Baby age calculator: days, weeks and calendar months
Updated
Enter your baby’s date of birth and the day you want to check. See their chronological age in days, complete weeks and completed calendar months, without treating every month as 30 days.
Calculate your baby’s age
Enable JavaScript to use the calculator. A week has seven days. Four weeks are 28 days, but a calendar month can have 28, 29, 30 or 31. A four-week-old baby has therefore not necessarily completed one calendar month.
Weeks and months measure time differently
A week has seven days. Four weeks are 28 days, but a calendar month can have 28, 29, 30 or 31. A four-week-old baby has therefore not necessarily completed one calendar month.
The three results describe the same interval; do not add them together. The day of birth is zero elapsed days. The calculator compares dates, not exact birth times.
How completed months are counted
Each month is completed on the day of the month your baby was born. If that day does not exist in the month being checked, this tool uses its last day. For example, January 31 to February 28, 2026 counts as one completed month and 28 elapsed days. The next monthly anniversary returns to March 31.
Leap years are included. February 29, 2024 to February 28, 2025 counts as 12 completed months and 365 elapsed days. This is the tool’s calendar convention, not a medical assessment.
Alongside your daily records
Age can help you place a recorded event in time. Prefer to start on paper? Find printable feeding, sleep and diaper logs on the Grow page.
This calculator shows chronological age only. It does not calculate corrected age for premature babies or recommend how much a baby should eat, sleep or grow.
Dates and privacy
The calculation date defaults to today on your device, and you can change it. The calculation runs on this page. The tool does not store the dates or send them to a server.
Sources and method
Intervals use the Gregorian calendar. The end-of-month rule is explained above so you can interpret the result.