Child · Growth
Child BMI Calculator (CDC + AAP 2023)
What is a healthy BMI for your child? CDC age-and-sex percentile bands with AAP 2023 obesity classes. Plus when to worry, how the adiposity rebound works, and how to handle weight conversations sensitively.
Last reviewed 28 May 2026
Child & teen BMI (CDC 2000)
BMI-for-age percentile, ages 2–20
Units
Sex assigned at birth
y
mo
cm
kg
Enter age, height and weight to see the CDC BMI-for-age percentile.
Uses the CDC 2000 BMI-for-age LMS reference (Kuczmarski 2002, Vital Health Stat 11/246). For under-2-year-olds the WHO 0–24 mo growth standards (weight-for-length) apply — use our Baby Growth Percentile calculator. From 20 y onward, the WHO/NIH adult BMI cut-offs apply (≥ 25 overweight, ≥ 30 obese).
What does this mean?
Unlike adult BMI’s fixed cut-offs, children’s BMI is age- and sex-specific. The CDC bands (AAP 2023): < 5th percentile = underweight, 5th–84th = healthy, 85th–94th = overweight, 95th–120 %× the 95th = obesity, 120–140 %× = Class 2 obesity, ≥ 140 %× = Class 3 (severe). One reading is screening, not diagnosis — what matters most is the growth trajectory across visits (a child tracking steadily on the 90th is healthier than one jumping from the 50th to the 90th). At any concerning percentile, AAP 2023 recommends offering Intensive Health Behaviour and Lifestyle Treatment (IHBLT) from age 6 — focused on family-level changes in food, sleep, screen time, and movement — and discussing meds/surgery only when comorbidities exist and structured treatment hasn’t worked.
What is a healthy BMI for my child?
Child BMI is interpreted against age and sex — not fixed adult cut-offs. The CDC 2000 charts (ages 2-20) give percentile bands:
- Underweight: below 5th percentile
- Healthy weight: 5th to under 85th
- Overweight: 85th to under 95th
- Obesity (Class 1): 95th and above
- Class 2 (AAP 2023): BMI ≥ 120% of 95th, or absolute ≥ 35
- Class 3 / severe (AAP 2023): ≥ 140% of 95th, or absolute ≥ 40
How is child BMI calculated?
Same formula as adult: BMI = weight (kg) ÷ height (m)². Then plot against the CDC age-and-sex chart to get the percentile. Imperial: BMI = weight (lb) ÷ height (in)² × 703.
Don’t compare a child’s raw BMI number to adult categories — a healthy 5-year-old can have BMI 14-17 (normal for that age) which would be underweight by adult standards. Always use the PERCENTILE, not the absolute number.
Why child BMI changes with age
Body composition changes throughout childhood:
- Infant (under 2): high body fat for survival/insulation.
- Age 2-5: BMI typically falls to lifetime low around 5-6 (“adiposity nadir”).
- Age 6+: gradually rises again — the “adiposity rebound”.
- Puberty: rapid changes diverging by sex.
- Adult: stabilises.
The percentile chart accounts for all of this — your child’s number is compared to same-age, same-sex peers.
What is the adiposity rebound?
The point around age 5-7 when BMI hits its lifetime low and starts rising again. Early adiposity rebound (before age 5) predicts higher adult adiposity and obesity risk (Whitaker 1998 NEJM). Late rebound (after age 7) is protective. One reason the BMI trajectory matters more than any single reading.
How is childhood obesity treated? (AAP 2023 thresholds)
- Below age 6: dietary/activity counselling for healthy-weight families. Intensive treatment only with significant complications or severe obesity.
- Ages 6+ with overweight or obesity: Intensive Health Behaviour and Lifestyle Treatment (IHBLT) — 26+ contact hours over 3-12 months, ideally family-based.
- Ages 12+ with obesity: weight-loss pharmacotherapy (semaglutide, liraglutide, orlistat) alongside IHBLT may be considered.
- Ages 13+ with class 2 or 3 obesity: metabolic / bariatric surgery referral appropriate.
NICE NG7 (UK) similar but more conservative on medication / surgery.
