# Baby Age Calculator

URL: https://speedycalculator.com/calculators/everyday-life/baby-age/
Last updated: 2026-10-25

> The baby age calculator works out how old your baby is in weeks and days, months and days, and total days – plus the corrected (adjusted) age for babies born prematurely, which is used to track development.

## Inputs

- Baby's date of birth
- Age on
- Gestational age at birth (weeks)

## Results

- Age in days
- Age in weeks
- and days
- Age in months
- and days
- Corrected age (weeks)
- Original due date
- Stage

## 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)
```

## Explanation

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.

### Tips

- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so "one month" is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.

### Common mistakes to avoid

- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).

### Inputs explained

FieldWhat to enterBaby's date of birthA dateAge onA dateGestational age at birthIn 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

## 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.

## FAQ

**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 – used for preemies until about two years old.

**When does a baby become a toddler?**
Usually at 12 months, when most begin walking.

**What formula does the Baby Age Calculator use?**
Age = today − date of birth (weeks and days, months and days).


## Sources

- [American Academy of Pediatrics – Corrected age for preemies](https://www.healthychildren.org/)