMI for various body types.

Health Fundamentals

What Is Body Mass Index (BMI)?

Body Mass Index (BMI) is an anthropometric index devised in the 19th century by Belgian mathematician and statistician Adolphe Quetelet. It evaluates the mathematical relationship between an individual’s weight and their height. In clinical and public health settings, BMI is widely recognized as a quick, non-invasive screening metric to identify weight categories that may correlate with health outcomes.

Rather than directly measuring adiposity (body fat), BMI measures excess weight relative to skeletal frame size. Medical organizations including the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) utilize BMI to monitor population trends and screen individuals who may benefit from further preventive health evaluations.

Mathematical Formulas

How Is BMI Calculated?

BMI calculations rely on standard mathematical equations based on whether measurements are recorded in metric or US customary units:

Metric Formula

BMI = weight (kg) ÷ [height (m)]²

Divide your weight in kilograms by your height in meters squared. For example, 70 kg ÷ (1.75 m × 1.75 m) = 22.86 kg/m².

US Customary Formula

BMI = 703 × weight (lb) ÷ [height (in)]²

Multiply your weight in pounds by 703, then divide by your height in inches squared. For example, (703 × 160 lbs) ÷ (69 in × 69 in) = 23.63.

Explore step-by-step formula explanations and worked examples
Adult Categories

Adult BMI Categories & What Is a Healthy BMI?

For adults aged 20 and older, standard clinical classifications group Body Mass Index values into four primary categories:

  • Underweight: BMI less than 18.5
  • Healthy / Normal Weight: BMI between 18.5 and 24.9
  • Overweight: BMI between 25.0 and 29.9
  • Obesity: BMI of 30.0 or higher (categorized into Class 1, Class 2, and Class 3)

A BMI within the 18.5 to 24.9 bracket is generally associated with optimal longevity and a reduced statistical incidence of cardiovascular and metabolic disorders in broad epidemiologic datasets. However, it is crucial to recognize that BMI is a screening metric, not an absolute indicator of personal metabolic wellness.

World Health Organization (WHO) Adult BMI Classifications

Standard international adult reference ranges. Note that BMI is an epidemiological screening metric and should be evaluated alongside individual clinical indicators.

Adult Body Mass Index classifications and health risk associations
CategoryBMI Range (kg/m²)Classification StatusGeneral Health Assessment
Severe Thinness< 16.0Underweight (Severe)Elevated risk of nutrient deficiencies and low bone mineral density
Moderate Thinness16.0 – 16.9Underweight (Moderate)Increased risk of nutritional compromise and fatigue
Mild Thinness17.0 – 18.4Underweight (Mild)Mild risk; may be typical for some lean body types
Normal / Healthy Weight18.5 – 24.9Healthy WeightStatistically lowest correlation with weight-related chronic conditions
Pre-Obesity / Overweight25.0 – 29.9OverweightSlightly elevated cardiovascular and metabolic risk; evaluate body composition
Obesity Class I30.0 – 34.9Obesity (Class 1)Moderate risk of hypertension, dyslipidemia, and type 2 diabetes
Obesity Class II35.0 – 39.9Obesity (Class 2)Severe risk of cardiovascular complications and sleep apnea
Obesity Class III≥ 40.0Obesity (Class 3 / Severe)Very severe health risk; comprehensive clinical management recommended

Clinical context: These cutoffs apply to adults age 20 and older. Some health organizations (such as the WHO Asian expert consultation) suggest alternative cutoffs (such as overweight beginning at 23.0 and obesity at 27.5) for populations with differing body fat distributions.

Important Scientific Context

Understanding the Limitations of BMI

While BMI is helpful for generalized population screening, it possesses inherent physiological limitations because it does not differentiate between lean body mass, bone structure, and fat tissue:

Muscular Individuals & Athletes

Muscle is significantly denser than adipose tissue. Athletes and heavy weightlifters frequently have high BMIs despite having single-digit body fat percentages.

Pregnancy & Nursing

BMI does not apply during pregnancy due to the weight of the growing fetus, amniotic fluid, and maternal fluid retention.

Older Adults & Sarcopenia

As individuals age, natural loss of skeletal muscle (sarcopenia) can mask increased body fat, meaning an older adult might have a “normal” BMI while carrying elevated visceral fat.

Ethnic & Genetic Diversity

Research indicates that health risks associated with body fat can manifest at lower BMI cutoffs in individuals of South Asian, East Asian, and other demographic backgrounds.

Clinical Guidance

When to Talk With a Healthcare Professional

If your calculated BMI falls outside the standard 18.5 – 24.9 range, there is no reason to panic. Rather, consider it an informative prompt to have a comprehensive conversation with your primary care physician or a registered dietitian.

A qualified medical professional will typically assess your overall health by reviewing:

  • Waist-to-hip ratio and abdominal adipose distribution
  • Blood pressure, fasting blood glucose, and HbA1c
  • Lipid profile (HDL, LDL, and triglycerides)
  • Personal and family cardiovascular history
  • Daily dietary patterns, sleep hygiene, and physical activity levels

Explore Targeted BMI Tools & Guides

Access specialized calculators and reference resources tailored to your measurement preference and informational needs.

US Customary

Pounds & Inches Calculator

Dedicated calculator optimized for standard US height (feet/inches) and weight (lbs).

Metric Standard

KG & CM Calculator

Streamlined calculator for international metric units (centimeters and kilograms).

Female Health

BMI Calculator for Women

Insights into female body fat distribution, hormonal cycles, and interpretation nuances.

Male Health

BMI Calculator for Men

Examines muscle density impact, visceral fat accumulation, and male fitness goals.

Helpful Reference

Frequently Asked Questions About BMI

Find clear, evidence-based answers to common questions about Body Mass Index calculation, accuracy, and interpretation.

Body Mass Index (BMI) is an anthropometric screening tool that estimates body mass relative to height. It is calculated by dividing weight in kilograms by height in meters squared (or 703 × weight in pounds divided by height in inches squared). It provides an easy, non-invasive method to categorize weight status across broad adult populations.