What is Age in Weeks Calculator?
age in weeks, pregnancy weeks This tool runs entirely in your browser. No data is sent to any server, and no signup is needed.
Age in Weeks Calculator
Your age in exact number of weeks. Useful for pregnancy tracking, infant development milestones, and meditation/habit counters.
About this tool
The Age in Weeks Calculator returns the total number of 7-day weeks elapsed between a date of birth and today, with the residual days in the trailing partial week. Weeks are the favoured unit for pregnancy dating, infant development, athletic macrocycles, and habit-tracking because they are constant in length unlike months.
How it works
Days elapsed = floor((Date.now() - parseDate(dob)) / 86,400,000) Weeks elapsed = floor(Days elapsed / 7) Residual days = Days elapsed mod 7 ISO week number = floor((ordinal_day - day_of_week + 10) / 7)
- Days elapsed derives from Julian day number subtraction: every date maps to a unique integer JDN, so the difference is a simple subtraction with no calendar quirks.
- Weeks elapsed divides the day count by 7, the only invariant unit in the Gregorian calendar.
- Leap-year adjustments are absorbed into the day count automatically. A century averages 5,217.86 weeks (365.2425 x 100 / 7).
- ISO 8601 weeks start Monday and number so week 1 contains the first Thursday of the year. US calendars commonly start the week on Sunday.
Worked example
Compute age in weeks for someone born July 15, 2010, evaluated on May 28, 2026:
- Anchor dates: birth 2010-07-15, evaluation 2026-05-28.
- Year span: 15 calendar years plus the period July 15 to May 28 (short by 48 days of a full 16th year).
- Leap days in span: 2012, 2016, 2020, 2024 = 4 leap days.
- Days elapsed: 15 x 365 + 4 - 48 = 5,475 + 4 - 48 = 5,431 days.
- Weeks: 5,431 / 7 = 775 weeks, remainder 6 days.
- Decimal years cross-check: 5,431 / 365.25 = 14.87 years (matches the calendar age).
Age-in-weeks milestones
Common reference points expressed in weeks, with the equivalent in years:
| Milestone | Weeks | Years (decimal) | Context |
|---|---|---|---|
| Full-term pregnancy | 40 | 0.77 | From LMP, ACOG definition |
| One year old | 52 | 1.00 | WHO infant feeding pivot |
| School entry (US K) | 260 | 5.00 | Kindergarten cutoff |
| 10,000 hour expert threshold | 250 | 4.81 | 40 hrs/week deliberate practice |
| Olympic cycle | 208 | 4.00 | Summer Games quadrennium |
| Driving age (US) | 832 | 16.00 | Most states issue full licence |
| Age of majority | 936 | 18.00 | Voting, contracts |
| 1,000-week mark | 1,000 | 19.18 | Often cited late-teens milestone |
| 4,000-week life span | 4,000 | 76.71 | Title of Oliver Burkeman's 2021 book |
| Centenarian | 5,218 | 100.00 | WHO centenarian threshold |
Common pitfalls
- Inclusive versus exclusive bounds. A child born on Monday and evaluated the following Monday has lived 7 days = 1 week, not 8 days. The standard convention excludes the start anchor.
- Timezone handling. If birth is stored as a date in one local zone and "now" is read in another, the day count can flip by one near midnight, shifting the week count when the residual day crosses 6 to 0.
- Daylight saving transitions. Spring-forward shortens one calendar day to 23 hours and fall-back stretches one to 25. Computing weeks from a millisecond difference and then dividing by 86,400,000 is off by one whole day if both endpoints are local and on opposite sides of a DST boundary.
- Calendar month confusion. A "month" is not 4 weeks. February is 4 weeks exactly only in non-leap years; the other months are 4 weeks plus 2 or 3 days. A year is 52 weeks plus 1 or 2 days.
- ISO week 53. Some years (about 71 in a 400-year cycle, including 2026) have 53 ISO weeks. Treating every year as 52 weeks understates the count.
- Pregnancy week labelling. Pregnancy is dated from LMP, not from conception. A "12-week pregnancy" really means 10 weeks since conception. Mixing the two conventions causes errors.
Related calculators and glossary
Frequently asked questions
How long is a pregnancy in weeks?
A standard pregnancy is dated 40 weeks from the first day of the last menstrual period (LMP). Conception happens around week 2 to 3 of those LMP-counted weeks. Full-term is 37 to 42 weeks, preterm is anything under 37, and post-term over 42. Ultrasound dating refines the LMP estimate within the first trimester to within 5 to 7 days.
Why is the first year of infant life tracked in weeks?
In the first year, US AAP and WHO vaccination, growth, and developmental milestones come on a 2, 4, 6, 9, and 12-week cadence. A week is short enough to flag failure-to-thrive early but long enough that small day-to-day variation does not noise the chart. After 12 months, pediatricians switch to months because growth slows.
How many weeks in a typical training or career cycle?
Four Olympic years equal 208 weeks. Elite endurance athletes build, peak, and taper across 4-week macrocycles, so a quadrennium is roughly 52 macrocycles. A 10-year pro cycling career (Tour de France winners average about this long) is 520 weeks of base, build, race, recover.
What is the difference between a calendar week and an ISO week?
ISO 8601 defines weeks as Monday through Sunday and numbers them so week 1 always contains January 4 (and the first Thursday). US and many UK calendars start the week on Sunday. The two systems disagree on which week certain dates fall in near year boundaries, which is why finance and engineering teams in Europe usually pin to ISO week.
Sources and further reading
- ISO 8601:2019 Date and time - Representations for information interchange - week numbering rules and ordinal arithmetic.
- IANA Time Zone Database (tzdata) - DST and offset rules used to normalise local-clock differences.
- American College of Obstetricians and Gynecologists, Methods for Estimating the Due Date (Committee Opinion 700, 2017) - LMP weeks definition.
- Burkeman, Oliver (2021) Four Thousand Weeks: Time Management for Mortals - the 4,000-week life-span framing.