Kids BMI calculator: child and teen BMI percentile

Why children use percentiles instead of a fixed BMI number

An adult's healthy BMI range stays the same from age 20 to age 70, but a child's body composition changes dramatically between toddlerhood and the end of puberty. Toddlers are naturally rounder, BMI dips through the preschool years, and then rises again from around age 5 or 6 in a pattern called "adiposity rebound". Because of this, a fixed number like "BMI 22 is fine" is meaningless for a child. Instead, the CDC's 2000 BMI-for-age growth charts compare a child's BMI with a large reference population of children the same age and sex, and express the result as a percentile: the proportion of that reference population with a lower BMI.

CDC BMI-for-age percentile categories

Source: CDC, About Child and Teen BMI and the American Academy of Pediatrics' 2023 Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents with Obesity.

How the UK, NHS and RCPCH view child BMI

The RCPCH and NHS use the UK90 (British 1990) growth reference rather than the CDC 2000 reference used by this calculator. The two references are broadly similar but not identical, since they were built from different national populations measured in different decades. For population level monitoring, such as the National Child Measurement Programme, the NHS uses the 85th and 95th centiles, the same bands used throughout this tool. For individual clinical assessment in clinic, the NHS and RCPCH instead flag overweight above the 91st UK90 centile and very overweight (obesity) above the 98th centile, a slightly stricter pair of thresholds designed to reduce false positives in one-off measurements. The toggle above explains this distinction so the figures here are not mistaken for an official UK90 clinical read-out.

A note for children of South Asian ancestry

BMI-for-age was built on data that under-represents South Asian children, and the relationship between BMI and body fat is not identical across ethnic groups. Hudda and colleagues (International Journal of Obesity, 2017) measured body fat directly in more than 5,000 UK children and found that, at the same BMI-for-age, South Asian children carried meaningfully more body fat than white European children, proposing lower, ethnicity-specific BMI cut-offs to flag excess adiposity fairly. In practice this means a South Asian child sitting comfortably in the "healthy" band on standard charts may still be carrying more fat mass than that label suggests, and some paediatric clinicians factor this in informally when assessing individual children. This calculator, like most public tools, applies the standard thresholds throughout; treat the healthy band with a little extra caution for children of South Asian heritage and mention it to your GP or paediatrician if you are unsure.

What to do with a high or low BMI percentile

A single BMI percentile is a snapshot, not a verdict on your child or your parenting. Growth is meant to be tracked over months and years, and it is completely normal for a child to move between percentile bands during growth spurts and puberty. If a reading is outside the healthy band, the evidence-based response is calm and whole-family, not a diet aimed at the child. The AAP's 2023 obesity guideline and WHO guidance both point to the same everyday levers: regular sleep, less recreational screen time, fewer sugary drinks, meals eaten together as a family, and more active play built into the day, with weight or appearance kept out of the conversation with the child. If you remain concerned, or a reading sits well outside the healthy band, a GP, health visitor or paediatrician can take proper serial measurements and, where appropriate, refer for structured family support.

A short note from us: as a parent it is very easy to fixate on one number from a calculator like this. Please treat it as a starting point for a kind conversation with a health professional, not as a label for your child. Kids hear far more about their bodies than we realise, even when we think we are being careful.

Frequently asked questions

How is BMI different for children than adults?

Adults use fixed BMI cut-offs (18.5, 25, 30), but children's bodies change shape constantly as they grow, so the same BMI number means something different at age 4 than at age 14. Paediatric BMI is instead expressed as a percentile: where a child's BMI sits compared with thousands of other children of the same age and sex, using reference curves such as the CDC's 2000 growth charts.

What counts as a healthy BMI percentile for kids?

The CDC defines underweight as below the 5th percentile, healthy weight as the 5th up to the 85th percentile, overweight as the 85th up to the 95th percentile, and obesity as the 95th percentile and above. Severe obesity is defined as a BMI at or above 120% of the 95th percentile value for age and sex, or an absolute BMI of 35 or higher, whichever is the lower number.

Does the NHS use the same percentiles as the CDC?

The NHS and RCPCH use the UK90 (British 1990) growth reference rather than the CDC 2000 reference, and the two are similar but not identical. For population monitoring, such as the National Child Measurement Programme, the NHS uses the 85th and 95th centiles, matching the bands used here. For individual clinical assessment, the NHS and RCPCH instead flag overweight above the 91st centile and very overweight (obesity) above the 98th centile on the UK90 chart.

Are BMI thresholds different for South Asian children?

Evidence suggests they should be treated with more caution. A UK study of over 5,000 children by Hudda and colleagues (International Journal of Obesity, 2017) found that South Asian children carried meaningfully more body fat than white European children at the same BMI-for-age, sex and age, and proposed lower ethnicity-specific BMI cut-offs to flag excess adiposity more fairly. Some paediatric clinicians in the UK now factor this in when assessing South Asian children individually, even though most public charts and calculators, including this one, still apply the standard thresholds throughout.

My child's BMI percentile looks high or low. What should I do?

One measurement is a snapshot, not a diagnosis. Growth is meant to be tracked over time on a chart rather than judged from a single number, and children naturally cross percentile lines during growth spurts and puberty. If you are concerned, a GP, health visitor, school nurse or paediatrician can plot proper serial measurements and advise calmly, without putting your child on a diet.

Should I put my child on a diet if their BMI percentile is high?

Paediatric guidance, including the American Academy of Pediatrics' 2023 obesity guideline, is clear that restrictive dieting and weight talk aimed at children can backfire and harm their relationship with food and their body image. The recommended approach is whole-family changes: more sleep, less screen time, fewer sugary drinks, regular meals together and more active play, with clinical support such as intensive behavioural therapy offered where appropriate rather than a diet handed to the child alone.