Baby age calculator

How to use the baby age calculator

1. Date of birth. Pick the day your baby was born.
2. Due date (optional). If your baby arrived before the due date, enter the original due date. Leave it empty for a baby born at term.

The large figure is your baby’s age in weeks and days as of today. Below it you see the age in months and days and the total number of days. With a due date filled in, the calculator also shows how early your baby was born and the adjusted age.

How baby age in weeks and months is worked out

Weeks come from a plain day count. Months follow the calendar, so a baby born on the 3rd turns one month older on the 3rd of each month.

Age in weeks = days since birth ÷ 7. Adjusted age = actual age − time born early. Time born early = due date − date of birth.

Worked example

A baby is born on 3 March 2026. The due date was 14 April 2026. Today is 12 May 2026.

• Days since birth: 28 days left in March, 30 in April and 12 in May. That is 70 days.
• Actual age: 70 ÷ 7 = 10 weeks, 0 days. On the calendar that is 2 months and 9 days.
• Time born early: 3 March to 14 April is 42 days, which is 6 weeks.
• Adjusted age: 70 − 42 = 28 days, or 4 weeks, 0 days.

Adjusted age is the same as counting from the due date instead of the birth date. 14 April to 12 May is 28 days.

Baby age in weeks and months

A month is a little longer than four weeks, so weeks and months drift apart as your baby grows. This table shows roughly where they line up.

Age in months	About this many weeks	About this many days
1 month	4 weeks	30 days
2 months	9 weeks	61 days
3 months	13 weeks	91 days
4 months	17 weeks	122 days
5 months	22 weeks	152 days
6 months	26 weeks	183 days
9 months	39 weeks	274 days
12 months	52 weeks	365 days
18 months	78 weeks	548 days
24 months	104 weeks	730 days

The exact figures depend on which months your baby has lived through, so use the calculator for a precise answer.

Adjusted age for premature babies

Adjusted age is also called corrected age. It is the age your baby would be if they had been born on the due date. Doctors and health visitors often use it to judge growth and milestones such as smiling, sitting and crawling, because a baby born early needs that time to catch up.

• When it is used. Corrected age is commonly used until a child is 2 years old. Your pediatrician may stop sooner or continue longer, so follow their advice.
• When actual age is used. Birthdays and most paperwork use the real date of birth. Ask your doctor which age applies to vaccinations and to starting solid food.
• Before the due date. If the due date has not arrived yet, there is no adjusted age to show. The calculator tells you how long is left until it starts.
• Born on time or late. No correction is needed. Leave the due date empty.

This page gives general information only. It is not medical advice. Talk to your pediatrician or health visitor about your baby’s growth and development.

For older children and adults, use the age calculator. To count down to a due date or an appointment, try days between dates. Working out a pet’s age instead? See the dog age calculator.

How this calculator was checked

Built by Rasel Hakkani, WordPress developer at Swift Web Dev. The results were checked against hand-counted dates, including a 29 February birth, a baby born today and due dates before and after the birth date. It is general information, not medical advice.

Sources

• American Academy of Pediatrics, HealthyChildren.org: Corrected Age For Preemies

Last checked: October 2026. Found a wrong answer? Tell us and we will fix it.