Dates & Time

Week Number of the Year (ISO & US)

Tells you which week of the year a date falls in. You can switch between ISO 8601 with a Monday start and the US convention with a Sunday start, or count weeks between two dates.

How to use
  1. Enter a date, or use today.
  2. Choose ISO 8601 (Monday start) or US (Sunday start).
  3. Switch to the two-date mode to count weeks between dates.
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Week number
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Date math uses your inputs as given; check time-zone and leap-year edge cases.
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How it's calculated

ISO week = ⌈(yearday of the week's Thursday + 1) ÷ 7⌉

yearday = day of year (Jan 1 = 1). ISO 8601 weeks start Monday and week 1 holds the first Thursday of the year. US weeks start Sunday with week 1 beginning Jan 1.

Common questions

What is the difference between ISO and US week numbers?

ISO weeks start on Monday and week 1 holds the year's first Thursday, while the US convention starts weeks on Sunday and counts week 1 from January 1.

Can week 1 fall in the previous year?

Under ISO rules, early January days can belong to the last week of the prior year if they fall before that first Thursday.

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