Age in months calculator
The tool gives the number of whole calendar months since birth and the days after the last full month, with a years and months split.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date of birth and age on this date.
- 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
Whole months = the largest number of months that can be added to the birth date without passing the calculation date (using the month's last day when the birth day does not exist). Days = what remains.
Example
Inputs: Date of birth = 2025-01-20, Age on this date = 2026-09-30. Result: Age in whole months: 20; Days after the last whole month: 10; Years: 1.
Limits and caveats
- Months differ in length, so a whole month is not a fixed number of days.
- A child born at the end of a month completes the month on the last day of a shorter month.
Questions about this tool
Why is a child's age given in months?
Growth is fast in the early years, and vaccination and growth charts usually use months up to 24 or 36 months.
How is a month counted for someone born on the 31st?
In a month without a 31st, the month completes on its last day, such as 28 February.
How many months are in 3 years?
36 whole months.