Baby Age in Weeks Calculator
Turn a birth date into an exact age in weeks and days, months and total days — on any date — with corrected age for premature babies, the next check-up or vaccination date for your schedule, and week-by-week and month-by-month birthday dates.
How it works
Weeks = ⌊days since birth ÷ 7⌋, remainder = days · Months = completed calendar months + leftover days · Corrected age = actual age − (40 weeks − pregnancy length at birth) · Postmenstrual age = pregnancy length at birth + actual age
For the first few months, age in weeks is the standard: feeding amounts, sleep patterns, growth-chart points and immunisation schedules are all set by completed weeks. The calculator counts the days from birth to the date you pick (today by default), splits them into whole weeks and days, and also counts calendar months — the same day-of-month each month, with month-end birthdays moved to the last day of shorter months — which is why “4 months” and “17 weeks” can both be right. For a baby born before 37 weeks, switch on “born early” and enter the pregnancy length at birth. Corrected (adjusted) age subtracts the weeks the baby arrived early from the actual age; it is what clinicians use for development milestones and growth until about age 2, while vaccinations follow the actual age. Postmenstrual age — pregnancy length at birth plus age since birth — is the figure neonatal units use in the first weeks. The schedule tables list the usual check-ups and vaccination appointments for the UK, the US or a 2-4-6-month calendar with their dates and how many days away they are, plus every week and month birthday.
Worked example
Born 15 June 2026, as of 26 September 2026: 103 days = 14 weeks 5 days, or 3 months 11 days. The baby turns 15 weeks on 28 September and 4 months on 15 October. On the UK schedule the 8- and 12-week vaccinations (10 August and 7 September) are done, and the next milestone is the 16-week vaccinations on 5 October — 112 days after birth, 9 days away. If the baby had been born at 32 weeks, it arrived 8 weeks early: corrected age is 103 − 56 = 47 days, 6 weeks 5 days.
Sources: NHS — vaccinations and when to have them · CDC — child immunization schedule · AAP HealthyChildren — corrected age for preemies
Frequently asked questions
When do people switch to months?
Usually around 12 weeks, though clinics often keep counting weeks until about 6 months and many parents switch at 3 months. After the first birthday ages are given in months up to about 2 years, then in years.
My baby was premature — which age do I use?
Use corrected age for development milestones, growth charts and feeding: it is the actual age minus the weeks born early (40 weeks minus the pregnancy length at birth). Switch on “born early” to see it. Vaccines still follow the actual age from birth, so don’t delay them.
How many weeks is a 3-month-old?
About 13 weeks, depending on the months involved: a baby born on 15 June turns 3 months on 15 September, which is 13 weeks and 1 day. A month averages 4.35 weeks, so 6 months is about 26 weeks and 1 year is 52 weeks and 1 day (2 in a leap year).
How are month birthdays counted for a baby born on the 31st?
In months without a 31st, the month birthday falls on the last day of that month — a baby born on 31 January turns 1 month on 28 February (29 in a leap year) and 2 months on 31 March.
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This calculator is for educational purposes. Double-check important results before acting on them.