Average Weight for a 1-Month-Old Boy

The average (50th-percentile) weight for a 1-month-old boy is 4.5 kg (9.9 lb), based on the WHO Child Growth Standards. Across the healthy range, most boys this age weighs between 3.4 kg (7.6 lb) and 5.7 kg (12.6 lb).

What's typical at this age

The first few months are the fastest weight gain of a baby's whole life — often around 150–200 g a week. Growth spurts around 2–3 and 6 weeks can mean a day or two of extra feeding and fussiness. Between month 1 and month 2, the median boy gains roughly 1097 g.

A percentile compares your boy to other boys the same age — the 50th is simply the middle. Being above or below it is usually completely normal; a baby who has always tracked the 15th or 85th percentile is typically growing exactly as they should. What pediatricians watch for is a sudden change in the pattern, not the number itself.

Percentiles calculated from the WHO Child Growth Standards (2006). For information only — not medical advice. If you're concerned about your child's growth, speak to your pediatrician or health visitor. Full disclaimer.