How should children eat?
Mediterranean-style pattern works well:
- Fruit and veg with every meal.
- Wholegrains over refined.
- Lean protein (chicken, fish, eggs, beans, tofu, paneer).
- Healthy fats (olive oil, nuts, avocado).
- Limit: sugary drinks, fruit juice, fast food, ultra-processed snacks, sweets.
- Portion sizes by age: roughly age in years = side-portion in tablespoons (5-year-old = 5 tbsp of veg).
- Family meals together — sit down, no screens.
- Don’t use food as reward or punishment.
- Don’t force clean plates.
- Don’t calorie-count for children (linked to disordered eating).
How much exercise do children need?
- Ages 5-17: at least 60 minutes/day of moderate-vigorous activity, plus 3 days/week of muscle/bone-strengthening (WHO / UK CMO).
- Under 5: at least 3 hours of varied activity spread through the day.
- Toddlers: shouldn’t be sedentary or restrained for more than 1 hour at a time (except sleep).
- Limit recreational screen time to under 1-2 hours/day for ages 5+.
- Active play beats structured exercise classes for younger children.
Different scenarios — common worries
Scenario 1: 7-year-old at 90th BMI percentile, very active in football
Worth a paediatric chat at next routine visit. Could be muscle, but BMI 90th + family history of CV disease still warrants assessment (BP, fasting lipids if indicated). Continue active lifestyle and healthy eating; don’t restrict food intake.
Scenario 2: 4-year-old, BMI was 50th at 2, now at 85th
Possible early adiposity rebound. Worth raising with HV/GP. Family-based discussion about food and activity patterns; don’t single out the child. Avoid weight-related conversations in front of the child.
Scenario 3: 12-year-old, BMI 35 (class 2 obesity), type 2 diabetes risk concern
AAP 2023: IHBLT recommended. Pharmacotherapy consideration (older adolescent). Endocrine workup if indicated. Family-based intensive intervention.
Scenario 4: 6-year-old at 3rd percentile (underweight), eats well, healthy
Some children are just small. Look at TREND — if always at 3rd-5th and growing along that line, likely constitutional small. Worth ruling out: coeliac (very common cause of failure to thrive at this age), thyroid, malabsorption, eating issues.
Scenario 5: 10-year-old South Asian, BMI 23 (95th percentile for age)
Ethnicity-adjusted thresholds matter — South Asian children show cardiometabolic risk at lower BMIs. Paediatric review with blood pressure, fasting glucose, HbA1c, lipids. Family-based intervention.
How do I talk to my child about weight?
AAP guidance is explicit:
- Avoid weight-related conversations in front of younger children.
- Focus on health behaviours, not body weight.
- Discuss food in terms of energy, growth, what helps body feel good — not “fattening” or “good/bad”.
- Don’t comment on appearance.
- Don’t restrict food intake harshly — linked to disordered eating later.
- Don’t single out one child’s eating in a family.
- Encourage activity in fun, family-based, non-competitive contexts.
- Stigma and weight-cycling are themselves health risks.
Care guidance — family-based approach
- Responsive feeding — parent decides WHAT to offer; child decides WHETHER and HOW MUCH (Satter division of responsibility).
- Eat together as a family. No phones at the table.
- Active family life — walks, parks, bike rides, swimming together.
- Healthy food available — fruit visible, water as default drink.
- Limit ultra-processed snacks in the home.
- Cook together — involves children, builds skills.
- Sleep matters — under-sleep linked to weight gain in children. School-age children need 9-12 hours/night.
- Mental health support if weight concerns are causing distress.
Limitations of BMI in children
- Doesn’t distinguish muscle from fat.
- Imperfect across ethnicities — CDC charts based on US sample.
- Single readings can be misleading — trend matters.
- Doesn’t measure cardiometabolic risk directly (BP, lipids, glucose still needed).
- Doesn’t apply under 2 years — use WHO weight-for-length and length-for-age.
- Should be combined with clinical assessment, not used alone.
Sources
- CDC. Growth Charts. 2000.
- Hampl SE, et al. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics 2023.
- NICE NG7. Preventing excess weight gain.
