Age in weeks calculator
Weeks old is days divided by seven, rounded down. A baby born eight weeks and three days ago is described as eight weeks old: the remainder is stated separately rather than rounded.
How to count weeks old
Weeks are the working unit for roughly the first three months, because feeding, sleep and growth guidance is all published that way and because the changes are weekly rather than monthly. Health services generally switch to months at around twelve weeks. The convention is always completed weeks. A baby is eight weeks old for the whole of the ninth week, in the same way an adult is 36 until their birthday. The switch to months is where a quiet mismatch appears: twelve weeks is 84 days, but three calendar months is 89 to 92 days, so "twelve weeks old" and "three months old" are five to eight days apart. It matters at the point where a chart indexed in months is being read against an age counted in weeks, and the safe move is to convert through the day count rather than assuming four weeks is a month.
Questions
Days since birth divided by seven, rounded down to completed weeks.
Because early development changes weekly, and most newborn guidance is written that way.
No. Four weeks is 28 days and no calendar month except a common-year February is that short. Over three months the gap reaches a week.
Around twelve weeks, which is where most health services change over.
No. Completed weeks is the convention, with the extra days stated separately.
Subtract the weeks born early from the chronological age. A baby born at 32 weeks is eight weeks early, so at 20 weeks old their corrected age is 12 weeks.
Commonly until around two years, by which point most children born preterm have caught up and the correction stops being meaningful.