Educational tool — not medical advice. Verify with a qualified healthcare professional before making clinical decisions.
How Weight-for-Age Percentiles Work
Weight-for-age percentiles compare your child's weight to a reference population of children the same age and sex. A child at the 50th percentile weighs more than 50% of children their age — this is the median, not a "target." Any percentile between the 3rd and 97th is considered within the normal range.
The two main reference datasets used worldwide are:
- WHO Growth Standards — based on healthy breastfed infants from six countries, representing how children should grow under optimal conditions. Recommended for children birth to 24 months.
- CDC Growth Charts — based on US national survey data, describing how American children actually grew. Recommended for children age 2 to 20 years.
This calculator shows both charts side by side when applicable, and marks the one recommended by the American Academy of Pediatrics (AAP) for your child's age.
What's More Important Than the Number
A single percentile reading is less important than the trend over time. A baby consistently tracking at the 15th percentile is growing normally — they're simply smaller than average. What pediatricians watch for is a child crossing two or more major percentile lines (called percentile crossing or growth faltering), which can indicate a feeding issue, illness, or other concern worth investigating.
Weight-for-Age FAQ
How do I use this weight percentile calculator?
Select your child's sex, enter their age (either by birth date or directly in years and months), and type their weight. Click "Calculate Percentile" to see the result on both WHO and CDC growth charts along with a plain-language interpretation.
What is a normal weight percentile for a baby?
Any percentile between the 3rd and 97th is within the normal range. A baby at the 20th percentile is just as healthy as one at the 80th — what matters is that they follow their own growth curve consistently over time rather than crossing multiple percentile lines.
Why does my baby's WHO percentile differ from the CDC percentile?
The WHO and CDC charts use different reference populations. WHO charts are based on healthy breastfed infants from multiple countries, while CDC charts reflect how US children actually grew. Breastfed babies often show higher percentiles on WHO charts in the first 6 months, then may appear to "fall" on WHO charts after 6 months compared to CDC — this is normal and reflects different growth patterns between breastfed and formula-fed infants.
When should I worry about my child's weight percentile?
Talk to your pediatrician if your child's weight is below the 3rd percentile, above the 97th percentile, or if their weight percentile has crossed two or more major percentile lines (e.g., dropping from the 50th to the 10th) over several visits. A single reading outside the typical range is rarely cause for concern on its own.
Is this calculator accurate for premature babies?
For premature infants, pediatricians typically use corrected age (adjusted for weeks of prematurity) until age 2. If your baby was born early, enter their corrected age rather than their actual age for a more accurate percentile reading. Specialized preterm growth charts (such as Fenton) may be more appropriate for very preterm infants — consult your pediatrician.
What is a z-score and how does it relate to percentile?
A z-score measures how far your child's weight is from the median in standard deviation units. A z-score of 0 means exactly at the 50th percentile (median). A z-score of +1 is roughly the 84th percentile, and −1 is roughly the 16th percentile. Clinicians often use z-scores because they're more precise at the extremes of the distribution where percentiles compress.