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Baby Age in Weeks Calculator

Enter the birth date and an as-of date to see the baby's age.

How it is calculated

The tool counts the whole calendar days between the birth date and the as-of date, then splits that count: weeks = floor(days / 7) and remaining days = days mod 7. The month figure is a simple conversion, months = days / 30.4375 (the average length of a month over a four-year cycle), shown with one decimal. Because it divides a day count rather than comparing calendar months, 1 March to 1 April gives about 1.0 months, not exactly one. Both dates must be real calendar dates in YYYY-MM-DD form, and the as-of date cannot be earlier than the birth date; otherwise no result is returned. Using the birth date itself as the as-of date gives 0 weeks 0 days. The age is for tracking only and is not a corrected age for premature babies.

Worked example

A baby is born on 2026-03-01 and you want the age on 2026-06-15. The gap is 106 days (31 + 30 + 31 + 14 days after 1 March). Dividing by 7 gives 15 full weeks with 1 day left over, so the result reads 15 weeks 1 days. For the month figure, 106 / 30.4375 = 3.48, shown as 3.5 months. The total day count is 106 days.

Try this example →

Common mistakes

  • Entering the due date instead of the actual birth date for a baby born early or late. The tool measures from the date you give, so the age will be off by the difference.
  • Reading the month figure as a calendar-month age. It is days divided by 30.4375, so a baby born on the 31st can show 3.5 months when a calendar count would say 3 months and some days.
  • Typing the date in day-month-year order. The tool expects year-month-day, and an impossible date such as 2026-02-30 gives no result.
  • Forgetting to change the as-of date when checking a past or future milestone, so the result reflects the wrong day.
  • Expecting a corrected age for a premature baby; the tool only measures time since birth.

About this calculator

In a baby's first year, age is most meaningfully tracked in weeks rather than months, since growth, feeding schedules, sleep patterns, and developmental milestones all shift noticeably from one week to the next in a way monthly tracking glosses over. This calculator works out a baby's age in weeks plus days, in months, and as a total day count from the birth date to any chosen as-of date.

Pediatric guidance — like well-baby visit schedules, vaccination timing, and developmental milestone checklists — is frequently expressed in weeks of age, especially in the newborn period. New parents and caregivers use a week-based age calculator to stay on top of that timeline and to answer the very common question of exactly how old a baby is on a given day.

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  • Editorial team

    For a baby born early, the weeks shown here are the age since birth, and a developmental chart may call for a corrected age instead. You can estimate it yourself: take 40 minus the gestational weeks at birth, then subtract that from the weeks shown. A baby born at 36 weeks is 4 weeks early, so 15 weeks of age becomes 11 weeks corrected. Ask your paediatrician which age a given chart or schedule uses. Which milestone or check-up are you lining up?

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