ISO week numbers for 2021

{"url":"https://calcdate.live/whatweek/2021/","path":"/whatweek/2021","kind":"week-year","title":"ISO Week Numbers 2021","h1":"ISO week numbers for 2021","generated_at":"2026-10-05T19:56:59+05:00","timezone":"Asia/Karachi","today":"2026-10-05","result":{"hero":{"value":"52","unit":"ISO weeks in 2021","sub":"Week 1 starts Monday, January 4, 2021"},"stats":[{"label":"ISO weeks","value":"52"},{"label":"week 1 starts","value":"Jan 4"},{"label":"days in year","value":"365"},{"label":"January 1","value":"Friday"},{"label":"December 31","value":"Friday"}]},"inputs":{"kind":"week-year","type":"whatweek","category":"calculator","year":"2021"},"method":"The result uses the exact inputs on this page and calendar/time arithmetic appropriate to this calculator. The result shown above is the value to use for this calculation.","milestones":[],"workdays":[],"accuracy":["The result uses the calendar rules and time zone shown on this page.","Leap years and actual month lengths are handled by the calendar arithmetic where they affect the calculation."],"excel":{"formula":"","note":"This calculation does not have a single reliable built-in Excel/Google Sheets formula without reproducing the page’s specific time-zone or calendar rules."},"sections":[["How many weeks are in 2021?",["2021 has 52 ISO weeks. Week 1 starts on Monday, January 4, 2021 because week 1 is the week that contains the first Thursday of January. The last week ends on Sunday, January 2, 2022. 2021 is a regular ISO year with 52 weeks. A 53rd week appears in roughly one ISO year out of every five or six."]],["How the weeks map to months",["Months and weeks do not line up neatly, so the table lists the first and last ISO week that touches each month. January begins in week 53 and December ends in week 52. A week that straddles two months is counted in both rows."]],["Why ISO weeks matter",["Manufacturing, retail, logistics, schools and government agencies in most of Europe and Asia plan in ISO weeks. A production run in week 32 or a report due in week 45 means the same span for everyone regardless of weekday. Using the week number avoids confusion between calendars that start weeks on Sunday and those that start on Monday."]],["Using this page",["Use the numbered links to open the exact dates of any week, or type a date into the form above to find its week number. The overview stays correct for any year because it is calculated from the ISO 8601 rule instead of typed in."]]],"summary":"ISO Week Numbers 2021: 52 ISO weeks in 2021 — Week 1 starts Monday, January 4, 2021","show_method":true}