BMI Calculator for Kids
Calculate body mass index, BMI Prime, and an age- and sex-specific BMI percentile for children ages 2 through 10 using CDC growth-chart parameters.
Measurements
Metric: 30.48 – 274.32 cm. US: 12 – 108 in. Use a decimal point, not a decimal comma.
Metric: 4.535 – 453.592 kg. US: 10 – 1,000 lb. Grouping commas are accepted only in US-style whole thousands.
Required for the sex-specific CDC BMI-for-age reference curves.
This kids-focused version uses completed integer ages 2 – 10.
Live results
At age 8 for a girl, this BMI is about the 72.36th percentile on the CDC 2000 BMI-for-age reference.
BMI 17.09 kg/m², BMI percentile 72.36th, CDC screening category healthy weight.
BMI position versus age-specific reference points
The bars compare the current BMI with the selected child's 5th, 50th, 85th, and 95th percentile BMI values. Every bar uses the same unit and a zero baseline.
Complete all inputs to display the reference comparison.
Current BMI 17.09 kg/m²; age-specific 5th percentile 13.54, 50th 15.83, 85th 18.32, and 95th 20.70 kg/m².
Reference details
| Reference point | BMI (kg/m²) | How it is used |
|---|---|---|
| Current BMI | 17.09 | Calculated from the entered height and weight. |
| 5th percentile | 13.54 | Below this percentile is the CDC underweight screening category. |
| 50th percentile | 15.83 | Median BMI on the selected age- and sex-specific reference curve. |
| 85th percentile | 18.32 | Starts the CDC overweight screening category. |
| 95th percentile | 20.70 | Starts the CDC obesity screening category. |
Percentile values use CDC 2000 BMI-for-age LMS parameters at the first half-month point for the selected completed year of age.
How to use this BMI calculator for kids
What this calculator does. This tool calculates a child's body mass index from height and weight, then places that BMI on an age- and sex-specific CDC BMI-for-age reference for children. It also shows BMI Prime, which is simply BMI divided by 25 and is included because it appears in many general BMI tools; BMI Prime is an adult-oriented comparison and should not be used by itself to judge a child's growth. The percentile and CDC screening category are the more relevant pediatric outputs. BMI is a screening measure, not a diagnosis, and it does not directly measure body fat, fitness, nutrition quality, or health. The CDC Child and Teen BMI Calculator guidance likewise emphasizes age- and sex-specific interpretation.
When to use it. Use the calculator to reproduce a BMI discussed at a routine visit, to understand how a child's height and weight combine mathematically, to see how the same BMI can fall at different percentiles for boys and girls or at different ages, or to prepare neutral questions for a pediatric clinician. It is most useful as one point in a growth record, not as a stand-alone verdict about a child.
How to calculate. The calculator opens with a ready-to-use demonstration: Metric units, height 128 cm, weight 28 kg, Girl, age 8. The live results and the example XLSX workbook are already available. To use your own measurements:
- Choose Unit system. Selecting Metric uses centimeters and kilograms. Selecting US customary converts the current height to inches and weight to pounds rather than merely changing the unit label.
- Replace Height and Weight with measured values. Results update immediately after each valid edit.
- Select Sex for CDC chart and Age in years. These choose the CDC reference curve used for percentile calculations; they do not change the BMI formula itself.
- Read BMI, BMI Prime, BMI percentile, and CDC screening category. The comparison chart and reference table use the same canonical model values.
- Select Download Excel to create a current-state workbook with Summary, Inputs, and Reference sheets. Reset clears the demonstration and all data fields, so export is disabled until a complete valid set of inputs is entered again.
Input guide. Unit system is required and accepts Metric or US customary. Example: Metric. Changing it converts the current measurements while preserving the underlying height and weight. Height is required and accepts a positive decimal using a period as the decimal separator; the supported range is 30.48 – 274.32 cm or 12 – 108 in. Example: 128 cm. A decimal comma such as “128,5” is rejected rather than silently reinterpreted. Higher height with unchanged weight lowers BMI because height is squared in the denominator. Weight is required and accepts 4.535 – 453.592 kg or 10 – 1,000 lb. Example: 28 kg. Higher weight with unchanged height raises BMI proportionally. Sex for CDC chart is required and accepts Boy or Girl because the CDC reference curves are sex-specific; example: Girl. Age in years is required and accepts completed integer ages 2 through 10; example: 8. Age affects the percentile reference but does not change the raw BMI. For more precise ages in months, use a clinical growth-chart tool that accepts month-level age.
Output guide. BMI is weight in kilograms divided by height in meters squared and is shown in kg/m². BMI Prime is BMI ÷ 25; a value of 1.000 therefore corresponds to BMI 25, but this adult benchmark is not a pediatric category. BMI percentile estimates the percentage of the CDC reference population of the same sex and age with a lower BMI. CDC screening category uses the standard pediatric cutoffs: underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at or above the 95th. The CDC child and teen BMI category definitions explain these thresholds. The chart compares Current BMI with the 5th, 50th, 85th, and 95th percentile BMI values for the selected age and sex; the Reference details table gives the exact same five numbers in text form.
Worked example. With the startup example of 128 cm and 28 kg, height is 1.28 m and BMI = 28 ÷ (1.28 × 1.28) = 17.09 kg/m². BMI Prime is 17.08984375 ÷ 25 = 0.684 after display rounding. For an 8-year-old girl, the CDC LMS parameters used here are L = – 2.617192, M = 15.827, and S = 0.117159. Applying the CDC LMS z-score transformation gives a z-score of about 0.594, corresponding to a BMI percentile of 72.36th. That falls between the 5th and 85th percentiles, so the displayed CDC screening category is Healthy weight. The same values appear in the first-open result cards, chart, table, and exported workbook.
Learn more. Accurate height and weight matter because small measurement errors can noticeably move BMI, especially near a percentile cutoff. CDC provides practical instructions for measuring children's height and weight. For the statistical model itself, the CDC growth-chart LMS data documentation explains the L, M, and S parameters and the z-score equations used for percentile calculations.
How the calculation works
BMI = weight (kg) ÷ height (m)². For US customary entries, the calculator first converts inches to meters and pounds to kilograms, so one canonical metric model drives every result and the workbook. The BMI percentile then uses CDC's LMS transformation: z = (((BMI ÷ M)L) – 1) ÷ (L × S), followed by the standard normal cumulative distribution. The selected age maps to the first half-month point of that completed year in the CDC 2000 BMI-for-age table.
Interpreting a child's BMI carefully
Children grow at different rates, and one measurement can be affected by recent growth, measurement technique, clothing, hydration, and ordinary biological variation. A percentile is a comparison with a reference population, not a target that a child must move toward. Trends across repeated, well-measured visits are typically more informative than a single point. If a result is unexpected, verify the measurements and discuss the growth pattern with a qualified pediatric health professional rather than changing a child's diet or activity solely because of this calculator.
Medical disclaimer: This calculator is educational and does not provide medical advice, diagnose a condition, or replace a pediatric assessment. BMI is one screening measure among many factors used to understand growth and health.