Baby Age Calculator
Enter your baby’s date of birth and see their age in days, weeks and months today, plus when the next milestones fall.
Days old counts the days since birth (the birth day is day 0). Age in months follows the calendar, counting a new month each time the same date comes around.
Set to today
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Months
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Weeks
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Years
Milestones
Months are counted by the calendar: each time the same day of the month comes around, your baby is one month older. Day milestones count from the birth date, so a baby born on June 1 is 100 days old on September 9.
How to use
How this baby age calculator works
This baby age calculator turns a date of birth into the numbers parents and caregivers use every day: how many days old the baby is, how many full weeks, how many months and days, and how old in years. Pick the birth date on the calendar and press Calculate age. You also get a list of upcoming milestones with a countdown for each: 100, 200 and 300 days old, 6 months, the first birthday, 18 months and the second and third birthdays. The next milestone is highlighted so you can see at a glance what is coming up. The calculation happens in your browser and the date is never sent anywhere.
How age is counted
Days old is the number of days since the day of birth, with the birth day itself counted as day 0. A baby born on June 1 is 100 days old on September 9. Weeks are whole weeks, which is the days old divided by seven and rounded down. Months follow the calendar: a baby is one month old when the same date comes around again, so a baby born on January 15 is 2 months old on March 15. If the next month is shorter than the birth date (for example a baby born on the 31st), the month rolls over on the last day of the shorter month. The tool also shows the extra days since the last full month, such as “3 months 12 days old.” Years are whole years, counted on each birthday. The result is a clear picture of age, not medical advice.
When weeks and when months?
Pediatricians and parents tend to use weeks for the first few months, because babies change quickly. Many people switch to months after about 12 weeks, and to months and then years after the first birthday. After 24 months, doctors often switch to years and months. Using the right unit makes it easier to compare your baby with the typical ranges in growth charts and developmental checklists, which are written in the same units.
Corrected age for premature babies
If a baby was born at least three weeks early, doctors often talk about a corrected (or adjusted) age, which counts from the original due date instead of the birth date. For example, a baby born 8 weeks early who is now 6 months old by birth date has a corrected age of about 4 months. Corrected age is commonly used for milestones and development until around age 2. This calculator uses the actual date of birth, so if your baby was premature, subtract the weeks born early to estimate corrected age, and ask your pediatrician which age to use for milestones.
Why parents track days, weeks and months
- Checkups. In the United States, well-baby visits typically happen in the first week, then at about 1, 2, 4, 6, 9, 12, 15, 18, 24 and 30 months. Knowing your baby’s exact age helps you schedule them. Your pediatrician’s office sets the actual schedule.
- Vaccines. Many routine vaccines are given at specific ages, such as 2, 4 and 6 months. Check the current schedule with your healthcare provider.
- Feeding and sleep. Guidance on solid foods, feeding amounts and sleep changes is usually given by age in months.
- Milestone photos and celebrations. Many families take monthly photos or celebrate 100 days, half-birthdays and the first birthday.
- Forms and apps. Daycare, insurance and growth-chart apps often ask for age in months.
A worked example
Imagine a baby born on March 10. On June 25 of the same year the baby is 107 days old. That is 15 full weeks and 2 days, and 3 months and 15 days, because three months have passed on June 10 and 15 more days have followed. The 100-days-old mark was June 18, and the next milestone is 200 days old on September 26. The 6-month mark falls on September 10. Enter your own baby’s birth date to see the real dates.
Growth charts and developmental checklists
Growth charts from the CDC and the World Health Organization, and developmental checklists such as the CDC’s milestone lists, are organized by age in months. A baby’s weight, length and head size are plotted against age, and a pediatrician looks at the pattern over time rather than a single number. Knowing the exact age in months and weeks makes it easier to read these charts and to answer a doctor’s question quickly. Remember that every baby develops at a different pace, and a calculator cannot say whether your baby is on track.
Fun ways to mark each milestone
- Take a photo with a sign showing the age every month, from the same spot.
- Write a short note or voice memo on the 100th day, the 6-month mark and the first birthday.
- Keep a small box for a keepsake each season, such as a hospital bracelet or a tiny sock.
- Share the next milestone date with grandparents so they can plan a call or a visit.
Frequently asked questions
How do I calculate my baby’s age in months?
Count a month each time the same date of the month comes around after the birth date. A baby born on May 20 is 1 month old on June 20 and 2 months old on July 20. Enter the birth date here and the calculator does it for you, including the extra days.
How many weeks old is my baby?
Divide the number of days since birth by seven and round down. The calculator shows the whole weeks. Many pediatricians describe babies in weeks for the first couple of months.
Does day one start on the day of birth?
Here the day of birth is day 0, so on the day after birth the baby is 1 day old. Some families count the birth day as day 1; if you do, the 100th day is one day earlier than the “100 days old” date shown here.
What age should I use for a premature baby?
Doctors usually use corrected age, which counts from the due date, for milestones until about age 2. Subtract the number of weeks your baby was born early from the actual age to estimate it, and confirm with your pediatrician.
Which milestones does the calculator show?
100, 200 and 300 days old, 6 months, the first birthday, 18 months and the second and third birthdays. Past milestones are marked with how long ago they were, and the next one is highlighted with a countdown.
Is this a medical tool?
No. It calculates age from a date, and it does not assess health, growth or development. Talk to your pediatrician about checkups, vaccines and developmental questions.
Is the birth date stored or shared?
No. The date is used only in your browser to calculate the result, and nothing is sent to a server.