Age in Weeks Calculator
Enter a birth date and a reference date (today by default) to find the exact number of complete weeks elapsed, plus the leftover days and totals in days, months, and years.
Date of birth
Reference date
Complete weeks elapsed since birth
- 1
Elapsed milliseconds
837,216,000,000Milliseconds from birth date to reference date. - 2
Total days
⌊837,216,000,000 ÷ 86,400,000⌋ = 9,690 - 3
Age in weeks
⌊9,690 ÷ 7⌋ = 1,384
How does this calculator work?
Age in weeks = floor(totalDays / 7), where totalDays is the number of whole days from birth to today. A 1-year-old is about 52 weeks old; a 25-year-old is about 1 304 weeks. Enter a birth date and today's date to get the exact count plus the leftover days, total days, months, and years.
Formula
How this is calculated
The calculator converts both dates to millisecond timestamps and subtracts to get the elapsed duration. Dividing by 86 400 000 (the number of milliseconds in a day) and flooring gives total whole days. Dividing that by 7 and flooring gives complete weeks, and the remainder (totalDays mod 7) gives the leftover days within the current week.
Because a 7-day week is a fixed unit independent of months or years, week-based age arithmetic is simpler and more uniform than month-based arithmetic. There are no month-length edge cases: every week is exactly 7 days. The calculation handles leap years automatically because the millisecond difference already captures extra days on Feb 29.
Week-based age is most commonly used for infants in the first two years of life, where developmental milestones are tracked by week. It is also used in obstetrics to express gestational age (counted from the last menstrual period, though this calculator counts from birth). The result is floored to whole weeks — partial weeks are not counted until the 7th day completes.
Frequently asked questions
Approximately 52 weeks (365 ÷ 7 = 52.14), so exactly 52 complete weeks with a few days left over depending on the exact calendar dates.
This calculator counts weeks from birth. For gestational age, you would enter the first day of the last menstrual period (LMP) as the "birth" date and today as the reference — gestational age is conventionally counted from LMP, not conception.
If the reference date falls exactly on the same day of the week as the birth date, the current week completes cleanly with 0 leftover days.
Also known as
TG we-Calculate Editorial Team. (2026). Age in Weeks Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/age-in-weeks-calculator
TG we-Calculate Editorial Team. "Age in Weeks Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/age-in-weeks-calculator.
TG we-Calculate Editorial Team, "Age in Weeks Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/age-in-weeks-calculator
@misc{wecalculate_age_in_weeks_calculator, title = {Age in Weeks Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/age-in-weeks-calculator}}, year = {2026}, note = {TG we-Calculate} }
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