Babies’ ages are usually counted in weeks for the first few months, then in months until two years. For premature babies, doctors use corrected age – the age they would be if born on the due date – when checking growth and milestones until about age two.
How to use the Baby Age Calculator
- Baby’s date of birth
- Age on
- Gestational age at birth
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the baby’s date of birth, the date to check, and – for a premature baby – the gestational age at birth in weeks.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Baby’s date of birth | A date |
| Age on | A date |
| Gestational age at birth | In weeks |
What each result means
- Age in days = [Age on] – [Baby’s date of birth]
- Age in weeks = floor([Age in days]÷7)
- and days = mod([Age in days], 7)
- Age in months = monthsbetween([Baby’s date of birth], [Age on])
- and days = [Age on] – addmonths([Baby’s date of birth], [Age in months])
- Corrected age (weeks) = max([Age in days]÷7 – max(40 – [Gestational age at birth], 0), 0)
- Original due date = [Baby’s date of birth] + max(40 – [Gestational age at birth], 0)×7
Formula
Age = today − date of birth (weeks and days, months and days)Corrected age = actual age − weeks born early (40 weeks − gestational age at birth)Worked example
Inputs — Baby's date of birth: 2026-06-17; Age on: 2026-10-25; Gestational age at birth: 40 weeks.
Results — Age in days = 130; Age in weeks = 18; and days = 4; Age in months = 4; and days = 8; Corrected age (weeks) = 18.6; Original due date = Wed, June 17, 2026; Stage = Infant.
Frequently asked questions
How do I calculate my baby's age in weeks?
Count the days since birth and divide by 7.
What is corrected age?
Age minus the weeks born before 40 weeks – used for preemies until about two years old.
When does a baby become a toddler?
Usually at 12 months, when most begin walking.
What formula does the Baby Age Calculator use?
Age = today − date of birth (weeks and days, months and days).