BMI Percentile Converter
Calculate a child’s or teen’s BMI percentile (ages 2 to 20) using the CDC 2000 growth-chart reference values for every month of age.
What the BMI percentile converter does
The BMI percentile converter estimates where a child’s body mass index falls compared with other children of the same age and sex. Enter the age in years, choose male or female, type height in centimetres and weight in kilograms, and press Calculate. The tool computes BMI, converts it to a z-score with the CDC LMS method using the reference values for the child’s exact month of age, and reports the percentile together with the 5th, 50th, 85th and 95th percentile BMI for that age and sex.
Why children use percentiles, not fixed cut-offs
Adults are classed by fixed BMI thresholds (18.5, 25, 30). Children’s bodies change shape as they grow, and a healthy BMI at age 5 (about 15.5) is very different from a healthy BMI at 15 (about 20). Growth charts therefore compare a child with the distribution of BMI values for children of the same age and sex, and express the result as a percentile: the percentage of reference children with a lower BMI.
How the calculation works
- BMI = weight (kg) ÷ height (m)²
- The BMI is converted to a z-score using the LMS method: z = ((BMI / M)L − 1) / (L × S), where M is the median BMI for the age and sex, S the coefficient of variation and L the power that makes the distribution symmetrical.
- The z-score is converted to a percentile with the normal distribution.
The converter carries the full CDC 2000 table of L, M and S values for every month of age from 24 to 240 months, for boys and girls separately, and interpolates between months. Enter whole years plus the extra months for the closest match. Percentile thresholds shown in the result (5th, 85th, 95th) are derived from the same values, and severe obesity is flagged at 120% of the 95th percentile, following current CDC and AAP practice.
Interpreting the percentile
| Percentile | CDC weight category (ages 2 to 19) |
|---|---|
| Below 5th | Underweight |
| 5th to below 85th | Healthy weight |
| 85th to below 95th | Overweight |
| 95th and above | Obesity |
| 120% of the 95th percentile or above | Severe obesity |
Worked example
A 10-year-old boy, 140 cm and 40 kg, the default values. BMI = 40 ÷ 1.40² = 20.4. On the CDC chart the median BMI for a 10-year-old boy is about 16.6 and the 85th percentile about 19.4, so a BMI of 20.4 sits near the 90th percentile, in the overweight band. A 10-year-old girl of the same height and weight would land slightly lower, near the 88th percentile, because girls’ median BMI at that age is a little higher.
Reference BMI values by age
| Age | Boys: 50th / 85th / 95th | Girls: 50th / 85th / 95th |
|---|---|---|
| 5 | 15.4 / 16.8 / 17.9 | 15.2 / 16.8 / 18.3 |
| 8 | 15.8 / 18.0 / 20.1 | 15.8 / 18.3 / 20.7 |
| 10 | 16.6 / 19.4 / 22.2 | 16.9 / 20.0 / 23.0 |
| 13 | 18.5 / 21.8 / 25.2 | 18.7 / 22.6 / 26.3 |
| 16 | 20.6 / 24.2 / 27.6 | 20.5 / 24.7 / 28.9 |
| 19 | 22.5 / 26.4 / 29.7 | 21.6 / 26.1 / 31.0 |
Values are computed from the CDC 2000 LMS parameters at the child’s birthday month, the same data the converter uses. Comparing a child’s BMI with the row for their age gives a quick manual check on the output.
What a high or low percentile means
A single reading is a screening result, not a diagnosis. BMI does not distinguish muscle from fat, and children go through normal periods of rapid change, particularly around puberty. A percentile above the 85th is a prompt for a health professional to look at growth over time, diet, activity and family history; a steady track along the same percentile is usually reassuring even if it is high, while a rapid crossing of percentile lines in either direction deserves attention.
Common mistakes
- Using adult thresholds. A BMI of 20 is healthy for a 16-year-old and overweight for an 8-year-old.
- Entering height in metres or weight in pounds. The fields expect centimetres and kilograms.
- Rounding the age. Reference values change every month in childhood; enter the extra months rather than the nearest whole year.
- Applying it under age 2. Infants are assessed with weight-for-length charts, not BMI.
Related tools
The BMI calculator handles adults, and the pediatric BMI percentile calculator is a companion tool for the same age range. The ideal weight calculator, waist to height ratio calculator and body fat calculator add other measures of body composition, and the age calculator gives a precise age from a birth date.
External references
Frequently asked questions
How accurate is this converter?
It uses the same CDC 2000 reference data and LMS method as the official CDC calculator, with month-level interpolation, so results agree to within rounding. Percentiles above 99 or below 1 are reported as limits rather than exact figures.
What percentile is healthy?
Between the 5th and the 85th according to CDC categories. Where a child sits within that band matters less than whether they stay on a steady track.
Why are boys and girls calculated separately?
Their growth patterns differ, especially in the timing of puberty, so the reference distributions are sex-specific.
Can I use this for adults?
No. Adults use fixed BMI categories; use the standard BMI calculator.
Does the converter store any data?
No. The calculation runs in your browser and nothing is sent to a server.