Health & Fitness
BMI Calculator for Children and Teens
Child BMI is read as a percentile for age and sex, not against adult thresholds — this works it out for ages 2 to 20.
BMI Calculator for Children and Teens
About the BMI Calculator for Children
A child's BMI is not read against the adult 18.5 / 25 / 30 thresholds. Children are growing, and the same BMI means very different things at five and at fifteen. Instead, BMI is converted to a percentile for the child's exact age and sex.
A percentile says where a child sits among children of the same age and sex. The 60th percentile means a BMI higher than 60% of them. This calculator works for ages 2 to 20 and uses the CDC 2000 BMI-for-age growth reference — the same dataset behind the printed growth charts used in clinics.
It reports the Z-score alongside the percentile. That is the statistical distance from the median, and it is more informative than a percentile at the extremes, where percentiles bunch together and stop distinguishing.
How to use the BMI Calculator for Children
- Choose the child's sex — the growth reference differs for boys and girls.
- Enter their age in years, to the nearest half-year.
- Enter height and weight in metric or imperial.
- Select Calculate BMI percentile.
- Read the percentile, not the raw BMI — that is what the categories are based on.
How a child percentile is worked out
BMI is calculated the usual way, then compared against the reference distribution for that exact age and sex using the LMS method — the standard approach behind growth charts worldwide.
1. BMI = weight (kg) ÷ height (m)²
2. Z = ((BMI ÷ M)L − 1) ÷ (L × S)
3. Percentile = Φ(Z), the standard normal distribution
L, M and S are the skew, median and variation published by the CDC for each age and sex.
Worked example
A boy aged 10, 139 cm tall and 32 kg.
BMI = 32 ÷ 1.39² = 16.6
The CDC median BMI for boys at exactly 10 years is 16.62, so this child sits almost exactly on the median.
Around the 49th percentile — healthy weight
The percentile categories
| Percentile | Category | What it means |
|---|---|---|
| Below 5th | Underweight | Lower BMI than 95% of peers |
| 5th – 84th | Healthy weight | The expected range |
| 85th – 94th | Overweight | Higher than 85% of peers |
| 95th and above | Obese | Higher than 95% of peers |
The percentages are not arbitrary. They come from a reference population measured in US national surveys, so the 50th percentile describes what was typical for that group — not necessarily what is ideal.
What a Z-score adds
A Z-score measures how many standard deviations a child sits from the median. Zero is exactly average, +1 is one standard deviation above, −1 one below.
| Z-score | Percentile |
|---|---|
| −2.0 | 2.3rd |
| −1.645 | 5th |
| −1.0 | 15.9th |
| 0 | 50th |
| +1.0 | 84.1st |
| +1.645 | 95th |
| +2.0 | 97.7th |
Z-scores matter most at the extremes. The gap between the 97th and 99th percentile looks small but covers a wide range of BMI, and clinicians track Z-scores over time because they keep discriminating where percentiles compress.
Reading a result properly
- One reading is a snapshot. Growth happens in spurts, and a child can cross percentile lines during normal development.
- The trend matters more. A child tracking steadily along the 70th percentile is a different picture from one who moved from the 50th to the 90th in a year.
- Age must be accurate. Percentiles shift with age, so a rounded age gives a misleading answer — particularly for younger children.
- Puberty changes things. Body composition shifts substantially, and timing varies enormously between healthy children.
Once a teenager reaches 20, the adult thresholds take over and the adult BMI calculator applies instead. For an estimate of body composition rather than weight-for-height, the body fat calculator uses tape measurements — though it is designed for adults.
If a result concerns you
Take it to a doctor or paediatrician rather than acting on it alone. They will look at the child's full growth history, height percentile, family build, activity and diet together — and a single BMI percentile tells them far less than that whole picture.
Do not put a child on a restrictive diet based on a calculator. Children need adequate energy and nutrients to grow, and unsupervised restriction can do real harm, including to bone development and to a child's relationship with food. Weight management in children is handled very differently from adults, and should be led by a health professional.
This is a screening tool, not a diagnosis. It cannot see your muscle mass, bone density, medical history or family background, and it does not replace an examination. If you are concerned about your weight or health, speak to a doctor who can assess you properly.
Z-score to percentile table
The result shows a z-score next to the percentile. They tell you the same thing in two ways. A z-score counts how many standard deviations your child is from the average for their age and sex. Here is how they line up:
| Z-score | Percentile | Meaning |
|---|---|---|
| −2.0 | 2.3rd | Very low for age |
| −1.645 | 5th | Underweight line |
| −1.0 | 15.9th | Below average |
| 0 | 50th | Exactly average |
| +1.0 | 84.1st | Above average |
| +1.036 | 85th | Overweight line |
| +1.645 | 95th | Obesity line |
| +2.0 | 97.7th | Very high for age |
CDC BMI percentile for kids and teens
This calculator follows the CDC BMI-for-age growth charts, the standard in US paediatric care for ages 2 to 20. Doctors look at the trend across several visits, not one reading. A child who moves steadily along the 70th percentile is growing normally. A sudden jump of two percentile bands is worth a chat with your doctor.
