Child & Teen BMI Calculator
Children's BMI is not the same as adults'. This tool uses CDC age-and-sex percentile bands — the same approach the NHS uses — to give you a clear, parent-friendly answer in seconds.
For children aged 2–19 only
Childhood BMI uses age-and-sex percentile bands, not the adult categories. Always discuss results with your child's GP or paediatrician before making changes.
Sex
What the percentile bands mean
Below the 5th percentile
Underweight
Lower BMI than 95% of children the same age and sex. Worth a GP review to check growth, eating, and absorption.
5th to 84th percentile
Healthy weight
Typical for age and sex. Keep up balanced meals, daily activity, and regular sleep — no clinical action needed.
85th to 94th percentile
Overweight
Heavier than 85 in 100 peers. Small family-wide changes (drinks, snacks, screen time, activity) often bring this back into the healthy band.
95th percentile or above
Obese
Clinically classified as obesity in children. Speak to your GP — NHS childhood weight-management services can help.
How the child BMI calculator works
Pick boy or girl
BMI percentile bands are sex-specific in childhood. Boys and girls develop at different rates, so the calculator uses two separate CDC tables.
Enter age, height, and weight
Use whole years for age (2 through 19). Height in cm or feet/inches; weight in kg or pounds — the calculator handles the conversion.
Read the percentile band
You'll see one of four bands: under-5th (underweight), 5th–84th (healthy), 85th–94th (overweight), or 95th+ (obese), with parent-friendly NHS-style guidance.
When to see your GP
- •BMI consistently in the 85th–94th band for more than 6 months
- •BMI at or above the 95th percentile at any age
- •BMI below the 5th percentile, especially with poor appetite or fatigue
- •Rapid changes in weight or height velocity
- •Any concerns about eating, body image, or growth
This calculator is a screening tool, not a clinical diagnosis. Your GP has access to your child's full growth chart and history.
Child BMI — Parent FAQs
Quick answers to the most common questions
A child's BMI uses age-and-sex specific percentile bands rather than the fixed adult cut-points of 18.5, 25, and 30. This is because healthy BMI ranges change as children grow and differ between boys and girls. The CDC growth charts (which the NHS broadly follows) classify children below the 5th percentile as underweight, 5th to 84th as healthy, 85th to 94th as overweight, and at or above the 95th percentile as obese.
This calculator covers ages 2 through 19 years, which is the standard CDC BMI-for-age range. Under 2, healthcare providers use weight-for-length charts instead, because BMI is not reliable for infants and toddlers. Above 19, adults should use our standard adult BMI calculator.
Being in the 85th to 94th percentile classifies a child as overweight but is not a cause for panic. Many children in this band naturally move back into the healthy range as they grow taller, especially during puberty. Focus on small, family-wide habits: 5 portions of fruit and vegetables a day, at least 60 minutes of activity, less screen time, and consistent sleep. If your child stays in this band for several months, speak to your GP.
Most paediatric guidance recommends not sharing exact BMI numbers with children, especially under age 12, because it can trigger body-image anxiety. Instead, focus on healthy behaviours — what the family eats, how active everyone is, and sleep — without making weight the explicit goal. For teenagers, an age-appropriate conversation with a GP or school nurse is usually better than a parent-led one.
The UK uses WHO growth charts from birth to age 4 and UK 1990 reference data from 4 to 18. CDC charts (used by this calculator) are very similar but use slightly different reference populations. For clinical decisions in the UK, the NHS will use UK-WHO charts. For a quick screening at home, CDC and UK charts give almost identical band classifications.
A BMI at or above the 95th percentile is classified as obesity in children and warrants a GP visit. Treatment focuses on the whole family, not just the child: reducing sugary drinks and ultra-processed snacks, increasing activity, and improving sleep. The NHS offers childhood weight-management programmes such as Healthier Families and tier 2 community services. Crash diets are never recommended in children.
BMI does not distinguish between muscle and fat in children either. A muscular teenage rugby player or gymnast may sit in the 85th–94th band but have very low body fat. In these cases, BMI alone is misleading — pair the result with waist measurement, body composition, and a GP review before assuming the child is overweight.
For most children, an annual BMI check (often done at school or at the GP) is enough. If your child is on the edges of the healthy band or is being monitored by a clinician, every 3 to 6 months is reasonable. Daily or weekly weighing is not recommended — it creates anxiety and does not change clinical decisions.
Have another question? Browse our full article library or try a free calculator.
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