How to use
Pick “Baby’s date of birth” and the result appears. “Reference date” starts out as today’s date on the device you are using. If you need the age on another day, such as a vaccination or a checkup, change the reference date to that day.
The result has five parts.
- Completed months: the number of full months your baby has lived. It is the largest figure, with the date your baby reaches the next month on a line below it.
- Exact age (months and days): the completed months plus the days left over.
- Total (weeks and days): the days since birth grouped into weeks of 7.
- Total days since birth: the days that have passed since the day of birth. The day of birth itself is not counted, so on that day it is 0 days.
- Upcoming milestones: a list of the next three months, plus whichever of 12, 18 and 24 months are still ahead, each with its date.
If your baby was born early, turn on “Show corrected age for premature babies (optional)” to see the corrected age as well. How to fill it in is covered under “Corrected age for premature babies” below.
Worked example
A baby born on March 15, 2026, with the reference date set to September 28, 2026:
| Item | Result |
|---|---|
| Completed months | 6 months |
| When the next month is reached | Your baby turns 7 months old on Thu, October 15, 2026. |
| Exact age (months and days) | 6 months 13 days |
| Total (weeks and days) | 28 weeks 1 day |
| Total days since birth | 197 days |
“Upcoming milestones” lists, in order, 7 months (October 15, 2026), 8 months (November 15), 9 months (December 15), 12 months (March 15, 2027), 18 months (September 15, 2027) and 24 months (March 15, 2028).
If you turn the same baby’s 28 weeks into months at four weeks each, you get 7 months, but by the calendar the baby is 6 months 13 days. A month is a little longer than four weeks, so an age estimated from weeks runs ahead of the calendar age. For anything scheduled by month, such as vaccinations and checkups, go by the dates under “Completed months” and “Upcoming milestones”.
How the months are counted
A month is complete each time the calendar reaches the same day number as the birthday. A baby born on March 15 is 1 month old on April 15 and 2 months old on May 15.
In a month that has no such day, the month is complete on its last day. A baby born on January 31, 2026 turns 1 month on February 28, 2 months on March 31 and 3 months on April 30. Being pulled back to the 28th in February does not lock the later months to the 28th, because every month is worked out again from the date of birth. For a baby born on January 31, 2024, a leap year, the 1-month date is February 29.
Corrected age for premature babies
If you judge development by actual age alone, a baby born before the due date can look behind other babies of the same age. Corrected age is counted from the due date instead of the actual birthday. HealthyChildren.org, from the American Academy of Pediatrics (AAP), describes it as the actual age in weeks minus “40 weeks minus the gestational age at birth”, and gives this example: a baby born at 32 weeks’ gestation is 8 weeks (2 months) early, so at 16 weeks (4 months) after birth the corrected age is 2 months. The same page advises using corrected age when you look at development during the first 2 years.
In this calculator you choose one of two options under “Method”.
- Use the due date: enter the “Due date” as a date.
- Use the gestational age at birth: enter a whole number from 20 to 42. The due date is taken to be the birthday plus (40 − weeks) × 7 days.
“Corrected age” in the result runs from the due date to the reference date and follows the same calendar rule as the completed months. Take a baby born on January 5, 2026 at 32 weeks’ gestation. The due date is 8 weeks later, March 2, 2026.
| Reference date | Exact age (months and days) | Corrected age |
|---|---|---|
| February 20, 2026 | 1 month 15 days | 0 months (before the due date) |
| April 27, 2026 | 3 months 22 days | 1 month 25 days |
| May 2, 2026 | 3 months 27 days | 2 months 0 days |
April 27 is exactly 16 weeks after the birth. Counted in weeks, the corrected age is 8 weeks. The calculator counts on the calendar from March 2, so it shows 1 month 25 days. The corrected age does not reach 2 months until May 2, five days later. If the reference date is before the due date, the corrected age is 0 months and a note is added: “The due date (Mon, March 2, 2026) hasn’t arrived yet, so the corrected age is 0 months.”
Turning corrected age on does not change “Completed months” or the dates under “Upcoming milestones”. Those are still based on the actual birthday. The results are for reference only and do not replace a medical diagnosis or advice.