Quick answer
A baby's age is the reference date minus the date of birth. Born 10 January 2025 and measured on 15 March 2025, a baby is 64 days old, which is 9 weeks 1 day, or 2 months 5 days.
Enter the date of birth and today's date. The calculator returns age in weeks and days, months and days, and total days.
Related tools: Age Difference Calculator: Time between two birth dates. School Age Calculator: Child's age on a school cutoff date. Birth Date from Age Calculator: Range of possible birth dates for someone of a given age.
How to use the Baby Age Calculator
- Pick date of birth.
- Pick reference date.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Date of birth | A calendar date | — |
| Reference date | A calendar date | — |
Formula and method
Formula
age = reference date − birth date
IEEE-754 double precision; shown to the decimals indicated.
Shown in weeks and days, and in calendar months and days.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Date of birth = 2025-01-10; Reference date = 2025-03-15.
| Result | Value |
|---|---|
| Age | 9 weeks, 1 days |
| In months and days | 2 months, 5 days |
| Total days | 64 |
Example 2
Inputs: Date of birth = 2024-06-01; Reference date = 2025-06-01.
| Result | Value |
|---|---|
| Age | 52 weeks, 1 days |
| In months and days | 12 months, 0 days |
| Total days | 365 |
Example 3
Inputs: Date of birth = 2025-02-28; Reference date = 2025-03-07.
| Result | Value |
|---|---|
| Age | 1 weeks, 0 days |
| In months and days | 0 months, 7 days |
| Total days | 7 |
The first line is the headline result (age). Below it the baby Age Calculator lists in months and days and total days, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Weeks, months, days
Parents use weeks for the first months and months afterwards. Month length varies, so '2 months 5 days' counts calendar months from the birth date, while '9 weeks 1 day' is exactly 64 days divided by 7.
Corrected age
For babies born early, health professionals often use corrected age (age minus weeks premature) when judging milestones during the first two years. This tool shows chronological age only.
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Reference date: entering "2025-01-01" shows "The reference date must not be before the birth date." beside that field and no result.
- Date of birth: left empty shows "Enter date of birth." beside that field and no result.
Choosing between the Baby Age Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Age Difference Calculator](/lifestyle/age-difference-calculator/) | first birth date and second birth date | Age difference |
| [School Age Calculator](/lifestyle/school-age/) | date of birth and school cutoff date | Age on the cutoff date |
| [Birth Date from Age Calculator](/lifestyle/age-to-birthday/) | age in years and reference date | Possible birth dates |
| [Birthday Calculator](/lifestyle/birthday/) | date of birth and reference date | Next birthday |
| [Half Birthday Calculator](/lifestyle/half-birthday/) | date of birth | Half birthday |
Assumptions and limitations
- Reference date cannot precede birth
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for reference date and date of birth to confirm this.
Frequently asked questions
How many weeks old is my baby?
Total days divided by 7, rounded down, plus remainder days.
Is a month always 4 weeks?
No, calendar months are 28 to 31 days.
Can I use it for pets?
Yes, it is just a date difference.
What does the Baby Age Calculator calculate?
A baby's age in weeks, months and days. It returns Age, In months and days and Total days.
What formula does the Baby Age Calculator use?
age = reference date − birth date. Shown in weeks and days, and in calendar months and days.
What do I need to enter in the baby Age Calculator?
You enter Date of birth and Reference date.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Reference date cannot precede birth.
What is an example of a baby Age result?
With date of birth 2025-01-10 and reference date 2025-03-15, the age is 9 weeks, 1 days.
Related calculators
- Age Difference Calculator — Time between two birth dates.
- School Age Calculator — Child's age on a school cutoff date.
- Birth Date from Age Calculator — Range of possible birth dates for someone of a given age.
- Birthday Calculator — Next birthday, days until it, and your current age.
- Half Birthday Calculator — The date six months after a birthday.
- Age Calculator — Calculate your exact age in years, months and days.
- Weekday Calculator — Look up the weekday (Monday to Sunday) of any past or future date.
Sources
- ISO 8601-1: Date and time, representations for information interchange (calendar dates, ordinal dates and week dates)
- Gregorian calendar reform, papal bull Inter gravissimas (1582): the leap-year rule used for all dates
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.