How old is my baby in weeks?

Count the days since birth and divide by 7. A baby born 60 days ago is 8 weeks 4 days old. The calculator does this for you.

How many weeks is 3 months?

About 13 weeks. Because months have different lengths, 3 calendar months is 12 to 13 weeks plus a few days.

Is a baby 1 month old at 4 weeks?

Not quite. By calendar, your baby turns 1 month on the same date the following month, which is 28 to 31 days. Many guides treat 4 weeks as roughly 1 month.

When should I use corrected age?

If your baby was born before 37 weeks, use corrected age for milestones and growth until about age 2-3, as the AAP advises. Your pediatrician will tell you which to use.

Does the calculator save my baby's birth date?

No. Everything runs in your browser and nothing is stored or sent.

Baby Age Calculator: Age in Weeks & Months / Tot

Medically reviewed by Safi Ullah Khan · Written by Muneeb Rana · Last updated 21 October 2026

Quick answer: A baby age calculator counts the days since birth and shows age in days, weeks and months. For example, a baby born 100 days ago is 14 weeks 2 days old, about 3 months. Doctors and guides use weeks in the first months, then months. Premature babies may also need a corrected age.

TL;DR

Enter a birth date; age shows in days, weeks + days and months + days.
Month age uses calendar months, so 3 months isn't always 12 or 13 weeks.
Born early? Use the corrected age calculator for milestones.

This baby age calculator turns a birth date into your baby's exact age in days, weeks and months, and points you to the feeding, sleep and development pages for that age. It also counts down to upcoming milestones like 6 weeks, 4 months and the first birthday.

How to use the baby age calculator

Enter your baby's birth date (and time, if you want the hour count).
Optionally pick a different 'as of' date, for example a checkup date.
Read age in days, weeks + days and months + days.
Follow the links to that age's feeding chart, sleep schedule and development pages.
If your baby was born before 37 weeks, also use the corrected age calculator.

How the baby age calculator works

days   = asOf - birthDate (calendar days)
weeks  = floor(days / 7), remainder days = days % 7
months = whole calendar months between dates (date-fns differenceInCalendarMonths, adjusted if the day of month isn't reached), remainder days

Age in weeks	About
4-5 weeks	1 month
8-9 weeks	2 months
13 weeks	3 months
17-18 weeks	4 months
26 weeks	6 months
39 weeks	9 months
52 weeks	12 months

Worked example

Sami was born on 3 July 2026. On 21 October 2026 he is 110 days old: 15 weeks 5 days, or 3 months 18 days. The tool links to the 3-month feeding chart and flags the 4-month checkup coming up in about 2 weeks.

Why weeks, then months

Babies change so fast early on that doctors and guides count in weeks for the first couple of months; growth charts and milestone lists switch to months. The CDC milestone checklists are at 2, 4, 6, 9 and 12 months. See the milestone checker.

Corrected age for preemies

The AAP recommends using corrected age (age from the due date) for growth and development until about age 2-3 for babies born preterm. The corrected age calculator does the maths.

Tot shows age automatically

Add your baby's birth date in Tot and every log shows their age, which helps when you share history with your pediatrician. See Tot features.

Download Tot free on the App Store or Google Play. Free with ads; optional Premium.

Sources

AAP HealthyChildren.org: Corrected Age for Preemies: www.healthychildren.org (accessed 21 October 2026)
CDC: Developmental Milestones (Learn the Signs. Act Early.): www.cdc.gov (accessed 21 October 2026)
WHO: Child Growth Standards: www.who.int (accessed 21 October 2026)

Medical disclaimer:

Tot and this website give general information only. Feeding, sleep and growth ranges vary; always follow your pediatrician's advice. If your baby is unwell, contact your doctor or emergency services.

Baby Age Calculator (Weeks, Months, Days)