Week number calculator
Look up ISO 8601 and US week numbers for any date, get the start and end of that week, convert a year plus week number back to a date range, and list every week of a year with its dates.
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ISO 8601: weeks start on Monday and week 1 is the week containing the first Thursday, so 2021-01-01 belongs to week 53 of 2020. US: weeks start on Sunday, week 1 is the week containing 1 January, and the week number follows the calendar year of the date.
Result
2020535 · Friday2020-12-28 ~ 2021-01-0353Every week of the chosen year in the current system; cross-year weeks show the end of December or the start of January.
W01 2019-12-30 ~ 2020-01-05 W02 2020-01-06 ~ 2020-01-12 W03 2020-01-13 ~ 2020-01-19 W04 2020-01-20 ~ 2020-01-26 W05 2020-01-27 ~ 2020-02-02 W06 2020-02-03 ~ 2020-02-09 W07 2020-02-10 ~ 2020-02-16 W08 2020-02-17 ~ 2020-02-23 W09 2020-02-24 ~ 2020-03-01 W10 2020-03-02 ~ 2020-03-08 W11 2020-03-09 ~ 2020-03-15 W12 2020-03-16 ~ 2020-03-22 W13 2020-03-23 ~ 2020-03-29 W14 2020-03-30 ~ 2020-04-05 W15 2020-04-06 ~ 2020-04-12 W16 2020-04-13 ~ 2020-04-19 W17 2020-04-20 ~ 2020-04-26 W18 2020-04-27 ~ 2020-05-03 W19 2020-05-04 ~ 2020-05-10 W20 2020-05-11 ~ 2020-05-17 W21 2020-05-18 ~ 2020-05-24 W22 2020-05-25 ~ 2020-05-31 W23 2020-06-01 ~ 2020-06-07 W24 2020-06-08 ~ 2020-06-14 W25 2020-06-15 ~ 2020-06-21 W26 2020-06-22 ~ 2020-06-28 W27 2020-06-29 ~ 2020-07-05 W28 2020-07-06 ~ 2020-07-12 W29 2020-07-13 ~ 2020-07-19 W30 2020-07-20 ~ 2020-07-26 W31 2020-07-27 ~ 2020-08-02 W32 2020-08-03 ~ 2020-08-09 W33 2020-08-10 ~ 2020-08-16 W34 2020-08-17 ~ 2020-08-23 W35 2020-08-24 ~ 2020-08-30 W36 2020-08-31 ~ 2020-09-06 W37 2020-09-07 ~ 2020-09-13 W38 2020-09-14 ~ 2020-09-20 W39 2020-09-21 ~ 2020-09-27 W40 2020-09-28 ~ 2020-10-04 W41 2020-10-05 ~ 2020-10-11 W42 2020-10-12 ~ 2020-10-18 W43 2020-10-19 ~ 2020-10-25 W44 2020-10-26 ~ 2020-11-01 W45 2020-11-02 ~ 2020-11-08 W46 2020-11-09 ~ 2020-11-15 W47 2020-11-16 ~ 2020-11-22 W48 2020-11-23 ~ 2020-11-29 W49 2020-11-30 ~ 2020-12-06 W50 2020-12-07 ~ 2020-12-13 W51 2020-12-14 ~ 2020-12-20 W52 2020-12-21 ~ 2020-12-27 W53 2020-12-28 ~ 2021-01-03
What this tool does
- Find out which days “week 41” covers in a planning sheet or a fiscal calendar: enter the year and week number and get the range.
- Confirm week numbers for ISO 8601 systems — 2021-01-01 belongs to week 53 of 2020, the kind of cross-year case that is easy to get wrong, and the week year is reported explicitly.
- Compare both conventions: Europe mostly uses ISO (weeks start Monday) while the US starts weeks on Sunday with week 1 containing 1 January — switch the system and compare.
- Grab the whole year at once: every week is listed with its start and end date, ready for planning or exporting.
Example
Input
Week system ISO 8601, date 2021-01-01
Output
Week date 2020-W53-5; week year 2020; week number 53; day of week 5 (Friday); week range 2020-12-28 ~ 2021-01-03; weeks in that year 53
Switching to the US system makes the same day 2021-W01-5 with the range 2020-12-27 ~ 2021-01-02 — the two systems diverge most around the turn of the year.
Frequently asked questions
Why is 2021-01-01 in week 53 of 2020?
ISO 8601 defines week 1 as the week containing the first Thursday of the year, with weeks starting on Monday. The week from Monday 2020-12-28 to Sunday 2021-01-03 has its Thursday on 2020-12-31, which falls in 2020, so the whole week belongs to 2020 and 2021-01-01 is the Friday of week 53 of 2020.
How do the ISO and US systems differ?
ISO 8601 starts weeks on Monday and anchors week 1 on the first Thursday, so the week year can differ from the calendar year. The US system starts weeks on Sunday, makes the week containing 1 January week 1, and numbers weeks by the calendar year of the date — which is why the end of December and the start of January can carry different numbers inside one Sunday-to-Saturday week.
How many weeks can a year have?
Both systems allow 53. In ISO that happens when 1 January is a Thursday, or when it is a Wednesday in a leap year; in the US system it depends on which weekday 1 January is. The “weeks in that year” row gives the exact number, and an out-of-range week number is rejected instead of being clamped.
How is the day of the week numbered?
ISO uses 1–7 with Monday as 1 and Sunday as 7; the US system uses 0–6 with Sunday as 0 and Saturday as 6. When converting a week date back to a calendar date the range is validated for the active system, so a bad value is reported rather than silently rounded.
Does the result depend on my time zone?
No. Everything runs on integer day numbers instead of Date objects, so the same input gives the same answer in every time zone and season — a daylight saving change can never shift the week.
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