Baby Weight Percentile Calculator

The Baby Weight Percentile Calculator shows where your baby's weight falls among other babies of the same age and sex using the World Health Organization (WHO) Child Growth Standards. A percentile tells you the percentage of babies who weigh less than yours. The 50th percentile is the median, but healthy babies can range from the 3rd to 97th percentile. What matters most is consistent growth along a stable curve, not hitting a specific percentile number.

How to Use This Calculator

1. Enter your baby's sex (male or female) for sex-specific WHO charts.
2. Enter your baby's age in weeks (0–13 weeks) or months (0–24 months).
3. Enter your baby's weight in kilograms or pounds.
4. Click Calculate to see the percentile and a brief interpretation.

What This Calculator Measures

This calculator outputs a weight-for-age percentile based on WHO standards.

• Percentile: The rank among 100 babies of the same age and sex; 50th = median.
• WHO standards: Based on data from healthy, breastfed babies across six countries.
• Weight-for-age: Compares weight to what's typical at a given age — not height-adjusted.
• Growth trajectory: More important than any single reading.

Formula or Logic

The calculator uses WHO LMS parameters (L = Box-Cox power, M = median, S = coefficient of variation) for each age in weeks or months. The z-score = ((Weight/M)^L − 1) ÷ (L × S). The percentile is read from the standard normal distribution corresponding to that z-score. Values below the 3rd or above the 97th percentile may warrant pediatric review.

Example Calculations

Example 1: A 3-month-old girl weighing 5.6 kg → approximately 50th percentile — right at the median for her age.

Example 2: A 6-month-old boy weighing 9.5 kg → approximately 90th percentile — larger than 90% of boys his age, but still within the normal range.

Understanding Your Results

Percentiles between the 3rd and 97th are generally considered within the healthy range. A single reading matters less than the growth trend over time. A baby who consistently tracks the 10th percentile is just as healthy as one tracking the 75th, as long as the curve is stable and rising.

Common Mistakes to Avoid

• Panicking over a low or high percentile without considering the growth trend over multiple visits.
• Comparing premature babies to full-term norms without adjusting for corrected gestational age.
• Confusing weight-for-age with weight-for-height, which measures whether a child is proportionate.
• Using adult BMI charts or non-WHO growth references for infants under 2 years.