The Baby Age Calculator works out a baby's exact age from their birth date, expressed the way parents and pediatricians actually use it: complete months plus the remaining weeks and days, alongside the totals in days, weeks, and months. Calendar months vary from 28 to 31 days, so counting a baby's age is not simple subtraction — the calculator does the calendar arithmetic properly, leap years included.
How It Works
The calculator first counts complete calendar months between the birth date and the comparison date (today, unless you choose another date). It then takes the days remaining after those whole months and splits them into weeks and leftover days. In parallel it reports the simple totals — total days lived, total weeks (complete weeks within those days), and total months — so you can answer "how old?" in whichever unit a form, a vaccination schedule, or a milestone chart asks for.
Input Parameters
| Input | Description | |---|---| | Birth date | The day your baby was born | | Comparison date | Optional; the date to calculate the age at. Defaults to today |
The birth date cannot be later than the comparison date, and both must be valid calendar dates.
Worked Example
A baby born on February 10, 2026, measured on August 23, 2026:
- Complete months: February 10 to August 10 is exactly 6 months.
- Remaining days: August 10 to August 23 is 13 days = 1 week and 6 days.
- Result: the baby is 6 months, 1 week, and 6 days old.
- Totals: 194 days lived, which is 27 complete weeks, and 6 total months.
That distinction between the two views matters: "6 months" and "27 weeks" describe the same baby on the same day, and neither is a rounding error — a calendar month averages about 4.35 weeks, which is why a 6-month-old is roughly 26 to 27 weeks old, not 24.
Why Months and Weeks Disagree
Four weeks is 28 days, but only February in a non-leap year is that short. A baby born on January 1 is one month old on February 1 (31 days later) but only four weeks old on January 29. Milestone guidance is published in both units — feeding and sleep advice often in weeks for the first months, developmental checks in months — so converting exactly, rather than treating a month as four weeks, keeps you on the right row of the chart.
Adjusted Age for Premature Babies
If your baby was born before 37 weeks, clinicians often track development by adjusted age: age counted from the original due date instead of the birth date, usually until about age two. To get the adjusted age with this calculator, simply enter the due date as the "birth date." A baby who is 12 weeks old by birth but was born 4 weeks early has an adjusted age of 8 weeks, and it is the 8-week milestones that apply.
Limitations
The calculator counts calendar age only; it does not assess development, and milestone ranges are wide and individual. Ages are computed at day precision — the time of day of the birth is not considered, so a baby measured on their birthday itself shows the completed age from the start of that day. For medical questions, including growth charts and vaccination timing, your pediatrician's records govern.
Frequently Asked Questions
Why does the calculator say 6 months but 27 weeks?
Because months and weeks are not interchangeable units. Six calendar months from February 10 reach August 10 — 181 days — and by August 23 the baby has lived 194 days, which is 27 complete weeks. Both figures are exact; they are just measuring in different units.
Can I calculate the age at a future date?
Yes. Set the comparison date to the future date — a first birthday party, a scheduled checkup, a flight — and the calculator reports the age the baby will be on that day, as long as it is after the birth date.
Should I use birth date or due date?
Use the birth date for legal age, forms, and everyday purposes. Use the due date (adjusted age) only for developmental milestone tracking when your baby was born prematurely and your clinician advises it.