Baby Growth Percentile Calculator
This calculator provides a simplified estimate and is not medical advice. Consult your pediatrician for an accurate growth assessment.
Length Percentile Moves Differently Than Weight
Length-for-age and weight-for-age percentiles don't always track together, and that's normal — a baby can sit at the 60th percentile for length while tracking the 40th for weight without either number being a concern on its own. Length reflects skeletal growth, which tends to be steadier over time, while weight can fluctuate more with feeding, illness, and growth spurts. This calculator estimates length/height percentile the same way official growth charts do conceptually: comparing a measurement to the age- and sex-specific median.
The Formula
z = (Length/Height − Median Length for Age) / Standard Deviation
Percentile = Normal CDF(z) × 100
This uses simplified median length-for-age anchor points approximating WHO Child Growth Standards, interpolated between ages, with an estimated standard deviation. It is a streamlined approximation, not the full WHO LMS statistical method used for official clinical growth charts.
Approximate Median Length by Age (0–24 months)
| Age | Boys (median) | Girls (median) |
|---|---|---|
| Birth | 49.9 cm | 49.1 cm |
| 3 months | 61.4 cm | 59.8 cm |
| 6 months | 67.6 cm | 65.7 cm |
| 12 months | 75.7 cm | 74.0 cm |
| 18 months | 81.7 cm | 80.0 cm |
| 24 months | 87.1 cm | 85.5 cm |
Ages between these anchor points are interpolated. A 6-month-old boy measuring 70 cm, for example, works out to roughly the 80th percentile using this method.
Where This Gets Used
- Comparing to weight percentile — checking whether length and weight percentiles are proportionate or diverging.
- Preemie catch-up tracking — parents of babies born early often watch length percentile closely as an indicator of catch-up growth.
- Well-child visit preparation — having a rough sense of the number before a pediatrician takes an official measurement.
How to Use This Calculator
- Enter the baby's age in months (0–24).
- Select Boy or Girl.
- Enter the baby's length/height and choose cm or in.
- Select Calculate to see the estimated percentile and category.
Related Calculations
Check weight percentile the same way with the Baby Weight Percentile Calculator, or estimate future adult height with the Child Height Predictor Calculator.