Child BMI Calculator - Percentiles and WHO Growth Charts

One number, seventeen age brackets, four weight categories

A child's BMI of 18.0 can mean perfectly normal at age 14 or clinically obese at age 5. Unlike adult BMI, which uses fixed thresholds (25 = overweight, 30 = obese), child BMI must be read against age- and sex-specific percentile charts. This calculator uses simplified WHO/CDC reference data for ages 2-18, interpolates between integer ages, and returns the child's percentile rank plus a clear category: underweight, normal, overweight or obese.

How to use the Child BMI Calculator - step by step

Sex - boys and girls have different BMI percentile curves at every age.
Age (years) - full years, from 2 to 18. For children under 2, consult your pediatrician directly.
Additional months - optional, for more precise interpolation between integer ages.
Weight (kg) - weigh without shoes, in light clothing, ideally in the morning.
Height (cm) - measure without shoes, standing straight against a wall.
Read your results - BMI value, percentile rank, weight category (with color coding) and a reference table for the child's age group.

BMI reference table by age and sex

Key BMI percentile values showing how "normal" changes with age.

Age / Sex	5th (underweight)	50th (median)	85th (overweight)	95th (obese)
Boy, 3 yr	13.8	15.5	16.9	17.9
Girl, 7 yr	13.1	15.3	17.0	18.4
Boy, 12 yr	14.8	17.5	20.2	23.0
Girl, 16 yr	16.2	19.4	22.0	23.8

BMI classification for children by percentile

BMI percentile	Category	What it means
Below 5th	Underweight	Weighs less than 95% of peers - pediatrician assessment needed
5th-85th	Normal	Healthy weight - child is developing properly
85th-95th	Overweight	Weighs more than 85% of peers - monitor diet and physical activity
Above 95th	Obese	Weighs more than 95% of peers - urgent pediatrician and dietitian consultation recommended

Practical examples

Example 1: Boy, 8 years old, 26 kg, 128 cm
BMI: 15.9 - 50th percentile - Normal. Right at the median for his age group.

Example 2: Girl, 12 years old, 55 kg, 155 cm
BMI: 22.9 - 96th percentile - Obese. Above the 95th percentile threshold of 22.0 for 12-year-old girls.

Example 3: Boy, 5 years old, 16 kg, 108 cm
BMI: 13.7 - ~15th percentile - Normal (lean). In the lower normal range, typical for active children.

Example 4: Girl, 15 years, 6 months, 62 kg, 165 cm
BMI: 22.8 - ~90th percentile - Overweight. Between the 85th (21.8) and 95th (23.7) thresholds.

Example 5: Boy, 3 years old, 12 kg, 95 cm
BMI: 13.3 - ~3rd percentile - Underweight. Below the 5th percentile (13.8). A pediatrician should evaluate growth and nutrition.

FAQ - Frequently asked questions

Why is child BMI different from adult BMI?

Children's body proportions change dramatically as they grow. A BMI of 18.0 is perfectly normal for a 14-year-old but signals obesity in a 5-year-old. That is why child BMI is always interpreted as a percentile - a ranking against same-age, same-sex peers - rather than against fixed thresholds like adults use.

What is the adiposity rebound?

Between ages 2 and 6, children's BMI naturally drops to its lowest lifetime level (~15.0). Around age 5-6, it begins rising again - this is the adiposity rebound. An early rebound (before age 5) is associated with higher obesity risk later in life. That is why tracking BMI from toddlerhood matters.

Can BMI tell me if my child has too much body fat?

Not precisely. BMI does not distinguish muscle from fat. A very athletic child can have a high BMI with low body fat. For a more complete picture, pediatricians may also measure waist circumference and skinfold thickness. BMI is a screening tool, not a diagnosis.

My child is at the 90th percentile - should I worry?

The 85th-95th range is classified as overweight, not obese. It means your child weighs more than 85-95% of peers. This does not necessarily mean a problem - but it warrants monitoring. Track the trend: if the percentile is stable or declining, there is usually no concern. If it is climbing, discuss diet and activity with your pediatrician.

How often should I check my child's BMI?

The AAP recommends checking BMI at every annual well-child visit from age 2. Between visits, checking every 3-6 months is reasonable if you have concerns. A single reading is less important than the trend over multiple readings.

Do boys and girls have different BMI norms?

Yes. At every age, boys and girls have different percentile curves. For example, a 12-year-old boy with a BMI of 20.2 is at the 85th percentile (overweight threshold), while a 12-year-old girl with the same BMI of 20.0 is also near the 85th. The numbers are close at age 12 but diverge more at other ages.

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