BMI Calculator for Kids & Teens (CDC BMI-for-age percentile)

Understanding the BMI calculator for kids and teens — concept and principle

For children and teenagers, BMI alone means little because it changes with age and differs between boys and girls. This calculator compares a child's BMI with the CDC BMI-for-age charts for ages 2 to 20 and gives the percentile, z-score and CDC weight category, including the 2022 extended method for very high BMI.

Boy, 8 y 6 m, 128 cm, 27 kg: with the default inputs the result is 61.2 percentile (healthy weight (5th–85th)).

How it is calculated

BMI = weight (kg) ÷ height (m)²

z = ((BMI ÷ M)^L − 1) ÷ (L × S), with L, M and S from the CDC table for the child's sex and age in months (taken at mid-month)

Percentile = share of the normal distribution below z

Above the 95th percentile, the CDC 2022 extended method is used: percentile = 90 + 10 × Φ((BMI − P95) ÷ σ).

Categories: under 5th underweight, 5th to below 85th healthy weight, 85th to below 95th overweight, 95th or above obesity, 120% of P95 severe obesity class 2, 140% class 3.

The principle

BMI normally falls in early childhood, reaches a low point around age 5–6 and then rises through adolescence. Comparing a child only with others of the same age and sex removes that pattern. The LMS method turns the skewed BMI distribution into a normal one so a z-score and percentile can be calculated. The original LMS curves fit poorly at very high BMI, which is why the CDC introduced an extended method for that range.

Worked example

Default inputs: boy, 8 years 6 months, 128 cm, 27 kg.

BMI = 27 ÷ 1.28² = 16.5.
Age = 102.5 months; CDC values L = −3.079, M = 15.96, S = 0.1070.
z = ((16.48 ÷ 15.96)^−3.079 − 1) ÷ (−3.079 × 0.1070) = 0.28 → percentile 61.2.
5th, 85th and 95th percentile BMI: 13.9, 18.3, 20.6; BMI is 80% of the 95th.
Category: healthy weight.

Each point is the calculator re-run with only “Child’s height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Child’s height (cm)	BMI-for-age percentile
32	100
64	100
96	99.8
128	61.2
160	0
192	0
256	0

How to read the result

Where does your result fall?

The percentile shows where the child sits among peers; a single value says little about health on its own. A doctor looks at growth over time, family history, diet, activity and, if needed, other measures. Weight concerns should never lead to restrictive dieting in children without professional guidance.

Common mistakes

Using adult BMI categories (25, 30) for a child.
Rounding age to whole years, which shifts the percentile.
Measuring height with shoes on.

Keep in mind: This is a screening measure, not a diagnosis. Discuss your child's growth with a doctor.

Frequently asked questions

What is a healthy BMI percentile for a child?

From the 5th to below the 85th percentile.

Why does the calculator use months?

BMI changes quickly with age, so the CDC tables are given by month.

Should I use WHO or CDC charts?

The CDC charts are used here for ages 2–20; many countries have their own national references.

Sources: Kuczmarski RJ et al., "2000 CDC Growth Charts for the United States: Methods and Development", Vital Health Stat 11(246), 2002 · Wei R et al., CDC extended BMI-for-age growth charts, Vital Health Stat 2(197), 2022

Checked 9 Nov 2026. CDC 2000 BMI-for-age growth charts, LMS parameters (Kuczmarski et al., Vital Health Stat 11(246), 2002); CDC 2022 extended BMI-for-age percentiles (Wei et al., NCHS Data Brief / Vital Health Stat 2(197), 2022). LMS and σ values as distributed in the RCPCH rcpchgrowth package.

Your numbers