Baby Age Calculator — weeks, months & corrected age
How many weeks and months old your baby is today, plus corrected age if they arrived before their due date.
What this calculates
Babies are counted in weeks at first, then months — and the two do not line up neatly, because months have different lengths. This calculator gives both: whole weeks and days since birth, and calendar months the way people say them, so a baby born on 15 January is one month old on 15 February. For babies who arrived before their due date there is also corrected age, sometimes called adjusted age: the age they would be if they had been born on the due date. Health visitors and doctors use corrected age to judge development and milestones until around the second birthday.
Formula
age in days = date − date of birth weeks = floor(days ÷ 7), remainder in days months = whole calendar months since birth, then days corrected age = date − due date (only if born early)
Example
A baby born on 1 January whose due date was 5 February arrived 5 weeks early. On 12 March they are 10 weeks old, and their corrected age is 5 weeks — the age to use when comparing them with developmental milestones.
Frequently asked questions
How many weeks is a three-month-old?
About 13 weeks — three months is roughly 90 to 92 days depending on which months they are. That mismatch is why baby books talk in weeks for the first months and switch to months later.
What is corrected age?
For a baby born before their due date, corrected (or adjusted) age is their age counted from the due date instead of the birth date. A baby born 8 weeks early who is 20 weeks old has a corrected age of 12 weeks, and that is the age to compare with typical milestones.
Until when should I use corrected age?
Generally until about two years old, after which most babies born early have caught up. Your health visitor or doctor will tell you which age they are using at check-ups.
Should vaccinations follow corrected age?
No — vaccination schedules normally follow the actual age from birth, even for babies born early. Follow the schedule your doctor or health service gives you.