How accurate is the weeks figure?

It is exact — it is derived directly from the precise number of calendar days between the two dates, not an approximation.

Age, Date & Calendar

Age in Weeks Calculator

Enter a date of birth to see your age in total completed weeks, plus the days left over.

Your calculation is performed in your browser.

Enter a date of birth to see the age in completed weeks.

The calculator finds the exact number of calendar days between your date of birth and the calculation date, then divides that exact figure by seven.

The total number of days is divided by 7. The whole part is the number of completed weeks, and the remainder is the extra days.

Total daysTotal days = Calculation date − Date of birth

Completed weeksWeeks = ⌊ Total days ÷ 7 ⌋

Extra daysDays = Total days mod 7

Someone exactly 100 days old is 14 completed weeks and 2 days old.

—Tracking a newborn’s age in weeks, which is how many early development milestones are described.
—Working out an exact weeks-old figure for feeding, sleep, or growth guidance that’s given by week.
—Satisfying curiosity about your own or someone else’s exact age in completed weeks.

—Because the underlying day count is exact, the weeks-and-remainder breakdown is exact too.