Week Number Calculator (ISO Week)

Week numbers are how planners, payroll, factories and much of Europe and Asia refer to a stretch of days: "delivery in week 45". This calculator gives the ISO 8601 week for any date, the Monday-to-Sunday range it covers, and the US-style week number, and it can turn a week number back into dates. The table lists every week of the year.

How to use this tool

1. Choose whether you have a date or a week number.
2. Pick the date — it starts on today — or enter the ISO year and week.
3. Read the ISO week, its Monday and Sunday, the US week number and the day of the year.
4. Use the table for the dates of every week in that year.

Formula

week = ⌊(day-of-year of that week's Thursday − 1) ÷ 7⌋ + 1

ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, 4 January). A week belongs to the year that holds its Thursday, so 29–31 December can be week 1 of the next year and 1–3 January can be week 52 or 53 of the previous one. A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. The US number starts weeks on Sunday and counts the week of 1 January as week 1.

Example

Sunday 3 January 2021: its Thursday is 31 December 2020, so the ISO week is 2020-W53 (Monday 28 December to Sunday 3 January). In the US system the same day starts week 2 of 2021, because 1 January 2021 was a Friday and week 1 was just 1–2 January.