Child & Teen BMI Calculator
Track growth percentiles accurately using official WHO, CDC, and IOTF international standards.
Child & Teen BMI Calculator
Understanding Your Child BMI Calculation(WHO)
Child and teen BMI cannot use adult fixed cutoffs. Based on the growth standard you selected (WHO or CDC) and your child's age, here is how the result maps to clinical weight status categories:
Severe Thinness
Z < −3 SDWell below the reference median — may signal nutritional deficiency or growth concerns. Follow up with a pediatrician.
Thinness
−3 SD ≤ Z < −2 SDBelow the healthy reference range for this age and sex. Trend tracking across visits matters more than a single reading.
Normal
−2 SD ≤ Z ≤ +1 SDWithin the WHO healthy growth reference for children 0–5 years.
Risk of Overweight
+1 SD < Z ≤ +2 SDEarly signal of upward weight trajectory. WHO uses this intermediate band only for children through 60 months.
Overweight
+2 SD < Z ≤ +3 SDAbove the healthy reference band. Discuss lifestyle and growth trends with your pediatrician.
Obesity
Z > +3 SDWHO applies the more conservative >+3 SD obesity threshold for children through 60 months.
* Band cards update automatically when you switch between WHO and CDC standards or change the child's age. WHO uses separate band logic for children through 60 months vs. ages 5–19.
Key Takeaways
- Age-and-sex context is mandatory: A raw BMI of 22 can mean healthy for a 16-year-old but obese for a 7-year-old — always interpret through percentiles or z-scores.
- BMI under 2 years: US and UK clinical practice recommends weight-for-length charts for infants 0–24 months rather than standard BMI-for-age.
- 2022 CDC Extended Charts: The Sigma half-normal model now tracks severe obesity up to the 99.99th percentile with Class 2 and Class 3 classification.
- CDC vs WHO differ in methodology: CDC charts describe how US children grow; WHO standards describe optimal international reference growth.
- Trend beats snapshot: A single percentile reading should never trigger dietary intervention — trajectory across well-child visits is the actionable metric.
- Screening, not diagnosis: BMI cannot distinguish muscle from fat; athletic children may register elevated BMI without excess adiposity.
- Adiposity rebound timing matters: Children whose BMI rises again before age 5 have statistically elevated later obesity risk.
01Why Is Child and Adult BMI Different, and How Do I Calculate It?
Adult BMI uses fixed thresholds — overweight at 25.0 and obesity at 30.0 regardless of age. Children and teens are still growing, so a raw BMI number must be compared against an age-and-sex-specific reference population using percentiles or z-scores.
1.1 The Formula Is the Same — the Interpretation Is Not
The BMI equation itself is identical for children and adults:
Formula Models
The Metric Formula
BMI = kg / m²
Where:
• kg Total body weight in kilograms
• m² Absolute physical height multiplied by itself (meters squared)
Example: 25 kg at 1.20 m height → 25 / (1.20 × 1.20) = 17.4 kg/m².
The Imperial Formula
BMI = (lbs × 703) / in²
Where:
• lbs Weight measurement in pounds
• in² Absolute height in inches, multiplied by itself
• 703 is the standard scaling conversion constant
Example: 55 lb at 47 in → (55 × 703) / (47 × 47) = 17.5 kg/m².
Source: CDC 2022 Growth Charts / WHO 2007 Growth Reference.
02What Should My Child's BMI Be, and What Is a Normal Range?
There is no single correct BMI for all children. A BMI of 16.0 may be healthy for a 4-year-old but underweight for a 14-year-old. Healthy ranges shift continuously with age as lean mass, fat distribution, and pubertal stage change.
2.1 Approximate Healthy BMI by Age (50th Percentile)
Source: CDC 2022 Growth Charts — midpoint values from LMS smoothing parameters. Normal is defined by percentile position (5th to 84th), not a fixed number.
03How Do I Calculate My Child's BMI Percentile, and What Does It Mean?
A BMI percentile shows where your child's BMI falls relative to peers of the same age and sex. The 75th percentile means their BMI equals or exceeds 75% of the reference group — not that 75% of their body is fat.
3.1 The LMS Method and Clinical Percentile Brackets
The CDC uses the LMS method (Lambda-Mu-Sigma) to model BMI distribution at each month of age. Your child's BMI is converted to a z-score, then mapped to a percentile.
Source: CDC 2022 Growth Charts / AAP 2023.
04Does Childhood BMI Change as the Child Ages, and Does It Apply to Babies?
Pediatric BMI follows a distinctive growth trajectory — and standard BMI-for-age charts are not recommended for infants under 24 months in US clinical practice.
4.1 BMI Does Not Apply to Infants Under 2
For infants birth to 24 months, clinicians track weight-for-length rather than BMI. The CDC BMI-for-age charts begin at age 2 years (24 months). WHO provides curves for 0–60 months, but AAP and NHS recommend weight-for-recumbent-length charts under age 2.
Source: WHO 2006 / CDC 2000 / AAP 2023.
4.2 The Adiposity Rebound
Childhood BMI follows a U-shaped curve: rapid rise in infancy, decline from ~12 months to ~5–6 years, then a second rise called the adiposity rebound. An early rebound (before age 5) is associated with increased later obesity risk — one reason continuous percentile tracking matters.
Source: WHO 2006 / CDC 2022 Growth Charts.
05What Is the Difference Between CDC, WHO, and NHS Child BMI Charts?
Source: CDC 2022 / WHO 2006 / WHO 2007.
06When Is a Child Considered Underweight, Overweight, or Obese?
Clinical weight status for children aged 2–20 is defined by BMI-for-age percentile or z-score position — not absolute BMI values.
6.1 CDC Clinical Cut-offs (United States, Ages 2–20)
Source: CDC 2022 Extended BMI Standard.
6.2 WHO Clinical Cut-offs (International Standard)
Ages 5–19 (excluding 60 months):
Ages 0–5 (including 60 months): adds a Risk of Overweight band (+1 SD to +2 SD) and sets obesity at > +3 SD.
Source: WHO 2006 / WHO 2007.
07Did the BMI Calculator Change Recently, and When Should I Worry About Severe Childhood Obesity?
In 2022, the CDC released Extended BMI-for-Age Growth Charts that fix a critical limitation of the 2000 charts: the inability to accurately track extremely high BMI values above the 97th percentile.
7.1 Severe Obesity Classification (2022 CDC Standard)
Source: CDC 2022 Extended BMI Standard.
08References
Show 7 references & sources
- Centers for Disease Control and Prevention (CDC). 2022 Extended BMI-for-Age Growth Charts for Children and Adolescents with Severe Obesity. National Center for Health Statistics. Link
- World Health Organization (WHO). WHO Child Growth Standards: Methods and Development. Geneva: WHO, 2006. Link
- World Health Organization (WHO). Growth Reference Data for 5–19 Years. WHO Reference 2007. Link
- Hampl SE et al. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics. 2023;151(2):e2022060640. American Academy of Pediatrics. Link
- Hales CM et al. Evaluation of Alternative Body Mass Index (BMI) Metrics to Monitor Weight Status in Children and Adolescents with Extremely High BMI Using CDC BMI-for-Age Growth Charts. Vital and Health Statistics, Series 1, No. 197. NCHS, 2022. Link
- Cole TJ, Lobstein T. Extended International (IOTF) Body Mass Index Cut-offs for Thinness, Overweight and Obesity. Pediatric Obesity. 2012;7(4):284–294. Link
- Harvard T.H. Chan School of Public Health. Obesity Prevention Source: Child Obesity. Link