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.
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. It is used for preemies until about two years old.
When does a baby become a toddler?
Usually at 12 months, when most begin walking.