- Enter the child's age in years (decimals allowed, e.g. 7.5). Valid range 2 to 20 y.
- Select sex at birth (used to choose the correct CDC growth curve).
- Enter height (choose cm or inches) and weight (choose kg or pounds). All four inputs are required before a result appears.
- The tool computes BMI = weight (kg) / height (m)², then looks up the CDC 2000 BMI-for-age L, M, S for the sex and age in months (linearly interpolated between adjacent monthly rows) and returns the z-score, percentile, and category. For BMIs at or above the 95th percentile it also reports %BMIp95 (BMI / BMI95th × 100), which discriminates severe obesity better than the z-score.
All computation runs in your browser; no values are stored or transmitted. For infants < 2 y, use the WHO weight-for-length charts instead.
When to Use
Use this calculator for any child or adolescent aged 2 through 19 years when a clinic visit or chart review needs a numeric BMI category. The CDC 2000 BMI-for-age z-score is the standard reference used by the American Academy of Pediatrics and the CDC for classifying underweight, healthy weight, overweight, and obesity in the United States. It is also the basis the CKD-in-children literature uses when correlating childhood adiposity with later proteinuria, hypertension, and incident kidney disease.
Appropriate population
Children and adolescents 2 through 19 years. Particularly useful at every well-child visit, before starting a weight-related medication, when counselling families about lifestyle change, and when screening for the metabolic comorbidities of childhood obesity — hypertension, dyslipidaemia, dysglycaemia, NAFLD, and obesity-related glomerulopathy.
When NOT to rely on it
Do not use this tool below 2 years of age — the WHO weight-for-length and length-for-age charts are preferred for 0–24 months. Do not use it as a stand-alone diagnosis: a single BMI value is a screen, not a verdict; it should be plotted on the growth chart and trended over time. In children with disproportionate stature (short stature from CKD, skeletal dysplasia, scoliosis, amputation, severe muscle wasting) BMI may be misleading, and in CKD growth failure can mask weight gain — interpret BMI alongside height-for-age and clinical context.
Pearls & Pitfalls
Categories (CDC / 2007 Expert Committee)
< 5th percentile — underweight. 5th to < 85th — healthy weight. 85th to < 95th — overweight. ≥ 95th — obesity (class 1). ≥ 120% of the 95th percentile or BMI ≥ 35 kg/m² — severe obesity (class 2). ≥ 140% of the 95th percentile or BMI ≥ 40 kg/m² — severe obesity (class 3). The tool flags class 2/3 severity automatically.
Use %BMIp95 for the heaviest children
Beyond about the 97th percentile the BMI-for-age z-score is mathematically compressed — very different high BMIs map to nearly identical z-scores, so the z-score cannot track change over time at the upper extreme (Freedman 2017). For any child whose BMI is at or above the 95th percentile, report %BMIp95 = (measured BMI / BMI95th) × 100 instead; this is the metric the CDC's Extended BMI-for-age chart and the AAP 2023 obesity guideline use to grade severity and follow response to treatment.
Pitfalls
(1) Self-reported height and weight systematically under-estimate BMI in adolescents — measure them. (2) A child whose BMI lands in the "healthy" band but who has crossed two percentile lines downward is failing to thrive — the trajectory matters more than the snapshot. (3) In CKD, chronic glucocorticoid use, growth-hormone deficiency, and prematurity, BMI may misclassify body composition; interpret with height-for-age and an honest look at habitus. (4) Pediatric obesity is associated with glomerular hyperfiltration, microalbuminuria, and earlier CKD onset — do not dismiss obesity as "just" a weight issue in a child with kidney disease.
Why Use It
Childhood and adolescent obesity are now among the most consistent modifiable risk factors for adult cardiovascular disease, type 2 diabetes, and chronic kidney disease. A numeric BMI z-score and percentile, mapped to the CDC 2000 growth charts and the 2007 expert-committee categories, is the standard language clinicians, payers, and public-health systems use to identify children at risk and to track response to intervention. For the nephrology population the stakes are higher still: pediatric obesity drives glomerular hyperfiltration, accelerates proteinuria, and predicts an earlier transition to CKD in vulnerable kidneys (low nephron endowment, prior AKI, congenital anomalies). Quantifying BMI with the CDC LMS method — rather than eyeballing the chart — gives a defensible, reproducible number that can be acted on, communicated to the family, and re-checked at the next visit.
