Baby Age Calculator

Enter your baby's date of birth to get their exact age in years, months, weeks, and days, plus a due-date-corrected age for babies born premature.

Quick Facts

Method
Calendar-based date subtraction
Counts full years, then full months, then remaining days between the date of birth and the selected date.
Corrected age
Birth date + weeks premature = adjusted start date
Full term is 37-42 weeks gestation; corrected age is generally used for babies born earlier, until about 24 months old.

Your Results

Calculated
Chronological Age
-
Since date of birth
Age in Weeks
-
Weeks + extra days
Total Days Old
-
Full days since birth
Corrected Age
-
Adjusted for prematurity

Ready

Enter the date of birth and press Calculate.

How the Baby Age Calculator Works

This tool finds your baby's exact age by subtracting the date of birth from a target date (today by default) using calendar-based date arithmetic — the same method used to describe age in everyday language, like "4 months, 2 weeks old." It also converts the result into total weeks and total days, and can shift the starting point for babies born before full term.

The calendar-based age formula

Given a date of birth and an end date, the calculator counts whole years elapsed, then whole months within the current year, then the remaining days — borrowing a month's worth of days whenever the day count would go negative (the same way "1 month after January 31" is treated as February 28, not February 31). The result reads like "1 year, 3 months, 12 days" rather than a single number.

Age in weeks and total days

  • Total days old is the exact number of calendar days between the date of birth and the selected date.
  • Age in weeks is total days divided by 7, shown as whole weeks plus the leftover days — the unit most newborn and infant milestones and appointments are tracked in.
  • Because calendar months run 28 to 31 days while a week is always 7 days, age in weeks and age in months will not always line up exactly. Both are correct; they are simply different units.

Corrected (adjusted) age for premature babies

Full term is generally considered 37 to 42 weeks of gestation. For babies born earlier, pediatricians often track a corrected (or adjusted) age that starts counting from the original due date instead of the actual birth date. This calculator estimates that due date as the date of birth plus the number of weeks born early, then applies the same calendar-based formula from that adjusted start date. Corrected age is typically used for growth and developmental comparisons until the child reaches about 24 months old; after that, chronological age alone is used.

Frequently Asked Questions

How is my baby's exact age calculated?
The calculator subtracts the date of birth from the date you want the age for using calendar-based date arithmetic: it counts full years, then full months, then remaining days, borrowing from the previous month when the day count goes negative. This matches how ages are normally described, such as "6 months, 3 days old."
What is corrected age and when should I use it?
Corrected age (also called adjusted age) shifts the starting point from the actual birth date to the expected due date, calculated as the birth date plus the number of weeks born early. Pediatricians commonly use corrected age instead of chronological age for growth and development comparisons in babies born before 37 weeks gestation, typically until the child reaches 24 months.
Why don't the age in weeks and age in months line up exactly?
Weeks are always 7 days long, but calendar months range from 28 to 31 days. A baby who is exactly 1 calendar month old is closer to 4 to just over 4.4 weeks old depending on the month, so the two units naturally diverge slightly — both are correct.
Can I calculate age as of a date other than today?
Yes. Change the "Calculate age as of" field to any date on or after the date of birth — useful for finding out how old a baby was, or will be, at a past or upcoming appointment, vaccination, or milestone date.

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