Baby Age Calculator
How old your baby is in weeks, months and days, and the corrected age if born early.
Good to know. Weeks until about three months, then months until two years, then years. Nobody says 104 weeks.
How it works
In plain words
Count the days from the date of birth. Divide by 7 for whole weeks, with the days left over. Months are counted by the calendar. For a baby born early, count from the due date instead for the corrected age.
The formula
age in weeks = ⌊days ÷ 7⌋ weeks + (days mod 7) days corrected age = age − weeks born early
Worked example
A baby born on 10 January 2026 is 64 days old on 15 March: 9 weeks, 1 day, or 2 months, 5 days.
Sources
No outside data or rates: this is plain arithmetic.
5 automated tests check its answers on every change.
About the baby age calculator
In the first months, a baby’s age changes so fast that weeks are the natural unit: check-ups, vaccinations and growth charts are often set at 6 or 8 weeks. After about three months people switch to months, and after two years to years. This calculator gives all of them for any date.
Months are counted on the calendar, like any age: from the 10th of one month to the 10th of the next is one month, whether it has 28 days or 31. Weeks are exact: every seven days.
For a baby born early, health visitors and doctors use the corrected age, counted from the due date, when they look at growth and development. A baby born 8 weeks early is 4 months old by the calendar at 4 months, but 2 months corrected, and is expected to do what a 2-month-old does. The correction is usually used until about two years, for babies born before 37 weeks.
Every baby develops at its own pace, so milestones are ranges, not deadlines; ask your health visitor or doctor if something worries you. For any age, use the age calculator; for the due date, the due date calculator.
Questions people ask
How many weeks old is my baby?
Count the days since birth and divide by 7. A baby born 64 days ago is 9 weeks and 1 day old.
What is corrected age?
The age counted from the due date instead of the birth date. It is used for babies born before 37 weeks, usually until age two.
When do you stop counting in weeks?
Usually around three months, then in months until two years.