Baby Growth Percentile Calculator
Compare a baby's weight, length and head circumference with the WHO growth standards from birth to 24 months.
About the Baby Growth Percentile Calculator
The WHO Child Growth Standards describe how healthy breastfed babies grow in good conditions; the CDC and AAP recommend them for children under 2 in the US. Each measurement is converted to a z-score with the WHO's LMS parameters (interpolated by age) and then to a percentile.
Doctors look for steady tracking along a percentile curve. Below the 2nd or above the 98th percentile, or crossing two major percentile lines, is worth discussing with your health visitor or paediatrician.
The formula
z = ((X ÷ M)^L − 1) ÷ (L × S); percentile = Φ(z)
Worked example
Example: Weight percentile = 99.7th
- Inputs: Sex: Girl; Date of birth: 1 January 2026; Date measured: 15 March 2026; Units: lb / in; Weight: 17 kg or lb; Length: 29 cm or in; Head circumference: 18 cm or in.
- Formula: z = ((X ÷ M)^L − 1) ÷ (L × S); percentile = Φ(z)
- Result: Weight percentile = 99.7th
Questions people ask
Is the 50th percentile the goal?
No; any percentile can be healthy if growth is steady.
Premature baby?
Use corrected age (see the baby age calculator) until about age 2.