Baby Age Calculator

Why baby age is counted in weeks

In the first months a baby changes so quickly that "three months old" is too rough a measure. Vaccination schedules, feeding advice and development checks are often given in weeks, and a few weeks can make a real difference to what a baby is ready for. This calculator turns a date of birth into an exact age in months and days, in weeks and days, and in total days, on today's date or any other date you choose, such as the day of a vaccination appointment.

Corrected age for babies born early

A baby born before 37 weeks of pregnancy is preterm. Such a baby has had less time to develop in the womb, so comparing them with babies of the same age since birth can make them look behind when they are developing normally. Corrected age, also called adjusted age, is the age the baby would be if born on the due date. It is commonly used for growth charts and development milestones until about two years of age, while vaccinations usually follow the actual age from birth. Your paediatrician decides which age to use for what.

How to use the calculator

1. Enter your baby's date of birth.
2. Leave Age on at today, or change it to a future appointment date.
3. If your baby was born early, enter the weeks of pregnancy at birth, and the extra days if you know them (for 32 weeks and 3 days, enter 32 and 3). For a baby born at 37 weeks or later, leave it at 40.

The formula

Chronological age is the calendar time since birth. Corrected age = chronological age - (40 weeks - weeks of pregnancy at birth). In practice the calculator moves the starting point from the birth date to the original due date, which is the birth date plus the weeks the baby was early, and counts from there. Months are counted as calendar months, so a baby born on the 31st completes a month on the last day of a shorter month.

A worked example

A baby was born on 10 June 2026 at 32 weeks. On 2 November 2026 the baby is 145 days old: 20 weeks and 5 days, or 4 months and 23 days. Because the baby arrived 8 weeks early, the original due date was 5 August 2026. Counting from that date, the corrected age on 2 November is 89 days: 12 weeks and 5 days, or 2 months and 28 days. For a development check, a doctor would usually compare this baby with babies around 3 months old rather than 5.

Limits

• If the due date has not yet been reached, there is no corrected age yet; the calculator shows how many weeks remain until the due date instead.
• The calculator uses 40 weeks as the full-term reference, as the corrected-age formula does.
• All dates stay in your browser and are not sent to Ur Next Door.
• The result is an estimate for general information and is not medical advice. Please consult your paediatrician about your baby's growth, feeding and development, and about which age to use for each check.