Week number calculator
A week number tells you which week of the year a date falls in. Two systems exist — ISO 8601 (Monday-first, used internationally) and the US system (Sunday-first) — and they can.
ISO week number
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US week number
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Day of week
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Day of year
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Quarter
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Weeks left in year
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ISO 8601 week numbering
Week 1 = the week containing the first Thursday of the year (equivalently, the week with Jan 4 in it).
- Weeks run Monday → Sunday
- The first and last few days of a year may belong to the adjacent ISO year
US week numbering
Week 1 = the week containing January 1.
- Weeks run Sunday → Saturday
- There is no ambiguity about which year a date belongs to
Frequently asked
What is the difference between ISO and US week numbers?
ISO 8601 weeks start on Monday; week 1 contains the year's first Thursday. US weeks start on Sunday; week 1 contains January 1. The two systems can disagree by up to 1 week near the start of year.
What is ISO week 1?
ISO week 1 is the week containing the first Thursday of the year. January 1 is not necessarily in week 1.
Can the ISO year differ from the calendar year?
Yes. Dates in late December that fall in ISO week 1 belong to the next ISO year. Dates in early January that belong to week 52 or 53 of the prior year show the previous ISO year.
How many weeks are in a year?
Most years have 52 ISO weeks. Years that start on Thursday, or leap years starting on Wednesday, have 53 weeks.
How do I share my calculation?
Click "Share with my numbers" to copy a URL with your date.