Baby Age Calculator

This infant age calculator will calculate the exact age of a baby from their date of birth. It calculates the number of days until today or any given date and displays the age in a format actually used by parents and pediatricians. Newborns are shown in weeks and days, older infants in months, toddlers in years. Once information is entered, the result will be immediately displayed in the results area.

To use this tool to calculate infant age:

Enter the baby's date of birth and check the age shown on the right. By default, the tool calculates the age until today so that it always shows the current age. To check the age at another date, for example for an upcoming appointment or a previous follow-up date, change the "Age reference day" to the desired date and then select the date.

If it is a premature birth, activate the "Premature" option and enter how many weeks before the due date the baby was born. The tool will also show the corrected age that has been calculated taking into account the premature birth.

Why is infancy measured in weeks?

In the first year of life, the week is the most natural unit. Immunization schedules, feeding amounts and developmental checkups are all based on infant age, which matters more for babies than it does for adults, even if they're small differences.

The tool will show the age either as whole weeks and remaining days e.g. 12 weeks 5 days or a single number which does not round up to the next whole number. It also shows the total number of days, whole months and decimal months that can be used in forms and charts where a single numeric entry is required.

Infant age calculation:

Age is based on the actual calendar and is the time span between two dates. When calculating age in weeks, the tool divides the number of days since birth by seven and keeps the remainder as extra days. When calculating age in months, it first counts full calendar months from the same date within a month then calculates remaining days.

Example calculation: Suppose a baby is born on day 1 in January and examined on day 10 in March of the same year. This is 68 days, or 9 weeks and 5 days. It is also 2 months and 9 days. The period from 1st January to 1st March is exactly two months, leaving 9 days remaining which brings us to the date of 10th March.

Weeks, months and developmental stages in pediatrics:

An average month is about 4.3 weeks, so a baby who is 3 months old would be approximately 13 weeks old, while a baby who is 6 months old would be approximately 26 weeks old. These are only estimates. As the number of days in a calendar month varies between 28 and 31 days, this tool calculates actual dates rather than fixed values. The table below serves as a handy reference for the first two years after birth.

Corrected age in premature babies

Because babies born less than 40 weeks from their due date have to catch up on developmental delays, the corrected age rather than actual postnatal age is used for development tracking. A full-term baby is considered 40 weeks and the number of weeks before the due date in a preterm birth is the difference between the number of gestational weeks at delivery and 40 weeks.

The corrected age is the actual postnatal age minus the number of weeks premature. For example, a baby who is 16 weeks old and was born 8 weeks early would have a corrected age of 8 weeks. The earlier a baby is born, the greater the difference between these two ages will be.

The corrected age is usually used until about 2 years of age, sometimes up to 3 years. After that the difference has no effect on the original guidelines. There is one exception: The immunization schedule is based on actual postnatal age. If a complete clinical record including gestational weeks and days is needed, use the separate adjusted age calculator.

Developmental milestones and next birthday

The results panel shows the baby's current stage of paediatric development (from newborn to toddler) and indicates the date and day of week for the next completed month of life. It also displays a countdown to the next birthday, and how much time has passed since the beginning of the current month of life.

Please note that developmental milestones are not set on specific days but should occur within a certain timeframe. The phases outlined here are just guidelines and every baby has their own pace of development.

Leap years and calendars.

All calculations use the actual calendar so February 29th, leap years and different number of days per month are automatically taken into account. The full months count is calculated by comparing date entries within each month which keeps the result correct for both month and day numbers instead of assuming every month has exactly 30 days.

This calculator is for everyday and educational purposes only and does not constitute medical advice. If you have any questions about your baby's growth, development or immunization schedule, especially if they were born prematurely, please consult a pediatrician, doctor or child care professional.