Baby Age Calculator

Enter your baby's date of birth to see their age in weeks, months and days, and when upcoming milestones fall. For adults, use the age calculator.

Baby's date of birth

Age on

Your baby's age

e.g. born Sep 15, 2026 → 7 weeks, 3 days on Nov 6, 2026

Your baby's age appears here as you type.

Calculated in your browser. The dates you enter never leave this device.

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How baby age is counted

Babies are usually counted in weeks for the first few months, in months until about age two, and in years after that. The calculator shows weeks, months and the total number of days, so you can use whichever your doctor or app uses.

A baby turns one month old on the same date the following month. Born on March 31? April has no 31st, so the one-month mark is April 30.

Example: born September 15, 2026. On November 6, 2026 the baby is 7 weeks and 3 days old, which is also 1 month and 22 days, or 52 days.

The 100th day

In Korea (baek-il) and in Chinese tradition, a baby's 100th day is celebrated with family. The milestone list shows when it falls, along with the 1, 3, 6 and 9-month marks and the first and second birthdays.

Born early?

For premature babies, doctors often track development using corrected age, counted from the due date. This calculator counts from the date of birth, so ask your pediatrician which age to use when comparing milestones.

Frequently asked questions

How do I count my baby's age in weeks?

Count the days since birth and divide by 7. The day of birth is day 0, so your baby is 1 week old on day 7.

When is my baby 1 month old?

On the same date the next month. If that date doesn't exist (for a baby born on January 31, there's no February 31), it's the last day of the shorter month.

Is 4 weeks the same as 1 month?

No. Four weeks is 28 days, while a month is 28 to 31 days. A baby born on the 1st is 4 weeks old on the 29th, but 1 month old on the 1st of the next month.

What if my baby was born early?

Doctors often follow a premature baby's development using corrected age, counted from the due date instead of the date of birth. Ask your pediatrician which age to use for milestones.