BMI-for-Age Percentile Calculator

What a BMI-for-age percentile means

BMI-for-age ranks a child's body mass index among children of the same sex and completed month of age on the CDC 2000 growth charts. The adult cutoffs of 25 and 30 do not apply. A BMI of 18 can sit in the healthy band at one age and the overweight band at another.

CDC categories for ages 2 through 19 are underweight below the 5th percentile, healthy weight from the 5th to under the 85th, overweight from the 85th to under the 95th, and obesity at or above the 95th. Severe obesity is 120% of the 95th percentile BMI or a BMI of 35 kg/m^2, whichever comes first.

The 2000 charts describe a US reference from surveys collected before the later rise in child obesity. A child at the 95th percentile today is not "average among classmates." The percentile is a historical reference, which is why clinics still use it as a screen.

AAP's 2023 obesity guideline tells primary care to measure height and weight, calculate BMI, and assess BMI percentile at least yearly from age 2 through 18. This page is that screen in a parent-facing box, not a diagnosis visit.

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How to use the BMI-for-age calculator

Pick US or metric units, then boy or girl. Type completed years and extra months. A child who turned 8 in March and you are measuring in June is 8 years and 3 months, not 8.0 and not 8.5 guessed from a report card.

Enter standing height without shoes and weight in light clothes. The highlighted line is the BMI-for-age percentile. BMI, z-score, CDC category, and the 95th percentile BMI for that sex and age sit under it.

If years plus months total fewer than 24 months, the tool stops and sends you to weight-for-length. CDC BMI-for-age tables start at 24 months. Infant BMI exists on WHO files. US clinics do not use it as the overweight screen before age 2.

Race is not an input. CDC BMI percentiles do not have a race selector. If a well-visit printout used a specialty chart, type those numbers into that chart, not this one.

You can switch units after you enter numbers. Hidden US fields keep their last value. If you typed 56 lb, then flipped to metric, type the kilogram value you actually measured. Do not assume the hidden 25.4 kg default is still your child.

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Formula, LMS tables, and a unit check

US BMI = 703 x pounds / inches squared. Metric BMI = kilograms / meters squared. Those two expressions match when pounds, inches, kilograms, and centimeters are the same child.

CDC publishes L, M, and S for each sex and half-month of age. Z = ((BMI / M)^L - 1) / (L x S). If L is 0, Z = ln(BMI / M) / S. Percentile is the standard normal area to the left of Z. A Z of 1.645 is the 95th percentile.

The 95th percentile BMI on the result card is M x (1 + L x S x 1.645)^(1/L). Severe obesity compares the child's BMI with 1.2 times that value, and also with 35. Either threshold is enough.

Age on CDC files is listed at the half-month point for that completed month. This calculator looks up 96.5 months for a child who has completed 96 months (8 years 0 months). Linear interpolation fills ages between published rows.

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Worked example: CDC's 12-year-old girl

CDC walks through a 12-year-old girl who is 5 feet tall (60 in, 152.4 cm) and weighs 136 lb (61.7 kg). BMI = 703 x 136 / 60^2 = 26.6. On the CDC BMI-for-age chart that BMI is about the 96th percentile, which is obesity.

Run the same inputs here: sex girl, 12 years, 0 extra months, 60 in, 136 lb. Expect BMI near 26.6, a percentile in the mid-90s, and the obesity category. The 95th percentile BMI printed beside it should sit below 26.6. If 26.6 is also at or above 1.2 times that 95th value, the severe-obesity line appears instead.

A different child, an 8-year-old boy at 50 in and 56 lb, has BMI = 703 x 56 / 2500 = 15.7. That is near the middle of the healthy band for many boys that age, not "skinny" because 15.7 would be underweight in an adult table.

AAP also publishes class 2 obesity (120% to under 140% of the 95th, or BMI 35 to under 40) and class 3 (140% of the 95th or BMI 40 and up). This result card collapses those into the severe-obesity line. A clinic that is staging treatment may still split class 2 from class 3 on the visit summary.

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Height and weight the same way the chart expects

Stand the child against a wall or stadiometer, heels together, head in the Frankfort plane, no shoes. Hair ties that add height stay off. A 1 inch error in height moves BMI more than a 1 pound error in weight at school age, because height is squared.

