Child BMI Percentile Calculator
BMI-for-age percentile and z-score for children and teens aged 2 to 20, from the CDC growth chart data, with the CDC weight-status category.
Result…
How it's calculated
BMI = weight (kg) ÷ height (m)² z = ((BMI ÷ M)^L − 1) ÷ (L × S), percentile = Φ(z) × 100 (L, M, S for the sex and age, interpolated between table rows) Above the 95th percentile (CDC extended): percentile = 90 + 10 × Φ((BMI − P95) ÷ sigma), z = Φ⁻¹(percentile ÷ 100) CDC categories: <5th underweight · 5th–<85th healthy weight · 85th–<95th overweight · ≥95th obesity · ≥120% of P95 or BMI ≥35 severe obesity
Children's BMI changes with age and differs between boys and girls, so it is judged as a percentile against children of the same sex and age. This uses the CDC 2000 BMI-for-age LMS data (2 to 20 years) as CDC publishes it, interpolated between its half-month rows, and CDC's 2022 extended method for BMIs above the 95th percentile, where the plain LMS curves are less reliable. Ages entered in years and months use the middle of that month, as CDC's tables do; dates give the exact age. Other countries use other references (for example the WHO 2007 references or the UK90 charts), so a percentile and category can differ from the chart your clinic uses. BMI does not measure body fat directly.
Your numbers, step by step
About the Child BMI Percentile calculator
A child's BMI cannot be read with the adult bands, because body shape changes as children grow and differs between boys and girls. This calculator works out the BMI from height and weight, in metric or imperial, then places it as a percentile and z-score against children of the same sex and age on the CDC 2000 BMI-for-age charts, which cover ages 2 to 20. Above the 95th percentile it switches to CDC's 2022 extended method, which CDC built because the original curves are less reliable at very high BMIs. You then get the CDC weight-status category. Enter the age in years and months, or use the date of birth and the date measured for an exact age. Every step is shown with your numbers, from the table row used to the final percentile.
How to use it
- Choose “Boy” or “Girl” under “Sex”.
- Under “Age from”, enter the age in years and months, or switch to “Dates” and enter the “Date of birth” and “Date measured”.
- Pick metric or imperial under “Units”, then enter the “Height” and “Weight”.
- Read the BMI-for-age percentile, the z-score and the CDC weight status; the steps show the L, M and S values used for that age.
Worked example
A boy aged 10 years 0 months is 140 cm (4 ft 7 in) tall and weighs 35 kg (77 lb). His BMI is 35 ÷ 1.4² = 17.9 kg/m². The calculator uses the half-month point, 120.5 months, and the CDC table for boys. That gives a z-score of 0.53 and a percentile of about 70.2. At this age the 5th, 85th and 95th percentiles are 14.2, 19.4 and 22.2 kg/m², so his BMI is in the healthy weight range.
Updated
Questions people ask
How do I calculate a BMI percentile for a child?
Work out the BMI as weight in kg ÷ height in metres squared. Then look up the L, M and S values for the child's sex and age in the CDC table and use z = ((BMI ÷ M)^L − 1) ÷ (L × S). The percentile is the share of the normal curve below that z-score.
What BMI percentile is healthy for a child?
CDC calls the 5th to under the 85th percentile healthy weight. Under the 5th is underweight, the 85th to under the 95th is overweight, and the 95th or above is obesity. Severe obesity is 120% of the 95th percentile or more, or a BMI of 35 or more.
Why does the calculator use 114.5 months for 9 years 6 months?
CDC's tables list each month at its half-month point, so 114.5 months stands for any age from 114.0 up to 115.0 months. CDC's own worked examples use that point. For an exact age, enter the dates instead.
Why can my child's percentile differ from the one at the clinic?
Countries use different growth references, such as the WHO 2007 references or the UK90 charts. The same BMI can fall at a different percentile, or in a different category, on another chart. Measurement errors of 1 cm or a few hundred grams also move the result.
Can I use the adult BMI calculator for my teenager?
Not before 20. Adult bands ignore age and sex. For adults, use the BMI calculator.
Is a high BMI percentile a diagnosis?
No. BMI does not measure body fat directly and does not tell muscle from fat. It is a screening number. A doctor, nurse or health visitor reads it alongside growth over time, so talk to one if you are worried. The method is on CDC's extended BMI data page.