A growth percentile is a number that tells you how your child's size compares to other children of the same age and sex. If your daughter is at the 70th percentile for weight, it means she weighs more than 70% of girls her age in the reference population — and less than 30%. It does not mean she is 70% of some ideal weight.
Percentiles are the standard language pediatricians use to track growth. At every well-child visit, your doctor plots your child's weight, height (or length for infants), and head circumference on a growth chart. The percentile tells you where that measurement falls on the chart.
How Growth Percentiles Are Calculated
Growth percentiles are not just simple rankings. They are derived using a statistical method called the LMS method, developed by British statistician Tim Cole in 1990. The method works like this:
- Reference data collection: Researchers measure thousands of healthy children at different ages. The World Health Organization (WHO) measured approximately 8,500 children across six countries for the WHO Child Growth Standards (2006). The US Centers for Disease Control and Prevention (CDC) compiled data from five national surveys spanning 1963–1994 for the CDC Growth Charts (2000).
- Three parameters per age: For each age and sex, statisticians calculate three values: L (a power that adjusts for skewness), M (the median, or 50th percentile), and S (a measure of spread). These are the LMS parameters.
- Z-score calculation: Your child's measurement is plugged into a formula with the LMS parameters for their exact age to produce a z-score — a number that indicates how many standard deviations they are above or below the median.
- Z-score to percentile: The z-score is converted to a percentile using the normal distribution. A z-score of 0 is the 50th percentile, +1 is roughly the 84th, and −1 is roughly the 16th.
This process ensures that percentiles are smooth and accurate even at the extremes of the growth curve. You can read the full calculation methodology on our About page, including the exact formulas used.
A Visual Way to Think About It
Imagine lining up 100 children of the same age and sex from lightest to heaviest. The child at position 25 is at the 25th percentile. The child in the exact middle — position 50 — is at the 50th percentile (the median). The child at position 90 weighs more than 89 of the other children.
The same concept applies to height. On a growth chart, the curved lines represent common percentile thresholds (typically the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles). Your child's measurement is plotted as a point on this chart, and where it falls relative to the curves tells you their percentile.
You can see this in action using our weight-for-age calculator or height-for-age calculator — both display interactive growth charts with your child's measurement plotted against the standard percentile curves.
Common Percentile Ranges and What They Mean
Most parents want a quick answer: "Is my child's growth normal?" Here is how pediatricians generally interpret percentile ranges:
| Percentile Range | What It Means |
|---|---|
| Below the 3rd percentile | Below the normal range. Your pediatrician may investigate further, but this can be normal for some children — especially if parents are also smaller than average. |
| 3rd to 15th percentile | Low-normal. Common and usually healthy, but your doctor may monitor trends more closely. |
| 15th to 85th percentile | Normal range. The vast majority of healthy children fall in this band. There is nothing special about the 50th percentile — a child at the 20th is just as healthy as one at the 80th, as long as they are following their own growth curve. |
| 85th to 97th percentile | High-normal. Perfectly fine for most children. For BMI specifically, above the 85th percentile is classified as "overweight" by the CDC. Use the BMI-for-age calculator for this assessment. |
| Above the 97th percentile | Above the normal range. Worth discussing with your pediatrician, particularly for weight and BMI. |
Key point: The specific percentile number matters much less than the trend over time. A child who has always tracked along the 15th percentile is growing perfectly normally. A child who drops from the 75th to the 15th percentile over several visits may need evaluation — not because the 15th percentile is bad, but because the change is significant.
When Percentiles Matter — and When They Don't
When percentiles are most useful
- Tracking trends over time. Two or more measurements plotted on a growth chart reveal a trajectory. A consistent curve — whether at the 10th or 90th percentile — is reassuring. Crossing two or more major percentile lines (called "percentile crossing" or "faltering growth") is what gets a pediatrician's attention.
- Screening for potential concerns. Extreme values (below the 3rd or above the 97th percentile) can be early signals of nutritional, hormonal, or genetic conditions. Percentiles help catch these early.
- Comparing different measurements. A child at the 90th percentile for height and the 90th percentile for weight is proportionally large. A child at the 90th percentile for weight but the 20th percentile for height is disproportionately heavy — a pattern that BMI-for-age percentiles can help identify.
When percentiles can be misleading
- Single point in time. One percentile reading tells you very little. Growth is a movie, not a photograph. Always look at the trend across multiple visits.
- Genetic context. A child of two parents who are both in the bottom 5th percentile for height is expected to be short. The 10th percentile for this child is perfectly on target.
- Premature infants. Growth charts assume full-term birth. For premature babies, clinicians use "corrected age" (age since due date, not birth date) until around age 2. Standard growth charts used with uncorrected age will make a preemie look smaller than they are. See our guide to premature baby growth charts for details.
- Puberty timing. Early or late puberty can make a child appear to be far above or below their expected percentile. Height predictions during this window are less reliable. See our height predictor for methods that account for current growth trajectory.
WHO vs. CDC: Two Sets of Growth Charts
You may notice that your child gets a different percentile depending on whether you use the WHO or CDC chart. This is normal and expected.
- The WHO Child Growth Standards (2006) are based on healthy breastfed children from six countries and represent how children should grow under optimal conditions. They are recommended for children birth to 24 months.
- The CDC Growth Charts (2000) are based on US national survey data and describe how American children actually grew. They are the primary reference for ages 2 to 20.
The American Academy of Pediatrics recommends using WHO charts for children under 2 and CDC charts for ages 2–20. Differences of 5–15 percentile points between the two sets of charts are common, especially in infancy. Neither is "wrong" — they simply use different reference populations.
What to Do With Your Child's Percentile
- Track it over time. Use our growth dashboard to record measurements at each well-child visit and watch for the trend.
- Don't panic over a single reading. One measurement that seems high or low is not an emergency. Growth charts are screening tools, not diagnostic instruments.
- Talk to your pediatrician. If your child's percentile has changed significantly, or if you have any concerns, bring it up at your next visit. Bring the chart.
- Remember that "average" is not "ideal." The 50th percentile is the median, not a target. Healthy children come in all sizes.