Baby Weight Percentile Calculator
This calculator provides a simplified estimate and is not medical advice. Consult your pediatrician for an accurate growth assessment.
What a Weight Percentile Actually Tells You
A weight percentile answers one narrow question: how does this baby's weight compare to a reference population of babies the same age and sex? A 40th percentile weight doesn't mean "underweight" — it means 40% of babies that age weigh less and 60% weigh more. Pediatricians watch the trend across visits far more closely than any single number, since a steady percentile over time usually matters more than the exact figure on one day.
The Formula
z = (Weight − Median Weight for Age) / Standard Deviation
Percentile = Normal CDF(z) × 100
This calculator uses simplified median weight-for-age anchor points approximating WHO Child Growth Standards, interpolated between the ages listed below, with an estimated standard deviation. It is a streamlined approximation, not the full WHO LMS statistical method clinicians use for official growth charts.
Approximate Median Weight by Age (0–24 months)
| Age | Boys (median) | Girls (median) |
|---|---|---|
| Birth | 3.3 kg | 3.2 kg |
| 3 months | 6.4 kg | 5.8 kg |
| 6 months | 7.9 kg | 7.3 kg |
| 12 months | 9.6 kg | 8.9 kg |
| 18 months | 10.9 kg | 10.2 kg |
| 24 months | 12.2 kg | 11.5 kg |
Ages between these anchor points are interpolated. A 6-month-old boy weighing 8.5 kg, for example, works out to roughly the 71st percentile using this method.
Where This Gets Used
- Between well-child visits — a quick sanity check on a home scale reading before the next pediatric appointment.
- Tracking trend over time — comparing percentile at 3, 6, and 12 months to see whether growth is tracking consistently rather than jumping around.
- Understanding pediatrician language — making sense of percentile numbers mentioned during checkups.
How to Use This Calculator
- Enter the baby's age in months (0–24).
- Select Boy or Girl.
- Enter the baby's weight and choose kg or lb.
- Select Calculate to see the estimated percentile and category.
Related Calculations
Check length/height percentile the same way with the Baby Growth Percentile Calculator, or estimate daily formula needs with the Formula Feeding Calculator.