How the Baby Age Calculator works
Enter your baby’s date of birth and the calculator counts the exact number of days since then, and turns it into weeks and days, months and days, and — once your baby is older — years, months and days. It also lists the dates when your baby reaches common age points such as 6 weeks, 2, 4, 6 and 9 months, and the first and second birthdays. You can also calculate the age on another date, for example on the day of an appointment.
Why baby age is counted in weeks, then months
In the first weeks, babies change so quickly that parents and professionals often count in days and weeks: “two weeks old”, “six weeks old”. From about two to three months, months become more natural, and after the first birthday, months are often used until about two years old (“18 months”). There is no strict rule — the calculator shows every format so you can use whichever is most helpful.
Weeks to months at a glance
| Age in weeks | Roughly |
|---|---|
| 4–5 weeks | 1 month |
| 8–9 weeks | 2 months |
| 13 weeks | 3 months |
| 17–18 weeks | 4 months |
| 26 weeks | 6 months |
| 39 weeks | 9 months |
| 52 weeks | 12 months |
A month is about 4.35 weeks, so weeks and months never line up exactly.
Using your baby’s age
Exact age is useful for health and development check-ups, vaccination appointments, feeding and sleep guidance written by age, and simply for celebrating milestones. Schedules for check-ups and vaccinations differ between countries, so follow the timetable your health service gives you.
Every baby develops at their own pace. If your baby was born early, the Corrected Age Calculator shows the adjusted age often used for development. If you have any concerns about your baby’s growth or development, talk to your doctor, health visitor or nurse.