Age in Months Calculator
Work out an exact age in months and days from a date of birth, with weeks, days, upcoming monthly birthdays and corrected age for preemies.
Babies and toddlers are measured in months: for check-ups, feeding and sleep advice, clothing sizes and developmental milestones. This calculator gives the exact age in completed months plus leftover days, along with weeks and total days, and lists the next monthly birthdays. For a baby born early, enter the weeks of pregnancy at birth to also get the corrected (adjusted) age.
How to use this tool
- Enter the date of birth.
- Keep the second date as today, or pick another date.
- If the baby was born before 37 weeks, enter the weeks of pregnancy at birth.
- Read the age in months and days, and the dates of the coming monthly birthdays.
Formula
Completed months = largest n with (birth date + n months) ≤ as-of date; leftover days = as-of date − (birth date + n months)
Adding a month keeps the same day of the month. When that day does not exist, for a baby born on 31 January for example, the last day of the shorter month is used instead. Corrected age = actual age − (40 − weeks of pregnancy at birth) weeks.
Example
A baby born on 15 January 2024 is 14 months and 5 days old on 20 March 2025: 15 March 2025 is the 14-month mark and five more days have passed. That is 430 days in total, or 61 weeks and 3 days. A baby born at 32 weeks on 1 January 2025 is 6 months old on 1 July 2025, but because it arrived 8 weeks early its corrected age is 4 months and 5 days.
Frequently asked questions
What is corrected age?
For a premature baby, corrected or adjusted age is how old the baby would be if born on the due date. Take the weeks born early, which is 40 minus the weeks of pregnancy at birth, away from the actual age. Pediatricians use it to judge milestones during the first two years.
When should I stop using corrected age?
Most children born early catch up by around age 2, so corrected age is normally used until then. Your pediatrician may suggest a different cut-off for a very premature baby.
Why does the monthly birthday sometimes fall on the 28th?
If a baby was born on the 29th, 30th or 31st, shorter months have no matching date, so that month’s birthday falls on the last day of the month.
How many weeks are in a month of age?
An average month is about 30.44 days, or 4.35 weeks, so 3 months is roughly 13 weeks. Use the dates for an exact figure.
Sources
Last updated: 2026-10-30