Child BMI Calculator — Percentile by Age & Sex

Child BMI Calculator — Enter Age, Sex, Height, and Weight

This BMI calculator by age uses CDC‑style references to show BMI, BMI‑for‑age percentile, and a plain pediatric weight category.

How to Use Child BMI Calculator — Percentile by Age & Sex

1. Step 1: Select sex and age

   Choose Male or Female, then enter age in years and additional months (ages 2–20 years).

2. Step 2: Pick your units

   Choose Metric (cm, kg) or US (ft/in, lb) based on your measurements.

3. Step 3: Enter height and weight

   Measure without shoes and with a reliable scale. Enter values carefully.

4. Step 4: See BMI and percentile

   The tool shows BMI, BMI‑for‑age percentile, and the CDC category.

5. Step 5: Compare ranges

   Review the 5th, 50th, 85th, and 95th BMI values for your child’s exact age.

Key Features

• Age and sex percentiles
• CDC charts
• Metric/Imperial
• Pediatric categories

Understanding Results

Formula

BMI is weight divided by height squared (kg/m²). The calculator converts feet/inches and pounds to metric under the hood so the same formula applies. For children and teens, BMI is compared with age‑ and sex‑specific references to report a percentile.

Reference Ranges & Interpretation

CDC categories for ages 2–20 are defined by percentile: underweight (<5th), healthy weight (5th–<85th), overweight (85th–<95th), and obesity (≥95th). Your result also shows the BMI values at the 5th, 50th, 85th, and 95th percentiles for your child’s exact age to make comparison easy.

Assumptions & Limitations

BMI is a screening tool. It does not directly measure body fat or account for body composition. Athletic children may have higher BMI from muscle. Always look at trends over time and discuss concerns with a pediatrician rather than drawing conclusions from a single reading.

Complete Guide: Child BMI Calculator — Percentile by Age & Sex

Estimate percentiles with the child BMI calculator using CDC charts. Input height, weight, age, and sex to see BMI, percentile, and weight category instantly.

The goal is simple: enter height, weight, age, and sex, and the tool outputs BMI, BMI‑for‑age percentile, and the pediatric weight category using CDC cut‑offs. Percentiles let you compare your child’s BMI to a large reference group of children the same age and sex.

Why BMI percentiles matter for children

Adults use fixed BMI thresholds to label categories. Children are different: they are growing, and BMI normally changes with age. A percentile places a child’s BMI within an age- and sex‑matched population. For example, the 70th percentile means the BMI is higher than 70% of peers and lower than 30%.

CDC categories for ages 2–20 are defined by percentile bands, not raw BMI values: underweight (<5th), healthy weight (5th–<85th), overweight (85th–<95th), and obesity (≥95th). These bands help clinicians decide when to take a closer look, monitor more closely, or discuss lifestyle supports.

How this child BMI calculator works

The app computes BMI from your entries, then estimates a BMI‑for‑age percentile using a smooth, CDC‑style model. For ages 2–20 years, BMI distributions vary by age and sex. We mirror that pattern using a standard LMS approach (see the formula section) with a log‑normal limit, which is a widely used technique for growth charts.

Results are designed for clarity on any device. You will see the BMI‑for‑age percentile, a color‑coded category, and a short note. We also show age‑specific BMI values at the 5th, 50th, 85th, and 95th percentiles to make interpretation straightforward.

For height percentiles specifically, you can use our Child Height Percentile Calculator and for infants see the Baby Weight Percentile Calculator. When a child reaches adulthood, compare with the Adult BMI Calculator.

Inputs, units, and best practices

You can enter height and weight in metric (centimeters and kilograms) or US units (feet/inches and pounds). For age, enter years and additional months. Accurate measurements improve consistency over time.

• Measure height without shoes, standing tall against a wall, head level, heels together.
• Weigh on a level surface, ideally the same time of day and similar clothing.
• Round sensibly (e.g., one decimal place). Consistency is more important than extreme precision.
• Re‑check entries if the result seems surprising; small typos can shift the percentile.

Reading your result and what the bands mean

The headline number is the BMI‑for‑age percentile. It is the most direct way to compare against peers. Alongside, you will see the CDC category:

• Underweight: below the 5th percentile.
• Healthy weight: 5th to less than 85th percentile.
• Overweight: 85th to less than 95th percentile.
• Obesity: at or above the 95th percentile.