- NICE NG189. Maintaining a healthy weight and preventing excess weight gain.
- Whitaker RC, et al. Predicting obesity in young adulthood from childhood and parental obesity. N Engl J Med 1997.
- WHO. Global recommendations on physical activity for health.
- Satter E. Division of Responsibility in Feeding.
Frequently asked questions
What is a healthy BMI for children?
Different from adults — child BMI is interpreted against AGE and SEX, not fixed cut-offs. CDC 2000 charts (ages 2-20) bands: UNDERWEIGHT: below 5th percentile. HEALTHY WEIGHT: 5th to less than 85th percentile. OVERWEIGHT: 85th to less than 95th. OBESITY: 95th percentile and above. AAP 2023 added Class 2 (BMI ≥ 120% of 95th or absolute ≥ 35) and Class 3 / severe obesity (≥ 140% of 95th or absolute ≥ 40). The percentile system handles the natural changes in body composition through childhood and adolescence.
How do I calculate child BMI?
Same formula as adult: weight (kg) ÷ height (m)². Then plot against the CDC age-and-sex-specific chart to get the PERCENTILE. The calculator above does both. Don't compare a child's raw BMI number to adult categories — a healthy 5-year-old can have BMI 14-17 (normal for that age) which would be underweight by adult standards. Always use the percentile, not the absolute number, in children.
Why does child BMI change so much with age?
Body composition changes throughout childhood. INFANT (under 2): high body fat for survival/insulation. AGE 2-5: BMI typically falls to lifetime low around age 5-6 ('adiposity nadir'). AGE 6+: BMI gradually rises again — the 'adiposity rebound'. PUBERTY: rapid changes diverging by sex. Adult BMI: stabilises. The percentile chart accounts for all this — your child's number is compared to same-age, same-sex peers from the reference population.
What is the adiposity rebound?
The point around age 5-7 when BMI hits its lifetime low and starts rising again. EARLY ADIPOSITY REBOUND (before age 5) predicts higher adult adiposity and obesity risk (Whitaker 1998 NEJM). LATE REBOUND (after age 7) is protective. This is one reason why a child's BMI trajectory matters more than any single reading — what your paediatrician sees on the growth chart over multiple visits tells more than one calculation.
Should I be worried about my child's BMI?
Look at three things together: (1) the percentile band; (2) the TREND over visits — has BMI crossed major percentile lines? (3) the CONTEXT — diet, activity, family history, growth trajectory. A single point on the chart rarely tells the story. Worth a paediatric conversation if: BMI ≥ 95th, BMI crossing two major percentile lines upward over a year or two, or family history of early cardiovascular disease / type 2 diabetes.
Can my child be 'overweight' just because they're muscular?
Yes — possible but uncommon at typical childhood activity levels. A child who's a serious athlete (gymnast, swimmer, footballer at competitive level) may have higher BMI from muscle. BMI is a screening tool, not a body-composition measurement. The next step in either case is the same: paediatric review, blood pressure, fasting lipids/glucose if 95th+. The label adjusts based on context; the safety-net of clinical assessment doesn't change.
How is childhood obesity treated?
AAP 2023 thresholds: BELOW AGE 6: dietary/activity counselling for healthy-weight families. INTENSIVE Health Behaviour and Lifestyle Treatment (IHBLT) only with significant medical complications or severe obesity. AGES 6+ with overweight or obesity: offer IHBLT — 26+ contact hours over 3-12 months, ideally family-based. AGES 12+ with obesity: weight-loss pharmacotherapy (semaglutide, liraglutide, orlistat) alongside IHBLT may be considered. AGES 13+ with class 2 or 3 obesity: metabolic / bariatric surgery referral is appropriate. NICE NG7 similar but more conservative on medication / surgery.
What should children eat?
Mediterranean-style pattern works well: fruit and veg with every meal; wholegrains over refined; lean protein (chicken, fish, eggs, beans, tofu); healthy fats (olive oil, nuts, avocado); limit sugary drinks, juice, fast food, ultra-processed snacks. NOT calorie-counting for children (associated with disordered eating). PORTION SIZES by age: roughly age in years = appropriate side-portion size in tablespoons (e.g. 5-year-old = 5 tbsp of veg). Sit together at family meals. Don't use food as reward or punishment. Don't force clean plates.