Pediatric BMI z-score & Percentile (CDC 2000)
Enter age, sex, height, and weight. The tool returns BMI, the CDC 2000 BMI-for-age z-score, the percentile, and the CDC / 2007 expert-committee weight category. For BMIs at or above the 95th percentile it also reports %BMIp95. Valid age range 2 to 19.99 years.
⚕ CDC 2000 BMI-for-age growth charts (Kuczmarski 2002). Categories from Barlow / Expert Committee 2007 and the AAP 2023 obesity clinical practice guideline. Severe-obesity classes use the CDC Extended BMI-for-Age convention (% of the 95th percentile). For ages under 2 y, use WHO weight-for-length. For licensed clinicians; not a substitute for individualized assessment.
Next Steps
Translate the BMI category into the next clinic action and the relevant comorbidity workup.
- Underweight (< 5th percentile): assess intake, growth trajectory, chronic disease (CKD, malabsorption, eating disorder, malignancy, social determinants). Plot weight-for-age and height-for-age; obtain a CBC, basic metabolic panel, urinalysis, and consider TTG-IgA if loose stools.
- Healthy weight (5th to < 85th): reinforce family-based healthy-lifestyle counselling (5-2-1-0: ≥5 servings of fruit / vegetables, ≤2 h screens, ≥1 h physical activity, 0 sugar-sweetened drinks). Recheck at each well-child visit and plot the trend.
- Overweight (85th to < 95th): family-based intensive health-behaviour and lifestyle treatment (IHBLT). Screen for hypertension (4-extremity BP plotted against the AAP table), dyslipidaemia (non-fasting non-HDL ≥ 9 y), and dysglycaemia (HbA1c or fasting glucose if additional risk factors). Recheck BMI in 3–6 months.
- Obesity (≥ 95th): AAP 2023 — offer IHBLT (≥ 26 contact hours over 3–12 months) as first-line, screen for hypertension, dyslipidaemia, dysglycaemia (HbA1c, fasting lipids), and NAFLD (ALT) starting at 10 y. Consider obstructive sleep apnoea and depression. In children with CKD or solitary kidney, monitor proteinuria and BP closely.
- Severe obesity (class 2 / 3): consider referral to a multidisciplinary paediatric weight-management programme; AAP 2023 supports anti-obesity pharmacotherapy from age 12 y and bariatric-surgery evaluation from age 13 y in appropriate candidates. Use %BMIp95, not the z-score, to track response.
- Pair this with the pediatric BP calculator (the AAP 2017 BP tables are sex-, age-, and height-specific) and an adult BMI / BSA / IBW reference for the transition visit.
Evidence & References
Formula
| Quantity | Formula |
|---|---|
| BMI (kg / m²) | weight (kg) ÷ height (m)² |
| BMI z-score, L ≠ 0 | z = ((BMI / M)L − 1) / (L × S) |
| BMI z-score, L = 0 | z = ln(BMI / M) / S |
| Percentile | Φ(z) × 100% (standard-normal CDF) |
| %BMIp95 | (measured BMI ÷ BMI95th) × 100 |
L, M, S are the sex- and age-specific Box-Cox power (L), median (M), and coefficient of variation (S) from the CDC 2000 BMI-for-age reference (bmiagerev.csv, ages 24–240 months, both sexes). The tool linearly interpolates between adjacent monthly rows. The standard-normal CDF is computed with a high-accuracy rational approximation (max absolute error ≈ 1.5 × 10⁻⁷).
Categories (CDC / 2007 Expert Committee)
| Category | Definition |
|---|---|
| Underweight | < 5th percentile |
| Healthy weight | 5th to < 85th percentile |
| Overweight | 85th to < 95th percentile |
| Obesity (class 1) | ≥ 95th percentile |
| Severe obesity (class 2) | ≥ 120% of the 95th percentile, or BMI ≥ 35 kg/m² |
| Severe obesity (class 3) | ≥ 140% of the 95th percentile, or BMI ≥ 40 kg/m² |
References
- Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC growth charts for the United States: methods and development. Vital Health Stat 11. 2002;(246):1–190.
- Barlow SE; Expert Committee. Expert Committee Recommendations Regarding the Prevention, Assessment, and Treatment of Child and Adolescent Overweight and Obesity: Summary Report. Pediatrics. 2007;120(Suppl 4):S164–S192.
- Freedman DS, Butte NF, Taveras EM, et al. The Limitations of Transforming Very High Body Mass Indexes Into z Scores Among 8.7 Million 2- to 4-Year-Old Children. J Pediatr. 2017;188:50–56.
