About the Baby Age Calculator
This baby age calculator turns a date of birth into every way people ask about a baby’s age: in calendar months and days, in weeks and days, and as a running total of days. It also lists the dates your baby reaches popular milestones — 1 month, 100 days, 6 months, the first birthday, 500 days, 18 months, 1,000 days and more — and shows which ones are still ahead.
Parents use it to answer “how many weeks old is my baby?” for feeding guides, sleep charts and growth trackers that work in weeks for the first few months, then in months. It is also handy for planning a 100-day celebration, monthly photos or a half-birthday, and for filling in forms that ask for an age in months.
Ages are counted in calendar months: a baby born on the 15th turns one month older on the 15th of each month (or the last day of a shorter month). The decimal month figure divides total days by the average month length of 30.44 days. For a premature baby, use the adjusted age calculator to see age corrected to the due date.
How to use the baby age calculator
- 1Enter your baby’s date of birth.
- 2Choose the date you want the age for — usually today.
- 3Read the age in months and days, or in weeks and days.
- 4Check the milestone table for 100 days, half-birthday and first-birthday dates.
Formula and method
The calendar age counts whole years and months from the birth date and then the leftover days, borrowing the length of the previous month when needed — the same way birthdays work. The week figure divides the total number of days by 7 and keeps the remainder as days, which is how “12 weeks 3 days” style ages are written.
The decimal month figure divides the total days by 30.4375, the average length of a month in the Gregorian calendar (365.25 ÷ 12), so it can differ slightly from the completed calendar months. Milestone dates add calendar months to the birth date (clamped to the last day of shorter months) or add a fixed number of days for day-count milestones like 100 and 1,000 days.
- Days
- Whole days from birth date to the as-of date
- 30.4375
- Average days per calendar month (365.25 ÷ 12)
Worked examples
Born March 15, 2026, checked September 29
From March 15 to September 15 is 6 calendar months, plus 14 more days. That is 198 days in total, or 28 weeks 2 days (198 ÷ 7 = 28 r 2). The next listed milestone is 9 months on December 15, 77 days away.
Newborn at 5 weeks
A baby born August 20 is 40 days old on September 29: one calendar month (to September 20) plus 9 days, or 5 weeks 5 days. The 2-month mark falls on October 20, 21 days later.
Almost 3 months old
A baby born July 1 is 90 days old on September 29 (31 + 31 + 28 days), which is 2 months 28 days or 12 weeks 6 days. The 3-month mark is October 1, two days away, and the 100-day mark follows on October 9.
Frequently asked questions
How do I calculate my baby’s age in weeks?+
Count the days since birth and divide by 7. The whole number is the weeks and the remainder is extra days — for example 40 days is 5 weeks 5 days. This calculator does the counting for you.
When does a baby count as one month old?+
On the same date in the following month — a baby born on March 15 is one month old on April 15. Babies born on the 29th–31st turn a month older on the last day of shorter months.
Is 4 weeks the same as 1 month for a baby?+
Not quite. Four weeks is 28 days, while a calendar month is 28 to 31 days (30.44 on average). That is why a baby can be 4 weeks old a few days before being 1 month old.
When should I stop giving my baby’s age in months?+
Doctors and growth charts use months up to about 24 months; after that most people switch to years, or years and months, for toddlers.
Should I use adjusted age for a premature baby?+
For development and growth checks, yes — premature babies are usually assessed by age corrected to the due date until about age 2. Vaccines still follow age from birth.