Baby age calculator: how old is she, exactly?

How old is she? The simplest question in the world, until a stranger at the grocery store asks it. You were there. You know the date. But somewhere between the 4am feed and the laundry, the arithmetic quietly left the building.

This baby age calculator counts it for you: her exact age in weeks and days, calendar months and days, and total days on earth, plus the little dates worth celebrating. Enter her birth date above and let your brain off the hook.

How many weeks old is my baby, and why weeks come first

In the beginning, everyone counts in weeks, for good reason. A newborn changes faster than at any other point in her life, and a 2 week old and a 6 week old can feel like entirely different babies. Weeks are the right size unit for that pace.

The medical world leans on small units early too:

• The first checkup typically happens 3 to 5 days after birth, with another around 1 month, per the AAP well-child visit schedule.
• Nap, wake window, and feeding guidance shifts week by week until about the 3 or 4 month mark, while her day and night rhythm is still being built.

Then the pace gently eases, and months take over. The CDC's developmental milestone checklists are organized by month, and checkups in the first two years follow a month-based schedule too, typically at 2, 4, 6, 9, 12, 15, 18, and 24 months. This calculator hands you both, always.

Baby age in months: how the calendar math actually works

Here is the part nobody warns you about: age in months is not one fixed formula, because months are not one fixed length. The standard way to count, and the way this tool works, is calendar month arithmetic: step forward one whole calendar month at a time, then count the leftover days.

A baby born on March 10 turns exactly 2 months old on May 10. But calendars have sharp edges: a baby born on January 31 cannot turn 1 month old on February 31, because that date does not exist. The usual convention, and ours, lands on the last day of the shorter month instead.

That edge case is one reason two tools can give different month ages for the same baby: some assume every month has 30 days, others quietly count 4 weeks as a month.

Week by week age, translated into months

When a sleep guide says 17 weeks and your brain is holding month ages, this chart translates. Month values are approximate.

After 24 months, week counting officially retires. Nobody says their preschooler is 178 weeks old, although now you could.

How old is my baby? When weeks become months become years

There is an unwritten schedule for how we say a child's age, and it maps onto how fast she is changing:

• Weeks from birth until around 12 weeks
• Months from there until the second birthday
• Years after that, with half years doing heavy lifting for toddler pride

It is why the growth charts used for the earliest stage run from birth to 24 months before switching to the big-kid charts.

The month habit sometimes puzzles grandparents, but there is real texture in it: a 14 month old and a 22 month old live in different worlds, and the word "one" flattens that. Past two, this calculator keeps counting in years and months up to age six.

Little dates worth circling on the calendar

Some of the sweetest milestones are not developmental at all. They are just dates.

The 100 day mark comes from East Asian traditions like the Korean baek-il, a celebration of a newborn making it safely through the most fragile stretch. This calculator hands you the exact date for the photo, the cake, or a quiet moment of "we made it".

Month-birthdays are the modern favorite: the monthly anniversary of her birth date, the day of the milestone-blanket photos. The half birthday at 6 months lands in a golden stretch. And the first birthday countdown is really for you. She will not remember the cake. You will remember everything.

Frequently asked questions