Why do weeks and months give different numbers?

Months vary in length (28–31 days) while a week is always 7 days. A 3-month-old baby is roughly 13 weeks old, not 12, because most months are longer than 4 weeks.

How is the months count calculated?

The calculator advances the birth date by one calendar month at a time until it would pass today's date. The remaining days are the difference between that last whole-month point and today.

Is this the same method pediatricians use?

Yes. Pediatric age is stated in weeks for the first two months, then in whole months up to 24 months, which matches what this calculator shows.

Can I use a future date as today?

No — the calculator only runs against the current date in your browser. For future estimates, count weeks/months manually from the birth date.

Does it work for premature babies?

It shows chronological age (days since birth). Corrected/adjusted age requires subtracting weeks of prematurity — that calculation is not included here.

Baby Age in Weeks and Months

Name: Baby Age in Weeks and Months
Availability: InStock
Author: Nham Vu

Enter your baby's birth date to instantly see their exact age in total weeks, whole months plus leftover days, and a day-by-day breakdown.

Enter Birth Date

Baby's date of birth

Calculate age as of

Defaults to today. Change for a past or future checkup date.

Select a birth date to see the age breakdown

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Summary

Enter your baby's birth date to instantly see their exact age in total weeks, whole months plus leftover days, and a day-by-day breakdown.

How it works

Enter your baby's date of birth.
The calculator counts the exact number of days since birth.
Days are converted to total complete weeks (days ÷ 7).
Whole months are counted by advancing the birth date month by month.
Remaining days after whole months are shown separately.

Use cases

Check if your baby has reached the 6-week or 12-week milestone.
Track when to schedule vaccine appointments at 2, 4, 6, and 12 months.
Confirm which developmental stage your baby is in for apps like Wonder Weeks.
Answer "how many weeks old is my baby?" for online parenting groups.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu