Baby Age & Milestone Dates Calculator

Enter a baby's date of birth to see the age in days, weeks and months, and the dates of milestones such as 100 days, the first birthday and 1,000 days. For a baby born early, it also shows the corrected age.

How to use

1. Pick the "Date of birth". "Child's name or nickname" is optional; use "+ Add child" to keep a second child separately.
2. "As of" is normally today. Change it to see the age on another date.
3. "Milestone dates" lists 50 to 1,000 days and the first three birthdays, with weekday and days left. "Save as calendar file (.ics)" adds them to a calendar.
4. If the baby was born early, switch on "Born before the due date (show corrected age)" and enter the "Due date".

How it works

• With "Count the birth day as day 1" on, day 100 is the birth day + 99 days, as in the Korean custom. Off, counting starts the day after birth.
• A baby turns one month older on the same day number each month. If that day does not exist, the last day of the month is used (born January 31: February 28).
• Corrected age is the age counted from the due date. Gestational age at birth is 40 weeks minus the time born early.
• The children you enter are saved only in this browser.

Questions and answers

When is my baby 100 days old?

Counting from the day after birth, a baby born January 1, 2026 is 100 days old on April 11. With the birth day as day 1, it is April 10.

How do you count a baby's age in months?

The baby gains a month each time the birth date comes round. A baby born March 15 is one month old on April 15 and two months old on May 15.

How many weeks old is my baby?

Divide the days since birth by 7. A baby born 45 days ago is 6 weeks and 3 days old, shown as "Age in weeks".

How is corrected age calculated for a premature baby?

Subtract the time born early from the actual age. A 4-month-old born 6 weeks early has a corrected age of about 2½ months.