Free BMI Calculator

Free BMI Calculator

Calculate Your Body Mass Index

Body Mass Index (BMI) is a simple measure of body fat based on your weight and height. Use this calculator to find out your BMI and see which category you fall into.
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Your BMI Result

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15 18.5 25 30 40
Underweight Normal Overweight Obese
Note: BMI is a useful screening tool, but it doesn't account for muscle mass, bone density, or fat distribution. Athletes or muscular individuals may have a high BMI without excess body fat. For a complete picture of your health, consider other factors and consult with a healthcare professional.

What is BMI?

Body Mass Index (BMI) is a numerical value calculated from your weight and height. It provides a simple, inexpensive screening method to categorize individuals into weight categories that may indicate health risks. BMI was developed in the early 19th century by Belgian mathematician Adolphe Quetelet and is sometimes called the Quetelet Index.

While BMI doesn't directly measure body fat, research has shown that BMI correlates moderately with more direct measures of body fat such as skinfold thickness, bioelectrical impedance, underwater weighing, and dual-energy X-ray absorptiometry (DEXA).

How is BMI Calculated?

BMI is calculated by dividing a person's weight by the square of their height. The formula differs slightly depending on whether you're using metric or imperial units:

Metric Formula:
BMI = weight (kg) ÷ height² (m²)
Imperial Formula:
BMI = (weight (lbs) ÷ height² (in²)) × 703

Example Calculation

For a person who weighs 160 lbs and is 5'10" (70 inches) tall:

BMI = (160 ÷ 70²) × 703
BMI = (160 ÷ 4900) × 703
BMI = 0.0327 × 703
BMI = 23.0

BMI Categories

The World Health Organization (WHO) and most health organizations use the following BMI categories for adults over 20 years old:

BMI Range Category Health Risk
Below 18.5 Underweight Increased risk of nutritional deficiency
18.5 – 24.9 Normal weight Lowest health risk
25.0 – 29.9 Overweight Moderate risk of health issues
30.0 – 34.9 Obese Class I High risk
35.0 – 39.9 Obese Class II Very high risk
40.0 and above Obese Class III Extremely high risk

Health Risks Associated with BMI

Underweight (BMI < 18.5)

Being underweight can indicate malnutrition, eating disorders, or other health conditions. Health risks associated with being underweight include:

  • Weakened immune system
  • Nutritional deficiencies
  • Osteoporosis and bone loss
  • Fertility problems
  • Increased risk of complications during surgery

Overweight and Obese (BMI ≥ 25)

Excess weight is associated with an increased risk of numerous health conditions:

  • Type 2 diabetes
  • Cardiovascular disease and stroke
  • High blood pressure (hypertension)
  • Certain cancers (breast, colon, kidney)
  • Sleep apnea and breathing problems
  • Osteoarthritis
  • Fatty liver disease
  • Gallbladder disease

Limitations of BMI

While BMI is a useful screening tool, it has several important limitations:

Doesn't distinguish fat from muscle

BMI cannot differentiate between body fat and lean muscle mass. Athletes and muscular individuals may be classified as overweight or obese despite having low body fat.

Doesn't account for fat distribution

Where fat is stored matters. Visceral fat (around organs) is more dangerous than subcutaneous fat (under skin). BMI doesn't measure this.

Age and sex differences

Women typically have more body fat than men at the same BMI. Older adults tend to have more body fat than younger adults at the same BMI.

Ethnic variations

Health risks associated with BMI may vary among different ethnic groups. For example, Asian populations may have higher health risks at lower BMI values.

BMI for Children and Teens

BMI is calculated the same way for children and teens, but the interpretation is different. Instead of fixed categories, BMI is compared to growth charts that account for age and sex:

Percentile Range Category
Below 5th percentile Underweight
5th to 84th percentile Healthy weight
85th to 94th percentile Overweight
95th percentile and above Obese

Other Ways to Assess Body Composition

BMI is just one tool for assessing health. Consider these additional measurements for a more complete picture:

Waist Circumference

Measures abdominal fat. High risk: >40" for men, >35" for women.

Waist-to-Hip Ratio

Compares waist to hip measurements. High risk: >0.90 for men, >0.85 for women.

Body Fat Percentage

Direct measure of fat. Can be measured via calipers, bioelectrical impedance, or DEXA.

Waist-to-Height Ratio

Simple rule: waist should be less than half your height.

Tips for Maintaining a Healthy BMI

  • Eat a balanced diet — Focus on whole foods, lean proteins, fruits, vegetables, and whole grains.
  • Control portion sizes — Use smaller plates and be mindful of serving sizes.
  • Stay physically active — Aim for at least 150 minutes of moderate aerobic activity per week.
  • Build muscle — Resistance training helps increase metabolism and improve body composition.
  • Get adequate sleep — Poor sleep is linked to weight gain and increased appetite.
  • Manage stress — Chronic stress can lead to overeating and weight gain.
  • Stay hydrated — Sometimes thirst is mistaken for hunger.
  • Track your progress — Regular monitoring helps you stay accountable.

When to See a Doctor

Consider consulting a healthcare professional if:

  • Your BMI is outside the normal range (below 18.5 or above 25)
  • You've experienced sudden, unexplained weight changes
  • You're having difficulty losing or gaining weight despite lifestyle changes
  • You have a family history of obesity-related conditions
  • You're experiencing symptoms related to your weight

A healthcare provider can perform additional assessments and help create a personalized plan for achieving and maintaining a healthy weight.