Children's BMI is interpreted using age- and sex-specific percentiles, not fixed adult thresholds. A healthy BMI for a child aged 2–12 is between the 5th and 85th percentile for their age and sex. Below the 5th is Underweight; 85th–95th is Overweight; 95th and above is Obese. This calculator uses the CDC 2000 growth chart LMS method. For teenagers aged 13–19, use the BMI Calculator for Teens.
How to use this calculator
Select Boy or Girl, enter your child's age in years and months (ages 2–12), then choose Metric or Imperial and enter weight and height. The calculator shows the BMI value, percentile rank, weight category, and the healthy weight range for that exact age and sex. If your child is 13 or older, the calculator will prompt you to use the BMI Calculator for Teens.
What is BMI-for-age?
BMI-for-age uses the same formula as adult BMI — weight (kg) divided by height (m) squared — but interprets the result as a percentile compared to a reference population of children the same age and sex. A fixed threshold like 25 cannot work for children because a normal BMI at age 6 would be overweight at age 10 and obese at age 14.
The BMI formula
$$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$$
$$\text{BMI} = \frac{703 \times \text{weight (lb)}}{\text{height (in)}^2}$$
BMI categories for children aged 2–12
| Category | BMI percentile | What it means |
|---|---|---|
| Underweight | Below 5th | BMI lower than 95% of peers; may indicate inadequate nutrition or underlying condition |
| Healthy weight | 5th to below 85th | Within the expected range for age and sex; lowest risk of weight-related complications |
| Overweight | 85th to below 95th | Higher than 85–94% of peers; increased metabolic and cardiovascular risk; lifestyle review recommended |
| Obese | 95th and above | Higher than 95%+ of peers; elevated risk of insulin resistance, hypertension, sleep apnoea, and joint complications |
BMI-for-age reference chart: ages 2–12
The tables below show BMI at the 5th, 50th, 85th, and 95th percentiles for boys and girls at ages 2–12, based on CDC 2000 LMS parameters. These values define the category boundaries at each age.
Boys (ages 2–12)
| Age | 5th %ile (Underweight) | 50th %ile (Median) | 85th %ile (Overweight) | 95th %ile (Obese) |
|---|---|---|---|---|
| 2 yr | 14.8 | 16.4 | 18.0 | 19.3 |
| 4 yr | 13.6 | 15.7 | 17.1 | 18.2 |
| 5 yr | 13.6 | 15.5 | 16.8 | 17.9 |
| 6 yr | 13.6 | 15.4 | 16.9 | 18.1 |
| 8 yr | 13.9 | 15.6 | 17.4 | 19.2 |
| 10 yr | 14.4 | 16.6 | 18.8 | 21.2 |
| 12 yr | 15.2 | 18.0 | 21.5 | 24.3 |
Girls (ages 2–12)
| Age | 5th %ile (Underweight) | 50th %ile (Median) | 85th %ile (Overweight) | 95th %ile (Obese) |
|---|---|---|---|---|
| 2 yr | 14.5 | 16.1 | 17.7 | 19.0 |
| 4 yr | 13.4 | 15.5 | 16.9 | 18.1 |
| 5 yr | 13.3 | 15.2 | 16.6 | 17.7 |
| 6 yr | 13.3 | 15.1 | 16.7 | 18.0 |
| 8 yr | 13.6 | 15.4 | 17.5 | 19.5 |
| 10 yr | 14.2 | 16.5 | 19.2 | 21.8 |
| 12 yr | 15.0 | 18.0 | 21.7 | 24.9 |
Adiposity rebound: the most important pattern in childhood BMI
Between ages 1 and 6, BMI naturally falls as children grow taller faster than they gain weight. It reaches a minimum — the adiposity nadir — typically between ages 4 and 6. It then rises again; this is the adiposity rebound.
The timing of rebound matters significantly. Research by Rolland-Cachera et al. (1984) showed that children who begin their rebound before age 5 have a substantially higher risk of adult obesity and metabolic disease, even when their BMI at the time appears normal. A child at the 50th percentile who rebounds early carries more future risk than a child at the 70th percentile who rebounds at age 7.
