BMI Calculator for Kids
As a parent, it is natural to wonder if your child is growing healthy and strong. BMI for kids works differently than for adults — instead of fixed cutoff numbers, children's BMI is compared to growth charts that account for age and sex. Our child BMI calculator does the math for you: enter your child's height, weight, age, and sex, then instantly see their BMI percentile. This tells you how your child compares to other children of the same age and sex. A percentile between the 5th and 85th means your child is in a healthy range. Below the 5th percentile suggests underweight, while above the 85th signals overweight. This tool uses the same CDC and WHO growth reference data that pediatricians use, giving you reliable results without the guesswork.
What Is
Body Mass Index for children and teens aged 2 through 19 uses age- and sex-specific percentiles rather than the fixed adult categories. The formula is the same — weight divided by height squared — but the result is plotted on a growth chart. A BMI at the 60th percentile means the child has a higher BMI than 60% of children of the same age and sex. The CDC defines: below 5th percentile as underweight, 5th to 84th as healthy weight, 85th to 94th as overweight, and 95th or above as obese. This approach is essential because children's body composition changes dramatically as they grow. A BMI that is perfectly normal for an 8-year-old boy might be concerning for a 14-year-old. Puberty timing, ethnicity, and family patterns all influence where a child falls on the curve. Pediatricians track BMI over time to spot trends — a child who crosses upward through percentiles may be gaining weight too rapidly relative to height.
How to Use
- Select your child's sex (male or female) — growth charts differ between boys and girls.
- Enter your child's date of birth or age in years and months for accurate percentile lookup.
- Measure your child's height. For best results, have them stand straight against a wall without shoes.
- Weigh your child on a reliable scale, preferably in light clothing.
- Click Calculate to see your child's BMI number and percentile ranking.
- Compare the result to the CDC categories and discuss any concerns with your pediatrician.
Examples
Input: Weight: 75 kg, Height: 175 cm
Process: BMI=75/(1.75)²=24.5
Result: BMI=24.5 → Normal
Input: Weight: 60 kg, Height: 162 cm
Process: BMI=60/(1.62)²=22.9
Result: BMI=22.9 → Normal
Input: Weight: 90 kg, Height: 170 cm
Process: BMI=90/(1.7)²=31.1
Result: BMI=31.1 → Obese
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Frequently Asked Questions
At what age should I start tracking my child's BMI?
The CDC recommends starting BMI screening at age 2. Before age 2, weight-for-length is measured instead of BMI. Regular annual BMI checks during well-child visits help identify trends early. If your child's BMI is trending upward rapidly or has jumped across percentile lines, talk to your pediatrician about nutrition and activity changes.
My child is very active in sports — does that affect their BMI?
Athletic children may have more muscle mass, which can push their BMI percentile higher even if they have low body fat. However, studies show most children who fall into the overweight or obese BMI categories do carry excess body fat regardless of activity level. If your child is highly athletic, consider pairing BMI with a waist circumference measurement or body fat assessment for a clearer picture.
What should I do if my child's BMI is above the 85th percentile?
First, don't panic. One measurement is just a snapshot — trends over time matter more. Focus on the 5-2-1-0 rule: 5 servings of fruits and vegetables, 2 hours or less of screen time, 1 hour of physical activity, and 0 sugary drinks daily. Avoid putting children on restrictive diets; instead, model healthy eating and make it a family effort. Your pediatrician can screen for related issues like high cholesterol or prediabetes and provide tailored guidance.
Is the BMI percentile different for boys and girls?
Yes — boys and girls have different growth patterns and body composition trajectories, especially after age 10 when puberty begins. Girls typically carry more body fat than boys of the same age. That is why the calculator requires sex input and uses sex-specific reference data from the CDC growth chartsCharts. A BMI at the 70th percentile for a boy is not the same numerical BMI as the 70th percentile for a girl.