Weigh on a floor scale that reads to at least 0.2 lb, after the child uses the toilet if you can wait. Heavy jeans and a hoodie can add a pound. That pound rarely flips a child across the 85th percentile by itself. Combined with a short height reading, it can.

Do not use a last-year well-visit height with this week's weight. Stretch during a growth spurt and the old height inflates BMI. Type numbers from the same day.

Ask the child to breathe out gently and stand tall, not to "make yourself bigger" by lifting the heels. Heel lift is cheating height and lowering BMI. A nurse who is plotting obesity care needs the honest number.

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What to do with the category the same week

A healthy-weight result is a screen, not a hall pass for unlimited juice. Pair it with the lunchbox sugar tally if snacks are the question you actually had.

Overweight or obesity on this page is the CDC label for that day's BMI. AAP still wants a clinician to look at trend, family history, and comorbidities. Referral talk in the 2023 guideline starts from BMI at or above the 95th percentile, not from a parent arguing with a percentile line at the kitchen table.

Underweight below the 5th percentile needs the same clinic visit energy as a high BMI: illness, intake, and height-for-age together. A short child with a low BMI is a different problem than a tall child who is thin and growing along the 4th percentile for years.

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Why this page refuses infants

CDC recommends WHO Child Growth Standards from birth to 2 years and CDC charts from 2 years on. Weight status under 2 is plotted on WHO weight-for-length. BMI-for-age is not the recommended infant overweight screen in US clinics, even though WHO publishes infant BMI files.

At the 2-year birthday a child can change category because the indicator changes (weight-for-length to BMI), the measurement changes (lying length to standing height, about 0.8 cm shorter), and the cutoff language changes (WHO 2nd/98th versus CDC 5th/85th/95th). A drop in percentile at that visit is often the chart switch, not overnight weight loss.

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Accuracy limits, edge cases, and safety

LMS z-scores flatten at very high BMI. That is why severe obesity uses 120% of the 95th percentile and why CDC added 2022 extended BMI-for-age charts above the 95th. This calculator uses classic LMS through the 95th and the 120%/BMI-35 rule for severe obesity. It does not load the extended sigma file, so a child far above the 95th gets a category, not a precise 99.9th extended percentile.

Down syndrome, skeletal dysplasia, and some endocrine conditions have separate charts. Do not treat a CDC percentile as the last word there.

Competitive swimmers and football linemen can plot high because of fat-free mass. The category still prints. The next step is a clinician, not deleting the result.

Ages 19 to 20 remain on CDC BMI-for-age tables. Many clinics switch to adult BMI at 20. This tool follows the table through 20 years (under 240 months). Past that, it stops.

Home numbers that jump two major percentile bands in a few months deserve a remeasure before anyone panics. Plotting errors are common. True rapid change is also a reason to call the office.

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How it works

BMI = 703 x lb / in^2 in US units, or kg / m^2 in metric. Z = ((BMI/M)^L - 1) / (L x S) using CDC L, M, and S for that sex and completed month of age. Percentile is the standard normal CDF of that z-score. Children under 24 months are sent to weight-for-length.

Frequently asked questions

• Is this the same as an adult BMI calculator?

  No. Adult cutoffs such as 25 and 30 do not apply. The result is a percentile among children of the same sex and age on the CDC 2000 charts.

• Can I use this for a 1-year-old?

  No. CDC BMI-for-age starts at 24 months. Use weight-for-length for infants.

• Does a high percentile diagnose obesity?

  It is the CDC screening category. A clinician still measures, plots a trend, and decides what follow-up is needed.

Related calculators

References

1. CDC. Child and Teen BMI Categories
2. CDC NCHS. CDC Growth Charts Data Files (LMS values)
3. CDC. Growth Charts
4. Hampl et al., AAP. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity
5. CDC NCHS. Data file for the Extended CDC BMI-for-age Growth Charts
6. CDC. SAS Program for CDC Growth Charts
7. Kuczmarski et al.. CDC growth charts: United States
8. CDC. Using WHO Growth Standard Charts