Baby Age Calculator

In a baby's first couple of years, age is tracked differently than it is for anyone older. Pediatrician visits, feeding guidance, and milestone checklists are usually organized by week or month, not by birthday — so this calculator leads with weeks and months, not just years.

Baby age in weeks

Age in weeks is especially useful during early infancy, since development changes quickly from one week to the next. It's calculated as exact elapsed calendar days divided by seven — for example, an 89-day-old baby is exactly 12 weeks + 5 days old (89 ÷ 7 = 12 remainder 5).

Baby age in months

Once a baby passes about three months, ages are more often described in months. This calculator uses exact calendar arithmetic — counting whole months and remaining days from the actual date of birth — rather than approximating a month as four weeks.

Weeks, months and days: what's the difference?

A calendar month isn't a fixed number of weeks — months run from 28 to 31 days, averaging about 4.35 weeks. That's why "3 months old" and "13 weeks old" don't always fall on the same day. For an exact figure, always calculate from the actual birth date rather than assuming 1 month = 4 weeks.

Corrected Age for Premature Babies

Chronological age is simply the time elapsed since the actual date of birth. Gestational age describes how far along a pregnancy was at birth, in weeks (40 weeks is generally treated as full term). Corrected age combines the two — it's chronological age adjusted for how many weeks early a baby was born, effectively counting age from the original due date instead.

For example: a baby born at 36 weeks gestation is 4 weeks early. At a chronological age of 20 weeks, their corrected age would be 16 weeks. Corrected age is commonly used when considering developmental expectations for babies born prematurely, generally up to around 24 months — confirm how to apply it for your baby with your pediatrician.

Baby Age Conversion Reference

Months are calendar-based and vary in length, so this table is an approximate reference only — for an exact figure on a specific baby, always use the calculator above rather than a flat weeks-per-month estimate.

Practical Examples

Born January 1, and it's now March 15? That's roughly 10 weeks and 3 days old on the Current Age tab — 2 months and 14 days on the calendar. Born 4 weeks early with a chronological age of 20 weeks? The Corrected Age tab would show a corrected age of 16 weeks. Want to know how old your baby was on a specific past date, like a family event, or will be on an upcoming one? The Specific Date tab handles both.

A note on vaccination schedules: these vary by country and are updated over time. Check the current schedule with your pediatrician or official public-health guidance rather than relying on any fixed list.

Related calculators

Comparing your baby's age against a sibling's? Try our Age Difference Calculator or Age Gap Calculator. Counting down to their first birthday? Use our Birthday Calculator. For age in the standard years/months/days format, our main Age Calculator covers that too. Or check the exact span between any two dates with Time Between Dates.