Calculator Use
The child bmi calculator is a screening tool used to estimate body fatness in children and adolescents ages 2 through 19. Unlike adult BMI, which is interpreted using fixed categories, a child's BMI must be compared against growth charts that account for age and biological sex.
To use this calculator effectively, simply follow these steps:
- Measurement System
- Choose between Standard (imperial) units or Metric units.
- Gender and Age
- Select the child's sex and age in years. Body composition changes significantly as children grow, and boys and girls develop at different rates.
- Weight and Height
- Enter the current weight and height accurately. Even small errors in height measurements can significantly impact the final BMI result.
How It Works
Body Mass Index (BMI) is a calculation based on mass and height. While the math is the same for children and adults, the interpretation for children is based on "BMI-for-age" percentiles. These percentiles show how a child's BMI compares to other children of the same age and sex.
Formula (Imperial): BMI = (Weight in lbs / (Height in inches)²) × 703
Formula (Metric): BMI = Weight in kg / (Height in meters)²
Once the raw BMI number is calculated, it is plotted on the CDC growth charts to find the percentile:
- Underweight: BMI less than the 5th percentile.
- Healthy Weight: 5th percentile to less than the 85th percentile.
- Overweight: 85th percentile to less than the 95th percentile.
- Obese: 95th percentile or greater.
Calculation Example
Example: A 10-year-old boy who weighs 75 pounds and stands 54 inches tall.
Step-by-step solution:
- Weight = 75 lbs
- Height = 54 inches
- Calculate Height Squared: 54 * 54 = 2,916
- Divide Weight by Height Squared: 75 / 2,916 = 0.02572
- Multiply by 703: 0.02572 * 703 = 18.08
- Resulting BMI: 18.1
For a 10-year-old boy, a BMI of 18.1 typically falls around the 75th percentile, which is within the Healthy Weight category.
Common Questions
Why is BMI interpreted differently for children than adults?
In adults, BMI categories (Underweight, Normal, Overweight, Obese) are fixed. However, children's body fat levels change throughout their development. Furthermore, girls and boys have different amounts of body fat as they mature. This is why a child bmi calculator uses percentiles to compare a specific child against a reference population of the same age and gender.
Does a high BMI mean a child is overweight?
BMI is a screening tool, not a diagnostic one. A high BMI can indicate high body fatness, but it can also be influenced by high muscle mass or a large frame. If a child has a high BMI-for-age percentile, a healthcare provider may perform further assessments like skinfold thickness measurements, diet evaluation, and physical activity reviews to determine if excess fat is a health concern.
At what age can you start using this calculator?
The standard BMI-for-age charts provided by organizations like the CDC are intended for children and adolescents between the ages of 2 and 19 years. For children under 2, doctors typically use weight-for-length charts rather than BMI.