This means that looking at BMI percentile over time — across multiple paediatric visits — is far more informative than any single measurement. A steady rise in percentile through ages 4–8 warrants attention even if the current percentile is below the 85th.
Supporting healthy growth in children aged 2–12
For children in the overweight or obese range, the clinical recommendation is not weight loss but weight maintenance while height increases. As a child grows taller without gaining excess weight, their BMI percentile naturally falls into the healthy range. This is a realistic and child-safe target that does not require caloric restriction.
Practical guidance for parents
- Serve regular meals and snacks at set times rather than continuous grazing — structure reduces overconsumption without restriction
- Offer vegetables and fruit first when children are hungry — this displaces calorie-dense snacks without banning them
- Limit sugary drinks including fruit juice — a glass of apple juice has as much sugar as a glass of fizzy drink
- At least 60 minutes of moderate-to-vigorous physical activity daily for children aged 6–12; active play covers most of this naturally
- Limit recreational screen time — sedentary hours displace active play and are independently associated with higher BMI regardless of diet
- Do not comment on a child's weight or body shape — children internalise body-image messages very early; focus on health and energy, not appearance
Never put a primary-school child on a calorie-counting or weight-loss diet without direct supervision by a paediatrician and registered dietitian. Caloric restriction can impair growth, bone mineralisation, and cognitive development in children.
Limitations of BMI for children
| Limitation | Why it matters |
|---|---|
| Cannot distinguish muscle from fat | A physically active, muscular child may be classified as overweight despite healthy body composition |
| Single measurement is limited | BMI percentile trajectory over time is far more meaningful than any one data point |
| Ethnicity differences | Children of South Asian and East Asian descent tend to carry more body fat at the same BMI percentile than European-descent peers |
| Normal variation in growth rates | During growth spurts, BMI may temporarily dip or rise in ways unrelated to actual fat change |
When to talk to your child's doctor
- BMI below the 5th percentile — especially if the child is eating poorly, losing weight, or has slowed linear growth
- BMI at or above the 85th percentile — especially if it has been rising across consecutive visits
- BMI at or above the 95th percentile — clinical evaluation is recommended regardless of other factors
- Any rapid shift of 10+ percentile points upward between routine visits
- Signs of early puberty (before age 8 in girls, before age 9 in boys) alongside rising BMI
- Acanthosis nigricans — darkened skin folds at the neck, armpits, or groin — which can indicate insulin resistance in overweight children
FAQs
What is a healthy BMI for a child aged 2–12?
Between the 5th and 85th percentile for the child's age and sex. For a 5-year-old boy, that is approximately 13.6–16.8. For a 10-year-old boy it is approximately 14.4–18.8. Use the calculator for exact values at any age.
What is a normal BMI for a 5-year-old?
Healthy weight (5th–85th percentile): approximately 13.6–16.8 for boys, 13.3–16.6 for girls. The median (50th percentile) is approximately 15.5 for boys and 15.2 for girls at age 5.
What is a normal BMI for a 10-year-old?
Healthy weight: approximately 14.4–18.8 for boys, 14.2–19.2 for girls. The overweight threshold (85th percentile) for a 10-year-old boy is approximately 18.8 and for a girl approximately 19.2.
Can I use an adult BMI calculator for my child?
No. Adult cutoffs (18.5 / 25 / 30) are not appropriate for children. A healthy 12-year-old girl may have a BMI of 20, which adult charts would classify as healthy by luck but for the wrong reason — and any younger child would be severely misclassified. Always use a child-specific percentile calculator.
Should I put my child on a diet if their BMI is high?
No — restrictive dieting is not recommended for growing children. The goal is typically to maintain current weight while height continues to increase, naturally lowering BMI percentile over time. A paediatrician or registered dietitian can guide appropriate changes.
Is this calculator accurate for a 12-year-old who has started puberty?
It gives a valid percentile result. However, puberty creates a specific context — body fat increases normally in girls and muscle mass in boys — that makes interpretation more nuanced for early adolescents. For children 13 and older, use the BMI Calculator for Teens, which addresses puberty-specific BMI interpretation.