# Age calculator of baby

Works out how old a baby is from the birth date, in months and days and in weeks and days, and the corrected age of a baby born before the due date.

- Page: https://www.acalculator.org/days/baby-age-calculator
- JSON spec: https://www.acalculator.org/days/baby-age-calculator.json
- Version: da9b975de0b6

## Default answer

Example with the default inputs (Birth date June 22, 2026, Age on September 30, 2026) on the example date Wednesday, September 30, 2026: A baby born on June 22, 2026 is 3 months, 8 days old on September 30, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| birth | Birth date | The day the baby was born. |
| on | Age on | The date to work out the age on. Today, unless you change it. |
| due | Due date (if born early) | The due date, for a baby born before it. Leave it empty for a baby born on time. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| age | Age | Whole years, whole months, and days from the birth date to the age-on date. |
| weeks | In weeks | The age as whole weeks and the days left. |
| days | Days old | Days from the birth date to the age-on date. |
| corrected | Corrected age | For a baby born early: whole years, months, and days from the due date to the age-on date (the age less the weeks born early). |
| correctedWeeks | Corrected age in weeks | The corrected age as whole weeks and the days left. |
| early | Born early by | Weeks and days from the birth date to the due date. |
| toDue | Days to the due date | When the age-on date is before the due date: the days left until the due date. |

## Method

age = the whole years, months, and days from the birth date to the age-on date; corrected age = the same from the due date (the age less the weeks born early)

## Assumptions

- Dates are whole calendar days with no time of day; the day of birth is day 0.
- Months are calendar months from the birth date; a day that a month does not have moves to its last day.
- Corrected age (AAP) counts from the due date. Use it for a baby born early, for the first 2 years.
- The age-on date is today unless you change it; the page reads today’s date from your device when it loads.

## Worked examples

1. birth = 2026-06-22, on = 2026-09-30 gives age = {"years":0,"months":3,"days":8}, weeks = {"weeks":14,"days":2}, days = 100. Source: Julian Day numbers (Meeus, Astronomical Algorithms, 1991, chapter 7): JD 2461313.5 − JD 2461213.5 = 100 days.
2. birth = 2026-01-01, on = 2026-04-23, due = 2026-02-26 gives weeks = {"weeks":16,"days":0}, early = {"weeks":8,"days":0}, correctedWeeks = {"weeks":8,"days":0}, corrected = {"years":0,"months":1,"days":28}. Source: American Academy of Pediatrics, HealthyChildren.org, Corrected Age For Preemies: corrected age = age in weeks − weeks born early (https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx).
3. birth = 2026-09-01, on = 2026-09-30, due = 2026-10-20 gives weeks = {"weeks":4,"days":1}, early = {"weeks":7,"days":0}, toDue = 20.
4. birth = 2024-02-29, on = 2026-02-28 gives age = {"years":2,"months":0,"days":0}, days = 730. Source: Gregorian leap-year rule (ISO 8601).

## FAQ

### How do I work out my baby’s age in weeks?

Count the days from the birth date to today and divide by 7. The whole number is the weeks and the remainder is the days. A baby born 100 days ago is 14 weeks and 2 days old, because 100 = 14 × 7 + 2.

### How do I work out my baby’s age in months?

Count calendar months from the birth date: the baby turns 1 month old on the same day of the next month. A baby born on June 22 is 3 months old on September 22, and 3 months and 8 days old on September 30.

### What is corrected age?

Corrected age (also called adjusted age) is the age of a baby born early counted from the due date instead of the birth date. The American Academy of Pediatrics says to take the baby’s age in weeks and subtract the weeks born early: a baby born at 32 weeks is 8 weeks early, so at 16 weeks old the corrected age is 8 weeks.

### How long should I use corrected age?

The American Academy of Pediatrics suggests using corrected age for growth and developmental milestones in the first 2 years. Vaccines are different: the CDC says babies born early get them at the same age from birth as other babies, not by corrected age. Ask your pediatrician if you are unsure.

### What if today is before my baby’s due date?

Then the corrected age has not started yet, and the calculator shows the days left until the due date instead.

### What happens with a February 29 birthday?

In a common year the birthday falls on February 28. A baby born on February 29, 2024 is 2 years old on February 28, 2026.

### Why does the age change when I open the page on another day?

The age-on date starts as today, read from your device when the page loads. The page itself was built on an earlier day, so it shows that day’s age until your browser works out today’s. Pick an age-on date for a fixed answer.

## Sources

- American Academy of Pediatrics, HealthyChildren.org, Corrected Age For Preemies: corrected age is the age in weeks less the weeks born early, used for the first 2 years. https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx
- CDC, Special Situations (General Best Practices for Immunization): preterm infants are vaccinated at the same chronological age and schedule as full-term infants. https://www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html
- Jean Meeus, Astronomical Algorithms (1991), chapter 7: Julian Day numbers, so the days between two dates are the difference of their Julian Days.