Birth Weight Percentile Calculator
Estimate where a baby’s birth weight falls compared to typical babies born at the same gestational age.
How this estimate works
Growth charts describe how birth weight is expected to vary at each week of gestation. This calculator uses approximate average birth weight and spread (standard deviation) figures for each gestational week, based on the general shape of published growth chart data, and calculates a z-score for the entered weight, then converts that z-score into an estimated percentile using the standard normal distribution.
What the percentile means
Babies between the 10th and 90th percentile are generally considered appropriately grown for their gestational age. Weight below the 10th percentile is often labeled ‘small for gestational age’ and above the 90th percentile ‘large for gestational age,’ though these labels alone don’t diagnose any condition.
This calculator uses simplified, general-purpose reference values and is intended for education and general interest only. Clinical growth assessments should use official sex-specific growth charts (such as Fenton, WHO, or Intergrowth-21st) reviewed by a pediatrician or obstetric provider.