These are screening thresholds, not a diagnosis. Growth patterns over several measurements are more informative than a single reading. Your pediatrician may consider height percentiles, family history, development, and other health markers.

Formula and method (LMS, CDC‑style)

BMI is calculated as weight in kilograms divided by height in meters squared. To convert US units, the tool transforms feet/inches to meters and pounds to kilograms, then applies the same formula. For percentiles, children use age‑ and sex‑specific curves. A standard way to model those curves is the LMS method, which approximates the distribution of a measurement using three parameters that vary smoothly with age: L (skew), M (median), and S (coefficient of variation).

In the log‑normal limit (L ≈ 0), the z‑score is computed as z = ln(x/M) / S, where x is the child’s BMI and M and S come from age‑ and sex‑specific reference values. The percentile is then the standard normal CDF of z. This is the same backbone approach used in many pediatric growth tools. Our implementation uses a smooth, offline approximation to keep the calculator fast and privacy‑first.

To help interpretation, the result also shows the BMI that corresponds to the 5th, 50th, 85th, and 95th percentiles at your child’s exact age. Those values are obtained by inverting the same formula (x = M · exp(S·z)).

Examples (how to use the child BMI calculator)

Example 1: A 10‑year‑old boy, 140 cm tall and 35 kg. BMI ≈ 17.9. The calculator compares 17.9 to the age‑matched distribution and returns a percentile and category. If the result is around the 50th–70th percentile, that fits within the healthy range.

Example 2: A 15‑year‑old girl, 162 cm and 68 kg. BMI ≈ 25.9. The tool maps this value to the 85th or higher percentile for age, which often falls in the overweight band. Growth history, height percentile, and lifestyle context should be discussed before drawing conclusions.

Example 3: A 6‑year‑old girl, 114 cm and 17.5 kg. BMI ≈ 13.5. If the percentile is below the 5th, it will appear in the underweight band. A clinician may review nutrition, recent illness, and measurement technique, and may look at height‑for‑age percentiles using the height percentile calculator.

Limitations, context, and when to follow up

BMI is a screening tool. It does not directly measure body fat, muscle mass, bone density, or distribution of weight. Athletic children with above‑average muscle may score higher percentiles without excess fat. Conversely, a BMI in the healthy range does not guarantee optimal diet, sleep, or cardiometabolic health.

• Look at trends over time rather than a single reading.
• Use consistent measuring methods to reduce noise.
• Consider height percentiles and development stage alongside BMI.
• Discuss concerns with a clinician; avoid making medical decisions from one number.

If you want to add body composition context for teens, you can compare with the Body Fat Percentage Calculator. And if you are tracking general growth, the Growth Calculator and Child Growth Calculator offer helpful cross‑checks.

Growth, habits, and next steps

What helps most is a calm, supportive environment built around routines: regular meals rich in whole foods, age‑appropriate activity, structured sleep, and limits around screens and sugary drinks. Small changes that children enjoy tend to stick. Praise effort and behaviors rather than body size.

• Offer vegetables, fruit, lean proteins, whole grains, and dairy (or fortified alternatives) at meals.
• Make active play the default: walks, playground time, biking, backyard games.
• Keep regular bedtimes; most school‑age kids need 9–12 hours of sleep.
• Model the habits you want to see — it’s more powerful than lecturing.

If weight is trending up quickly relative to height, a pediatrician can help rule out medical issues and offer family‑centered strategies. If weight is trending low, they can screen for feeding, absorption, or other concerns — again, with a supportive, non‑judgmental approach.

Related calculators

• Child Height Percentile Calculator — compare stature by age and sex.
• Baby Weight Percentile Calculator — for infants 0–24 months (WHO‑style curves).
• Adult BMI Calculator — fixed adult categories for ages 20+.
• Body Fat Percentage Calculator — estimate composition context for teens.
• Growth Calculator — quick percentile snapshot.

References

• MedlinePlus. Body mass index in children (overview).
• Centers for Disease Control and Prevention. Growth Chart Percentile Data Files.

This page is for education only. It does not provide medical advice and does not replace guidance from your child’s clinician.

Frequently Asked Questions