Baby Age Calculator

Written byOleh Soloninupdated 29 August 2026

Date of birth

Measured on

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Weeks at birth

Result

Age17 months and 29 days
Whole weeks78
Days since birth547
Corrected ageborn at term — no correction needed
Next month milestone2026-09-15
Days until it2

This result is informational only and is not professional advice.

Paediatrics counts in weeks, the calendar in months

Works out a baby's age in months, weeks and days, and gives a corrected age for babies born before term.

In the first year a baby's age is tracked in both weeks and months, and the two scales do not line up: a month is not four weeks but about 4.35. By the first birthday the gap is nearly three weeks, which shows up when reading development tables.

Babies born early get a corrected age: the weeks missing from forty are subtracted from the actual age. A baby born at 32 weeks is, at four months from birth, only two months corrected — and that is the figure their development should be judged against.

What this calculator shows

Age in months and days, in whole weeks, and in days since birth
The corrected age, if the baby was born before 37 weeks
The date of the next month milestone and the days until it

What to keep in mind

Months are counted by the calendar, from a date to the same date the following month, rather than in blocks of thirty days.
Age correction is generally applied to around two years, after which the difference in development usually evens out.

FAQs

How many weeks are in six months?

About 26. A calendar month averages 4.35 weeks, so six months comes to 26 weeks rather than 24.

What is corrected age?

Age counted from the due date rather than the birth date. Subtract the weeks of prematurity from the actual age: born at 32 weeks means minus eight weeks.

Until what age is correction applied?

Usually to about two years. By then most children born preterm have caught up with their peers on the main developmental markers.

Why do clinics use months and charts use weeks?

Months suit appointments and vaccination schedules; weeks are more precise for assessing development in the first year, when change is rapid.

Worked example

Six months from birth

Input: Born 2025-03-15, measured on 2025-09-15

Output: 6 months exactly — 184 days and 26 whole weeks

Note: Six months is 26 weeks, not the 24 you would get counting four weeks to a month. That is exactly why weaning and vaccination schedules are kept in weeks.

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