Baby age calculator

Your baby’s age, the way doctors count it

In the first months, a baby’s age is usually given in weeks, then in months. The calculator shows both from the date of birth, along with the exact age in years, months and days, the total days, and the dates of the next month birthday, day 100 and the first birthday.

A baby age calculator for infant age in weeks and months, with a corrected age calculator for premature babies.

Weeks and months

Age in weeks is the days since birth divided by 7. A baby born on 1 August is 9 weeks and 6 days old on 9 October, and 2 months and 8 days old. Month birthdays fall on the same date each month; a baby born on the 31st has its month birthday on the last day of shorter months.

Set Age on to a future date to see how old your baby will be at a check-up or vaccination appointment.

Corrected age for premature babies

A baby born before 37 weeks has had less time to grow than a baby born on time. Corrected age counts from the original due date instead of the birth date:

Corrected age = age since birth − weeks born early

A baby born 8 weeks early is 4 months old at 4 months after birth, but about 2 months corrected. Paediatricians use corrected age to judge growth and milestones such as smiling, sitting and crawling, usually until about age two. Vaccinations normally follow the age since birth. Switch on Born early and enter the due date to see both.

Little milestones

Many families mark special days: the 100th day, the first month birthdays, the first birthday. The calculator gives the date of the next one. Day 1 is the day of birth, so day 100 is 99 days later.

For older children and adults, the age calculator gives the exact age on any date, and if you’re planning ahead, see the pregnancy due date calculator. To count down to the first birthday, try the birthday countdown.