Adult BMI uses fixed cutoffs — under 18.5 is underweight, 25 and over is overweight. But a 7-year-old girl with a BMI of 17 is perfectly normal, while the same BMI on a 16-year-old would raise a concern. That is why the Centers for Disease Control and Prevention (CDC) stopped judging children by adult BMI numbers and instead uses BMI-for-age percentiles, which compare a girl's BMI against other girls of the same age. This calculator computes that pediatric BMI, looks up the CDC percentile for girls aged 2 to 20, and tells you whether your daughter falls in the underweight, healthy, overweight, or obese range.
Enter her age in years, her weight in pounds, and her height in inches. The result updates instantly, showing the BMI number, the percentile, the weight-status category, and the approximate healthy weight range for her height.
How to Use
- Enter the girl's Age in whole years. The CDC growth charts cover girls from 2 to 20 years old. If your daughter is younger than 2, use a weight-for-length chart instead — BMI-for-age is not the right tool.
- Type her current Weight in pounds. For the most accurate reading, weigh her on a digital scale first thing in the morning, without shoes or heavy clothing.
- Enter her Height in inches. Measure her against a wall without shoes, standing straight with heels together.
- Read the result card. The BMI is the body mass index number (kg/m² equivalent), the CDC Percentile tells you where she ranks among girls her age, and the Weight Status label translates that percentile into a category.
- Use the Healthy Weight range as a target band, not a strict prescription. It shows roughly what she would weigh at the 5th and 85th percentiles for her current height.
What the weight-status code means. The result card returns a small number that the page maps to a plain label: 0 = Underweight (below the 5th percentile), 1 = Healthy weight (5th to below the 85th), 2 = Overweight (85th to below the 95th), and 3 = Obese (95th or above). The percentile is shown as a whole number followed by a percent sign; the healthy-weight low and high lines together show the pound band that matches the 5th–85th percentile range for her current height.
A single measurement is a snapshot, not a verdict. Growth is a curve — pediatricians track BMI percentiles over months and years on a growth chart, and they look at the trend, not one point.
How Pediatric BMI Differs from Adult BMI
The formula itself is the same Quetelet index:
BMI = weight in pounds ÷ (height in inches)² × 703
What changes is how the number is interpreted. For adults, the categories are fixed thresholds. For children, the CDC plots BMI on a percentile curve built from large national survey data. The categories are:
- Underweight — below the 5th percentile.
- Healthy weight — from the 5th percentile to below the 85th.
- Overweight — from the 85th to below the 95th percentile.
- Obese — at or above the 95th percentile.
So a BMI of 16.9 at age 10 lands near the middle of the healthy range for a girl, but that same BMI at age 16 would sit closer to the low end. Age is not a footnote — it is the whole point of the percentile lookup.
The percentile is computed with the LMS method, which captures the skewed shape of growth data: L (the power transformation), M (the median), and S (the coefficient of variation) are looked up at the girl's exact age, and the BMI is converted to a z-score and then to a percentile. This is the same method the CDC's Epi Info software and the WHO growth standards use. Girls' curves are published separately from boys' because girls' body composition — especially as puberty begins — changes on its own timeline.
Examples
Example 1 — A typical 10-year-old. A girl weighs 70 lbs and is 54 in tall. BMI = 70 ÷ 54² × 703 = 70 ÷ 2916 × 703 ≈ 16.88. On the CDC girls chart at age 10, that is around the 57th percentile (57%) — right in the middle of the healthy range, which spans the 5th percentile (5%) up to the 85th percentile (85%). The healthy weight band for 54 in is about 53 to 75 lbs.
Example 2 — An underweight 8-year-old. A girl weighs 45 lbs and is 52 in tall. BMI = 45 ÷ 2704 × 703 ≈ 11.70. At age 8, that falls below the 5th percentile (5%), so the calculator labels her Underweight. Her parents should discuss this with her pediatrician, because crossing down two major percentile lines over time is the kind of trend that warrants attention.
Example 3 — A 10-year-old who is obese. A girl weighs 120 lbs at 54 in. BMI = 120 ÷ 2916 × 703 ≈ 28.93. At age 10, the 95th percentile (95%) is around BMI 21.5, so 28.9 is well above it — the calculator flags Obese (about the 99th percentile, 99%). This is the category where early intervention matters most, because childhood obesity tracks into adulthood in a large share of cases.
Example 4 — The percentile, not the raw BMI, is what matters. Two girls both have a BMI of 18. A 7-year-old at BMI 18 is around the 90th percentile (90%, overweight). A 14-year-old at BMI 18 is around the 50th percentile (50%, healthy). Same number, very different story — which is exactly why this calculator asks for age.
FAQ
What is a normal BMI for a 10-year-old girl?
A 10-year-old girl's median (50th percentile) BMI is about 16.6. A reading between roughly 13.7 (5th percentile) and 19.2 (85th percentile) is considered a healthy weight. Above 19.2 she would be classified as overweight, and at or above about 21.5 she would be in the obese range.
How is BMI-for-age different from adult BMI?
Adult BMI uses fixed cutoffs (18.5, 25, 30) for everyone. Pediatric BMI uses the same formula but plots the result on a CDC percentile curve that compares the child to other children of the same age and sex. A BMI that is normal for a teenager would be overweight for a toddler, so age percentiles are essential.
What BMI percentile is overweight for a girl?
The CDC defines overweight as the 85th percentile up to, but not including, the 95th percentile for girls the same age. Obese is the 95th percentile or above. Underweight is below the 5th percentile.
My daughter's BMI says she is obese — should I panic?
No, but do not ignore it either. The percentile is a screening tool, not a diagnosis. Childhood obesity is best addressed with small, sustainable changes — more sleep, less screen time, water instead of sugary drinks, daily active play, and family meals. A pediatrician can look at the growth trend over time and decide whether referral to a dietitian or weight-management program is appropriate.
Can I use this calculator for a boy?
No. The CDC publishes separate percentile curves for boys and girls because their body composition changes differently as they grow. A BMI of 17 means slightly different things on the girls' and boys' charts at the same age. Use a boys' BMI calculator for your son.
At what age does BMI-for-age stop applying?
The CDC BMI-for-age growth charts run from age 2 to 20. At age 20, you switch over to the adult BMI categories. This calculator is intended for girls in that 2–20 year window.
Is BMI a direct measure of body fat?
No. BMI is a ratio of weight to height, and it does not distinguish muscle from fat. A dancer or runner can have a higher BMI with very little body fat. Pediatricians use BMI-for-age as a quick screening tool, and they follow up with waist circumference, skinfold measurements, or clinical judgment when the number looks off.
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Disclaimer
This BMI for girls calculator is provided for informational purposes only and is not medical advice. It is a screening estimate based on self-reported height, weight, and age and on CDC BMI-for-age percentile data; it does not replace a diagnosis, growth-chart assessment, or evaluation by a licensed pediatrician. Children grow at different rates, and a single percentile reading does not determine health on its own. Always discuss your daughter's growth and weight with her doctor, who can interpret trends over time in the context of her overall development, family history, and diet.