Babies and toddlers under 2 (baby weight percentile)
Looking for a baby weight percentile calculator or an infant growth chart calculator? This tool starts at age 2. BMI is not used before age 2. For babies, doctors use the WHO growth charts and look at weight-for-length, weight-for-age and length-for-age percentiles. Your baby's percentile is recorded at each check-up — ask your health visitor or paediatrician to show you the chart.
Average weight by age for boys and girls
Parents often ask how much a child of a certain age should weigh. There is no single right number — healthy children vary a lot — but here are the approximate median (50th percentile) weights from the CDC growth charts:
| Age | Boys | Girls |
|---|---|---|
| 2 years | about 28 lb (12.7 kg) | about 27 lb (12.1 kg) |
| 4 years | about 36 lb (16.3 kg) | about 35 lb (15.9 kg) |
| 5 years | about 41 lb (18.4 kg) | about 40 lb (18.0 kg) |
| 6 years | about 46 lb (20.7 kg) | about 44 lb (20.1 kg) |
| 7 years | about 50 lb (22.9 kg) | about 50 lb (22.6 kg) |
| 8 years | about 57 lb (25.6 kg) | about 56 lb (25.4 kg) |
| 9 years | about 63 lb (28.6 kg) | about 63 lb (28.6 kg) |
| 10 years | about 70 lb (31.9 kg) | about 71 lb (32.4 kg) |
| 12 years | about 88 lb (40.0 kg) | about 92 lb (41.5 kg) |
| 13 years | about 100 lb (45.3 kg) | about 101 lb (45.8 kg) |
| 14 years | about 112 lb (50.8 kg) | about 109 lb (49.4 kg) |
| 15 years | about 123 lb (56.0 kg) | about 115 lb (52.0 kg) |
| 16 years | about 134 lb (60.8 kg) | about 118 lb (53.5 kg) |
How much should a 5 year old weigh? About 40–41 lb is the median. So how much should a 10 year old boy weigh? About 70 lb is the middle of the range, but anything between roughly the 5th and 85th percentile for BMI counts as healthy. Weight alone says little without height, which is why this calculator uses BMI-for-age. A tall child will weigh more than the median and still be perfectly healthy.
The same idea works as a height and weight percentile calculator check: enter the child’s age, height and weight above, and the result tells you where their BMI sits compared with other children of the same age and sex.
Other ways to use this calculator
This tool works out percentiles from CDC growth data, which answers several common searches.
- Z-score to percentile calculator — the result shows both; the table above converts any z-score.
- CDC BMI percentile calculator for kids and teens — built on CDC BMI-for-age charts for ages 2 to 20.
- Growth chart percentile calculator — for BMI-for-age; height and weight percentiles for babies come from WHO charts at your clinic.
- Child BMI calculator and teen BMI calculator — works for ages 2 to 20 using BMI-for-age percentiles.
- BMI for children vs adults — the same formula, but children are compared with others of the same age and sex.
- CDC BMI pediatric calculator — uses the CDC growth reference, the same as kids BMI chart lookups.
Frequently asked questions
Because children are still growing and the same BMI means different things at different ages. A BMI of 18 is healthy for a fifteen-year-old but high for a five-year-old, so fixed adult thresholds cannot work. A percentile compares the child against others of the same age and sex, which is the only way to read a growing child's BMI meaningfully.
Below the 5th percentile is classed as underweight, 5th to 84th is healthy weight, 85th to 94th is overweight, and 95th or above is obese. The percentile says how the child compares with peers of the same age and sex — the 90th percentile means a higher BMI than 90% of them. These are screening categories, not diagnoses.
A Z-score measures how many standard deviations a child's BMI sits from the median for their age and sex. Zero is exactly average, +1.645 is the 95th percentile. It is more useful than a percentile at the extremes, where percentiles bunch together — the 97th and 99.5th look close but cover very different BMIs. Clinicians track Z-scores over time for that reason.
Speak to your doctor or a paediatrician, and do not start any restrictive diet on your own. A single reading is a snapshot; they will look at the whole growth history, height percentile, family build and activity together. Children need adequate energy to grow, so weight management in childhood is handled very differently from adults and should be professionally supervised.
Use the normal distribution. A z-score of 0 is the 50th percentile, +1 is about the 84th, +1.645 is the 95th and +2 is about the 97.7th. Negative z-scores mirror these below the 50th percentile.
No. BMI-for-age charts start at age 2. For babies and toddlers, doctors use WHO growth charts based on weight-for-length, weight-for-age and length-for-age.
The median weight for a 13 year old girl on the CDC growth charts is about 101 lb (45.8 kg). Healthy girls of that age vary widely with height and puberty timing, so use the BMI-for-age percentile on this page rather than weight alone.
Sources and further reading
- CDC — BMI-for-age growth charts (2000 reference) — the LMS reference values this calculator uses
- CDC — About child and teen BMI — the percentile categories and how to read them
- American Academy of Pediatrics — Clinical practice guideline on childhood obesity — how paediatricians assess and manage child weight
- NHS — Child BMI and healthy weight — practical guidance for parents