Baby Age Calculator
How old is your baby in weeks, months and days — with corrected age for babies born early, and the dates of milestones like 100 days.
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How to work out your baby’s age
- Enter your baby’s date of birth, and the time if you know it.
- If your baby was born early, add the due date to see their corrected age.
- Read their age in weeks, months and days and the upcoming milestones; Share / Copy link lets family see it too.
How baby age is counted
Babies' ages are usually given in weeks for the first few months, then in months until about two, and in years after that, so the headline follows the same pattern. A week is counted every seven days from the birth date, so a baby born on a Tuesday turns one week old the following Tuesday. A month is counted on the same date each month; a baby born on the 31st has their monthly birthday on the last day of shorter months (28 or 29 February, 30 April and so on).
Corrected age (also called adjusted age) is for babies born early: it is their age counted from their due date instead of their birthday. A baby born 8 weeks early who is now 20 weeks old has a corrected age of 12 weeks, which is the age to compare with when thinking about growth and development. Health professionals generally use corrected age until about two years old. Enter the due date to see it, along with how many weeks pregnant the birth was (gestational age): before 37 weeks is preterm, 37 to 41 weeks is term, and 42 weeks or more is post-term.
The milestone table lists weekly and monthly birthdays, the popular 100-day and 1,000-day celebrations, the half birthday and the first few birthdays, and can add the upcoming ones to your phone's calendar. This tool is for interest and planning — always follow the advice of your midwife, health visitor or doctor about your baby's health and development. Everything is worked out in your browser; if you tick "Remember on this device", the details are kept only in this browser and never uploaded. Share link gives you a link with the date of birth in it (plus the time, due date and name, if you've filled them in), so a partner or grandparent can open it and see the baby's age on the day they look. The details travel only in the link itself; opening one doesn't replace a baby you've saved on that device unless you change something.
Questions about baby age
How old is my baby in weeks?
Count every seven days from the birth date: a baby born on a Tuesday turns one week old the following Tuesday. Ages are usually given in weeks for the first few months, then in months.
What is corrected age?
For babies born early, their age counted from the due date instead of the birthday. A baby born 8 weeks early who is 20 weeks old has a corrected age of 12 weeks. It’s generally used until about age two.
When is my baby 100 days old?
Enter the date of birth and the 100-day milestone is listed, along with the half birthday, 1,000 days and the monthly birthdays.
Is my baby’s information saved?
Only if you tick Remember on this device, and then only in your browser. Nothing is uploaded; the share link carries the details in the link itself.