Toolvica

BMI Calculator

Calculate Body Mass Index for adults and children. Free, fast, and accurate.

Adult (20+)Child (2–19)
US unitsMetric units
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Table of Contents

How to Use Our BMI Calculator

  1. Choose between Adult (20+) or Child (2–19) mode using the toggle switch.
  2. Select your preferred unit system: US (feet/inches and pounds) or Metric (centimeters and kilograms).
  3. Enter your height and weight in the input fields. Results update instantly as you type.
  4. For children and teens, also enter age (years) and select sex — these are required for accurate percentile comparison.
  5. View your BMI result with a color-coded category and visual chart showing where you fall.
  6. Use the Try buttons for quick examples, or the copy and share buttons to save or send your result.

What is BMI?

Body Mass Index (BMI) is a numerical value calculated from your weight and height. It is one of the most commonly used screening tools to estimate whether a person's weight is in a healthy range.

BMI provides a quick, inexpensive, and non-invasive way to categorize individuals into weight status categories. For adults, BMI is interpreted using standard categories that apply to both men and women.

For children and teens aged 2 to 19, BMI is interpreted differently — it is compared against age-and-sex-specific growth charts to determine a percentile ranking.

How to Calculate BMI

BMI = Weight (kg) ÷ Height² (m²)

Metric example:

  1. Convert height to meters: 170 cm = 1.70 m
  2. Square the height: 1.70² = 2.89
  3. Divide weight by height squared: 65 ÷ 2.89 = 22.5
  4. BMI = 22.5 (Healthy Weight)

US example:

  1. Multiply weight by 703: 160 × 703 = 112,480
  2. Square the height in inches: 70² = 4,900
  3. Divide: 112,480 ÷ 4,900 = 22.96
  4. BMI = 23.0 (Healthy Weight)

Adult BMI Categories (CDC)

ClassificationBMI Range (kg/m²)
Underweight< 18.5
Healthy Weight18.5 – 24.9
Overweight25.0 – 29.9
Obesity — Class 130.0 – 34.9
Obesity — Class 235.0 – 39.9
Obesity — Class 3≥ 40

Standard adult BMI categories used by the CDC for ages 20 and older.

Child & Teen BMI (Ages 2–19, CDC)

For children and teens, BMI is interpreted differently than for adults.

CategoryPercentile Range
Underweight< 5th percentile
Healthy Weight≥ 5th to < 85th percentile
Overweight≥ 85th to < 95th percentile
Obesity≥ 95th percentile

CDC BMI-for-age categories based on sex- and age-specific percentile charts for ages 2–19.

BMI Risks and Limitations

BMI is a screening measure and a surrogate marker of body fat, not a diagnosis. The conditions below are associated with higher or lower BMI in population studies — they are not guaranteed outcomes for any individual. Additional measurements, such as waist circumference or body fat percentage, can provide a more complete picture.

Risks of Being Overweight

  • •Type 2 diabetes
  • •Heart disease and stroke
  • •Joint problems

Risks of Being Underweight

  • •Malnutrition
  • •Weakened immune system
  • •Bone density loss
  • •Growth issues in children
  • •Fertility problems

Limitations of BMI

While BMI is a useful screening tool, it does not directly measure body fat. It cannot distinguish between muscle mass and fat mass.

Athletes and muscular individuals may have a high BMI due to muscle weight rather than excess fat. Older adults tend to have more body fat than younger people with the same BMI.

For children and teens, BMI must be interpreted using age- and sex-specific percentiles because body composition changes as they grow. For a complete health assessment, BMI should be used alongside other measurements.

Frequently Asked Questions

Is BMI accurate for athletes?
BMI can overestimate body fat in athletes because muscle is denser than fat. Athletes often have a high BMI but low body fat. Consider using waist measurement or body fat percentage alongside BMI for a more complete picture.
Does BMI differ by gender or age?
For adults, BMI is interpreted the same way for men and women. For children and teens aged 2–19, BMI is compared against age-and-sex-specific percentile charts from the CDC. Older adults tend to have more body fat than younger people with the same BMI.
Is BMI the same as body fat percentage?
No. BMI is a screening measure based only on height and weight — it estimates whether a person's weight is in a healthy range but does not directly measure body fat. Two people with the same BMI can have very different body fat levels. Body fat percentage is a more direct measure of body composition, so it can give a more complete picture alongside BMI. Try our Body Fat Calculator to estimate your body fat percentage.
What if my BMI is normal but I look overweight?
BMI does not distinguish between muscle and fat. A normal BMI with a rounded appearance may indicate higher body fat percentage or different fat distribution. Waist circumference and body fat measurements can provide a clearer picture.
Can BMI be used during pregnancy?
Standard BMI categories are not reliable during pregnancy because weight naturally increases. Healthcare providers use gestational weight gain guidelines instead of BMI to monitor healthy weight changes during pregnancy.

References

1. World Health Organization (WHO). Obesity and Overweight — Fact Sheet.

2. Centers for Disease Control and Prevention (CDC). About Body Mass Index (BMI).

3. Centers for Disease Control and Prevention (CDC). Adult BMI Calculator.

4. Centers for Disease Control and Prevention (CDC). CDC Growth Charts.

The BMI calculator on this page is for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for personalized guidance.

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