Baby age calculator in weeks and months
Vaccine and growth charts use weeks or months, and this gives you both, counted on the real calendar.
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How to use it
- Enter date of birth and age on.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Total days = chosen date - birth date. Weeks = days ÷ 7 with the remainder in days. Months are full calendar months from the birth day; a 31st maps to the last day of a shorter month.
Example
Inputs: Date of birth = 2026-01-15, Age on = 2026-04-27. Result: Age in months and days: 3 months 12 days; Age in weeks and days: 14 weeks 4 days; Age in days: 102 days.
Limits and caveats
- For babies born early, doctors track development by corrected age until 2 years; use the corrected age calculator.
- Months differ in length, so months will not always equal weeks ÷ 4.
Questions about this tool
When is my baby one month old?
On the same date of the next month, so a baby born on January 15 turns one month on February 15.
Why do doctors use weeks at first?
Growth is fast in the early months, and weeks give more precision for weight and feeding checks.
Is the birth day counted?
No. Age is zero on the day of birth and one day on the next day.