How much exercise do children need?
WHO and UK CMO: AGES 5-17 should get at least 60 MINUTES of moderate-vigorous activity DAILY, plus 3 days/week of muscle/bone-strengthening. UNDER 5: at least 3 hours of varied physical activity spread through the day; toddlers should not be sedentary or restrained for more than 1 hour at a time (except sleep). Limit recreational screen time to under 1-2 hours/day for ages 5+. The active-play approach beats structured exercise classes for younger children.
Is BMI fair across different ethnicities for children?
Imperfect. CDC 2000 charts were based on a US sample of mostly European-descent children. South Asian children show cardiometabolic risk at lower BMIs than European children. The IDF (International Diabetes Federation) uses ethnicity-adjusted thresholds for adult metabolic syndrome but childhood adjustments are less established. Some UK guidelines use lower percentile cut-offs for high-risk ethnic groups. Always combine BMI with clinical assessment including BP, lipids, glucose.
Why does the calculator not work under 2?
Body composition and length measurement in the first 2 years are different. CDC and WHO recommend WEIGHT-FOR-LENGTH (not BMI-for-age) for infants and toddlers. Length is also measured SUPINE (lying down) until age 2, then HEIGHT standing — different measurements. For 0-24 months use /calculators/baby-percentile (WHO growth standards). CDC BMI charts pick up at exactly 24 months.
How do I talk to my child about weight?
AAP explicit: avoid weight-related conversations in front of children (especially younger ones). Focus on HEALTH BEHAVIOURS not body weight. Discuss food in terms of energy, growth, what helps body feel good — not 'fattening' or 'good/bad'. Don't comment on appearance. Don't restrict food intake harshly (linked to disordered eating later). Don't single out one child's eating in a family. Encourage activity in fun, family-based, non-competitive contexts. Stigma and weight-cycling behaviours are themselves health risks.
What about the parents' role in childhood weight?
Huge. Family lifestyle is the strongest predictor of childhood weight. RESPONSIVE FEEDING (parent decides WHAT to offer; child decides WHETHER and HOW MUCH) is the evidence-based approach (Satter Division of Responsibility). Eating together as a family. Modeling healthy food relationships. Active family life. Limiting screens. Not using food as reward. Not making one child's food different from siblings. Parents' own BMI is the strongest single predictor of child's adult BMI (genetic + behavioural).
What conditions can cause unusual BMI in children?
Most child BMI variation is multifactorial (genetics + behaviour + environment). But certain conditions can cause unusual patterns: HYPOTHYROIDISM, Cushing's syndrome, Prader-Willi syndrome, growth hormone deficiency, certain genetic syndromes. CONSIDER REFERRAL for: BMI crossing 2+ major percentile lines rapidly without obvious explanation; very early-onset obesity (under age 5 with no family pattern); short stature with obesity (consider endocrine causes); developmental delay + obesity (consider syndromic causes). Most children referred turn out to have lifestyle-multifactorial obesity — but the workup is worth it.
What's the difference between BMI and body fat percentage?
BMI = mass/height² — a screening proxy for adiposity. BODY FAT % = directly measured (DEXA, BIA, calipers) — actual body composition. DEXA is gold standard but expensive and uses tiny radiation dose. BIA (bioelectrical impedance) is OK but affected by hydration. Skinfold calipers need trained practitioner. For most clinical purposes, BMI percentile + clinical assessment is sufficient. Body fat % rarely changes the management plan but can be useful in specific athletic / academic / clinical contexts.
How does this relate to other calculators on BumpBites?
Companion: /calculators/baby-percentile for under-2 (WHO growth); /calculators/child-height-predictor for adult-height estimate; /calculators/milestone-tracker for developmental tracking; /calculators/toddler-feeding for nutrition guidance 1-3; /calculators/food-intro-tracker for weaning history; /calculators/pregnancy-bmi for the maternal side that influences child BMI risk.